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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/37777-0.txt b/37777-0.txt new file mode 100644 index 0000000..8169f94 --- /dev/null +++ b/37777-0.txt @@ -0,0 +1,6853 @@ +The Project Gutenberg EBook of Darwin, and After Darwin (Vol 3 of 3), by +George John Romanes + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Darwin, and After Darwin (Vol 3 of 3) + Post-Darwinian Questions: Isolation and Physiological Selection + +Author: George John Romanes + +Release Date: October 17, 2011 [EBook #37777] + +Language: English + +Character set encoding: UTF-8 + +*** START OF THIS PROJECT GUTENBERG EBOOK DARWIN, AFTER DARWIN (VOL 3 OF 3) *** + + + + +Produced by Marilynda Fraser-Cunliffe, LN Yaddanapudi and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + + + + + + + + +DARWIN, AND AFTER DARWIN + +III + +POST-DARWINIAN QUESTIONS + +ISOLATION AND PHYSIOLOGICAL SELECTION + + +BY THE SAME AUTHOR. + +DARWIN, AND AFTER DARWIN. An Exposition of the Darwinian Theory and a +Discussion of Post-Darwinian Questions. + + 1. THE DARWINIAN THEORY. With portrait of Darwin. 460 pages. 125 + illustrations. Cloth, $2.00. + + 2. POST-DARWINIAN QUESTIONS. Edited by Prof. C. Lloyd Morgan. With + portrait of G. J. Romanes. 338 pages. Cloth, $1.50. + + 3. POST-DARWINIAN QUESTIONS. ISOLATION AND PHYSIOLOGICAL SELECTION. + Edited by Prof. C. Lloyd Morgan. With portrait of Mr. J. T. + Gulick. 181 pages. Cloth, $1.00. + + All three volumes together, $4.00 net. + +AN EXAMINATION OF WEISMANNISM. With portrait of Weismann. 236 pages. +Cloth, $1.00. + +THOUGHTS ON RELIGION. Edited by Charles Gore, M.A., Canon of +Westminster. Third Edition. 184 pages. Cloth, gilt top, $1.25. + +THE OPEN COURT PUBLISHING COMPANY, +324 DEARBORN STREET, CHICAGO. + +[Illustration: Frontispiece--John J. Gulick] + + + + +DARWIN, AND AFTER DARWIN + +_AN EXPOSITION OF THE DARWINIAN THEORY +AND A DISCUSSION OF +POST-DARWINIAN QUESTIONS_ + +BY THE LATE + +GEORGE JOHN ROMANES, M.A., LL.D., F.R.S. + +_Honorary Fellow of Gonville and Caius College, Cambridge_ + + +III + +POST-DARWINIAN QUESTIONS + +ISOLATION +AND PHYSIOLOGICAL SELECTION + + +Chicago +THE OPEN COURT PUBLISHING COMPANY +1906 + +CHAPTER 1. COPYRIGHTED BY +THE OPEN COURT PUBLISHING CO. +1897. + + + +The Lakeside Press +R. R. DONNELLEY & SONS CO., CHICAGO + + + + +PREFACE + + +Of the six chapters which constitute this concluding volume of G. J. +Romanes' _Darwin, and after Darwin_, three, the first two and the last, +were in type at the time of his death. I have not considered myself at +liberty to make any alterations of moment in these chapters. For the +selection and arrangement of all that is contained in the other three +chapters I am wholly responsible. + +Two long controversial Appendices have been omitted. Those marked A and +B remain in accordance with the author's expressed injunctions. In a +third, marked C, a few passages from the author's note-books or MSS. +have been printed. + +The portrait of the Rev. J. Gulick, which forms the frontispiece, was +prepared for this volume before the author's death. Mr. Gulick's chief +contributions to the theory of physiological selection are to be found +in the Linnean Society's _Journal_ (_Zoology_, vols. xx and xxiii), and +in four letters to _Nature_ (vol. xli. p. 536; vol. xlii. pp. 28 and +369; and vol. xliv. p. 29). + +I have to thank Mr. Francis Galton, D.C.L., F.R.S. and Mr. F. Howard +Collins for valuable assistance generously rendered for the sake of one +whom all who knew him held dear. For he was, if I may echo the words of +Huxley, "a friend endeared to me, as to so many others, by his kindly +nature, and justly valued by all his colleagues for his powers of +investigation and his zeal for the advancement of science." + +C. LLOYD MORGAN. + +BRISTOL, _May 1897_. + + + + +CONTENTS + + +CHAPTER I. +ISOLATION 1 + +CHAPTER II. +ISOLATION (_continued_) 28 + +CHAPTER III. +PHYSIOLOGICAL SELECTION 41 + +CHAPTER IV. +EVIDENCES OF PHYSIOLOGICAL SELECTION 62 + +CHAPTER V. +FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION 81 + +CHAPTER VI. +A BRIEF HISTORY OF ISOLATION AS A FACTOR IN +ORGANIC EVOLUTION 101 + +GENERAL CONCLUSIONS 144 + +APPENDIX A. MR. GULICK'S CRITICISM OF MR. WALLACE'S +VIEWS ON PHYSIOLOGICAL SELECTION 151 + +APPENDIX B. AN EXAMINATION BY MR. FLETCHER +MOULTON OF MR. WALLACE'S CALCULATION TOUCHING +THE POSSIBILITY OF PHYSIOLOGICAL SELECTION +EVER ACTING ALONE 157 + +APPENDIX C. SOME EXTRACTS FROM THE AUTHOR'S +NOTE-BOOKS 169 + + + + +_ISOLATION_ + + + + +CHAPTER I. + +ISOLATION. + + +This treatise will now draw to a close by considering what, in my +opinion, is one of the most important principles that are concerned in +the process of organic evolution--namely, Isolation. I say in _my_ +opinion such is the case, because, although the importance of isolation +is more or less recognized by every naturalist, I know of only one other +who has perceived all that the principle involves. This naturalist is +the Rev. J. Gulick, and to his essays on the subject I attribute a +higher value than to any other work in the field of Darwinian thought +since the date of Darwin's death[1]. For it is now my matured conviction +that a new point of departure has here been taken in the philosophy of +Darwinism, and one which opens up new territories for scientific +exploration of an endlessly wide and varied character. Indeed I believe, +with Mr. Gulick, that in the principle of Isolation we have a principle +so fundamental and so universal, that even the great principle of +Natural Selection lies less deep, and pervades a region of smaller +extent. Equalled only in its importance by the two basal principles of +Heredity and Variation, this principle of Isolation constitutes the +third pillar of a tripod on which is reared the whole superstructure of +organic evolution. + + [1] It will be remembered that I regard Weismann's theory of + heredity, with all its deductive consequences, as still _sub + judice_. + +By isolation I mean simply the prevention of intercrossing between a +separated section of a species or kind and the rest of that species or +kind. Whether such a separation be due to geographical barriers, to +migration, or to any other state of matters leading to exclusive +breeding within the separated group, I shall indifferently employ the +term isolation for the purpose of designating what in all cases is the +same result--namely, a prevention of intercrossing between A and B, +where A is the separated portion and B the rest of the species or kind. + +The importance of isolation as against dissimilar forms has always been +fully appreciated by breeders, fanciers, horticulturists, &c., who are +therefore most careful to prevent their pedigree productions from +intercrossing with any other stock. Isolation is indeed, as Darwin has +observed, "the corner-stone of the breeder's art." And similarly with +plants and animals in a state of nature: unless intercrossing with +allied (i.e. dissimilar) forms is prevented, the principle of heredity +is bound to work for uniformity, by blending the dissimilar types in +one: only when there is exclusive breeding of similarly modified forms +can the principle of heredity work in the direction of change--i.e. of +evolution. + +Now, the forms of isolation--or the conditions which may lead to +exclusive breeding--are manifold. One of the most important, as well as +the most obvious, is geographical isolation; and no one questions that +this has been an important factor in the process of evolution, although +opinions still vary greatly as to the degree of its importance in this +respect. At one end of the series we may place the opinion of Mr. +Wallace, who denies that any of what may be termed the evolutionary +effect of geographical isolation is due to "influence exerted by +isolation _per se_." This effect, he says, is to be ascribed exclusively +to the fact that a geographically isolated portion of a species must +always encounter a change of environment, and therefore a new set of +conditions necessitating a new set of adaptations at the hands of +natural selection[2]. At the other end of the series we must place the +opinion of Moritz Wagner, who many years ago published a masterly +essay[3], the object of which was to prove that, in the absence of +geographical isolation (including migration), natural selection would be +powerless to effect any change of specific type. For, he argued, the +initial variations on which the action of this principle depends would +otherwise be inevitably swamped by free intercrossing. Wagner adduced a +large number of interesting facts in support of this opinion; but +although he thus succeeded in enforcing the truth that geographical +isolation is an important aid to organic evolution, he failed to +establish his conclusion that it is an indispensable condition. +Nevertheless he may have been right--and, as I shall presently show, I +believe he was right--in his fundamental premiss, that in the presence +of free intercrossing natural selection would be powerless to effect +divergent evolution. Where he went wrong was in not perceiving that +geographical isolation is not the only form of isolation. Had it +occurred to him that there may be other forms quite as effectual for the +prevention of free intercrossing, his essay could hardly have failed to +mark an epoch in the history of Darwinism. But, on account of this +oversight, he really weakened his main contention, namely, that in the +presence of free intercrossing natural selection must be powerless to +effect divergent evolution. This main contention I am now about to +re-argue. At present, therefore, we have only to observe that Wagner did +it much more harm than good by neglecting to perceive that free +intercrossing may be prevented in many other ways besides by migration, +and by the intervention of geographical barriers. + + [2] _Darwinism_, p. 150. + + [3] _The Darwinian Theory, and the Law of Migration_ (Eng. Trans., + Stanford, London, 1873). + +In order that we may set out with clearer views upon this matter, I will +make one or two preliminary remarks on the more general facts of +isolation as these are found to occur in nature. + +In the first place, it is obvious that isolation admits of degrees: it +may be either total or partial; and, if partial, may occur in numberless +grades of efficiency. This is so manifest that I need not wait to give +illustrations. But now, in the second place, there is another general +fact appertaining to isolation which is not so manifest, and a clear +appreciation of which is so essential to any adequate consideration of +the subject, that I believe the reason why evolutionists have hitherto +failed to perceive the full importance of isolation, is because they +have failed to perceive the distinction which has now to be pointed out. +The distinction is, that isolation may be either discriminate or +indiscriminate. If it be discriminate, the isolation has reference to +the resemblance of the separated individuals to one another; if it be +indiscriminate, it has no such reference. For example, if a shepherd +divides a flock of sheep without regard to their characters, he is +isolating one section from the other indiscriminately; but if he places +all the white sheep in one field, and all the black sheep in another +field, he is isolating one section from the other discriminately. Or, if +geological subsidence divides a species into two parts, the isolation +will be indiscriminate; but if the separation be due to one of the +sections developing, for example, a change of instinct determining +migration to another area, or occupation of a different habitat on the +same area, then the isolation will be discriminate, so far as the +resemblance of instinct is concerned. + +With the exception of Mr. Gulick, I cannot find that any other writer +has hitherto stated this supremely important distinction between +isolation as discriminate and indiscriminate. But he has fully as well +as independently stated it, and shown in a masterly way its far-reaching +consequences. Indiscriminate isolation he calls Separate Breeding, while +discriminate isolation he calls Segregate Breeding. For the sake, +however, of securing more descriptive terms, I will coin the words +Apogamy and Homogamy. Apogamy, of course, answers to indiscriminate +isolation, or separate breeding. Homogamy, on the other hand, answers +to discriminate isolation, or segregate breeding: only individuals +belonging to the same variety or kind are allowed to propagate. +Isolation, then, is a genus, of which Apogamy and Homogamy are +species[4]. + + [4] I may here most conveniently define the senses in which all the + following terms will be used throughout the present + discussion:--_Species_ of isolation are, as above stated, homogamy + and apogamy, or isolation as discriminate and indiscriminate. + _Forms_ of isolation are modes of isolation, such as the + geographical, the sexual, the instinctive, or any other of the + numerous means whereby isolation of either species may be secured. + _Cases_ of isolation are the instances in which any of the forms of + isolation may be at work: thus, if a group of _n_ intergenerants be + segregated into five groups, _a_, _b_, _c_, _d_, _e_, then, before + the segregation there would have been one case of isolation, but + after the segregation there would be five such cases. + +Now, in order to appreciate the unsurpassed importance of isolation as +one of the three basal principles of organic evolution, let us begin by +considering the discriminate species of it, or Homogamy. + +To state the case in the most general terms, we may say that if the +other two basal principles are given in heredity and variability, the +whole theory of organic evolution becomes neither more nor less than a +theory of homogamy--that is, a theory of the causes which lead to +discriminate isolation, or the breeding of like with like to the +exclusion of unlike. For the more we believe in heredity and variability +as basal principles of organic evolution, the stronger must become our +persuasion that discriminate breeding leads to divergence of type, while +indiscriminate breeding leads to uniformity. This, in fact, is securely +based on what we know from the experience supplied by artificial +selection, which consists in the intentional mating of like with like +to the exclusion of unlike. + +The point, then, which in the first instance must be firmly fastened in +our minds is this:--so long as there is free intercrossing, heredity +cancels variability, and makes in favour of fixity of type. Only when +assisted by some form of discriminate isolation, which determines the +exclusive breeding of like with like, can heredity make in favour of +change of type, or lead to what we understand by organic evolution. + +Now the forms of discriminate isolation, or homogamy, are very numerous. +When, for example, any section of a species adopts somewhat different +habits of life, or occupies a somewhat different station in the economy +of nature, homogamy arises within that section. There are forms of +homogamy on which Darwin has laid great stress, as we shall presently +find. Again, when for these or any other reasons a section of a species +becomes in any small degree modified as to form or colour, if the +species happens to be one where any psychological preference in pairing +can be exercised--as is very generally the case among the higher +animals--exclusive breeding is apt to ensue as a result of such +preference; for there is abundant evidence to show that, both in birds +and mammals, sexual selection is usually opposed to the intercrossing of +dissimilar varieties. Once more, in the case of plants, intercrossing of +dissimilar varieties may be prevented by any slight difference in their +seasons of flowering, of topographical stations, or even, in the case of +flowers which depend on insects for their fertilization, by differences +in the instincts and preferences of their visitors. + +But, without at present going into detail with regard to these different +forms of discriminate isolation, there are still two others, both of +which are of much greater importance than any that I have hitherto +named. Indeed, these two forms are of such immeasurable importance, that +were it not for their virtually ubiquitous operation, the process of +organic evolution could never have begun, nor, having begun, continued. + +The first of these two forms is sexual incompatibility--either partial +or absolute--between different taxonomic groups. If all hares and +rabbits, for example, were as fertile with one another as they are +within their own respective species, there can be no doubt that sooner +or later, and on common areas, the two types would fuse into one. And +similarly, if the bar of sterility could be thrown down as between all +the species of a genus, or all the genera of a family, _not otherwise +prevented from intercrossing_, in time all such species, or all such +genera, would become blended into a single type. As a matter of fact, +complete fertility, both of first crosses and of their resulting +hybrids, is rare, even as between species of the same genus; while as +between genera of the same family complete fertility does not appear +ever to occur; and, of course, the same applies to all the higher +taxonomic divisions. On the other hand, some degree of infertility is +not unusual as between different varieties of the same species; and, +wherever this is the case, it must clearly aid the further +differentiation of those varieties. It will be my endeavour to show that +in this latter connexion sexual incompatibility must be held to have +taken an immensely important part in the differentiation of varieties +into species. But meanwhile we have only to observe that _wherever_ such +incompatibility is concerned, it is to be regarded as an isolating +agency of the very first importance. And as it is of a character purely +physiological, I have assigned to it the name Physiological Isolation; +while for the particular case where this general principle is concerned +in the origination of specific types, I have reserved the name +Physiological Selection. + +The other most important form of discriminate isolation to which I have +alluded is Natural Selection. To some evolutionists it has seemed +paradoxical thus to regard natural selection as a form of isolation; but +a little thought will suffice to show that such is really the most +accurate way of regarding it. For, as Mr. Gulick says, "Natural +selection is the exclusive breeding of those better adapted to the +environment: ... it is a process in which the fittest are prevented from +crossing with the less fitted, by the exclusion of the less fitted." +Therefore it is, strictly and accurately, a mode of isolation, where the +isolation has reference to adaptation, and is secured in the most +effectual of possible ways--i.e. by the destruction of all individuals +whose intercrossing would interfere with the isolation. Indeed, the very +term "natural _selection_" shows that the principle is tacitly +understood to be one of isolation, because this name was assigned to the +principle by Darwin for the express purpose of marking the analogy that +obtains between it and the intentional isolation which is practised by +breeders, fanciers, and horticulturists. The only difference between +"natural selection" and "artificial selection" consists in this--that +under the former process the excluded individuals must necessarily +perish, while under the latter they need not do so. But clearly this +difference is accidental: it is in no way essential to the process +considered as a process of discriminate isolation. For, as far as +homogamous breeding is concerned, it can matter nothing whether the +exclusion of the dissimilar individuals is effected by separation or by +death. + +Natural selection, then, is thus unquestionably a form of isolation of +the discriminate kind; and therefore, notwithstanding its unique +importance in certain respects, considered as a principle of organic +evolution it is less fundamental--and also less extensive--than the +principle of isolation in general. In other words, it is but a part of a +much larger whole. It is but a particular form of a general principle, +which, as just shown, presents many other forms, not only of the +discriminate, but likewise of the indiscriminate kind. Or, reverting to +the terminology of logic, it is a sub-species of the species Homogamy, +which in its turn is but a constituent part of the genus Isolation. + +So much then for homogamy, or isolation of the discriminate order. +Passing on now to apogamy, or isolation of the indiscriminate kind, we +may well be disposed, at first sight, to conclude that this kind of +isolation can count for nothing in the process of evolution. For if the +fundamental importance of isolation in the production of organic forms +be due to its segregation of like with like, does it not follow that any +form of isolation which is indiscriminate must fail to supply the very +condition on which all the forms of discriminate isolation depend for +their efficacy in the causing of organic evolution? Or, to return to our +concrete example, is it not self-evident that the farmer who separated +his stock into two or more parts indiscriminately, would not effect any +more change in his stock than if he had left them all to breed together? + +Well, although at first sight this seems self-evident, it is in fact +untrue. For, unless the individuals which are indiscriminately isolated +happen to be a very large number, sooner or later their progeny will +come to differ from that of the parent type, or unisolated portion of +the previous stock. And, of course, as soon as this change of type +begins, the isolation ceases to be indiscriminate: the previous apogamy +has been converted into homogamy, with the usual result of causing a +divergence of type. The reason why progeny of an indiscriminately +isolated section of an originally uniform stock--e.g. of a species--will +eventually deviate from the original type is, to quote Mr. Gulick, as +follows:--"No two portions of a species possess exactly the same average +character, and, therefore, the initial differences are for ever reacting +on the environment and on each other in such a way as to ensure +increasing divergence as long as the individuals of the two groups are +kept from intergenerating[5]." Or, as I stated this principle in my +essay on _Physiological Selection_, published but a short time before +Mr. Gulick's invaluable contributions to these topics:-- + + [5] _Divergent Evolution through Cumulative Segregation_ (_Zool. + Journal, Linn. Soc._, vol. xx. pp. 189-274). + + As a matter of fact, we find that no one individual "is like + another all in all"; which is another way of saying that a specific + type may be regarded as the average mean of all its individual + variations, any considerable departure from this average being, + however, checked by intercrossing.... Consequently, if from any + cause a section of a species is prevented from intercrossing with + the rest of its species, we might expect that new varieties should + arise within that section, and that in time these varieties should + pass into new species. And this is just what we do find[6]. + + [6] The passage proceeds to show that in view of this consideration + we have a strong additional reason for rejecting the _a priori_ + dogma that all specific characters must necessarily be useful + characters. For it is evident that any divergence of specific + character which is brought about in this way need not present any + utilitarian significance--although, of course, natural selection + will ensure that it shall never be deleterious. + +The name which I gave to this cause of specific change was Independent +Variability, or variability in the absence of overwhelming +intercrossing. But it now appears to me that this cause is really +identical with that which was previously enunciated by DelbÅ“uf. +Again, in his important essay on _The Influence of Isolation_, Weismann +concludes, on the basis of a large accumulation of facts, that the +constancy of any given specific type "does not arise suddenly, but +gradually, and is established by the promiscuous intercrossing of all +individuals." From which, he says, it follows, that this constancy must +cease so soon as the condition which maintains it ceases--i. e. so soon +as intercrossing (Panmixia) between all individuals ceases, or so soon +as a portion of a species is isolated from its parent stock. To this +principle he assigns the name of Amixia. But Weismann's Amixia differs +from my Independent Variability in several important particulars; and on +this account I have designedly abstained from adopting his term. Here +it is enough to remark that it answers to the generic term Isolation, +without reference to the _kind_ of isolation as discriminate or +indiscriminate, homogamous or apogamous. On the other hand, my +Independent Variability is merely a re-statement of the so-called "Law +of DelbÅ“uf," which, in his own words, is as follows:-- + + One point, however, is definitely attained. It is that the + proposition, which further back we designated paradoxical, is + rigorously true, A constant cause of variation, however + insignificant it may be, changes the uniformity [of type] little by + little, and diversifies it _ad infinitum_. From the homogeneous, + left to itself, only the homogeneous can proceed; but if there be a + slight disturbance ["léger ferment"] in the homogeneous, the + homogeneity will be invaded at a single point, differentiation will + penetrate the whole, and, after a time--it may be an infinite + time--the differentiation will have disintegrated it altogether. + +In other words, the "Law," which DelbÅ“uf has formulated on +mathematical grounds, and with express reference to the question of +segregate breeding, proves that, no matter how infinitesimally small the +difference may be between the average qualities of an isolated section +of a species compared with the average qualities of the rest of that +species, if the isolation continues sufficiently long, differentiation +of specific type is necessarily bound to ensue. But, to make this +mathematical law biologically complete, it ought to be added that the +time required for the change of type to supervene (supposing apogamy to +be the only agent of change) will be governed by the range of individual +variability which the species in question presents. A highly stable +species (such as the Goose) might require an immensely long time for +apogamy alone to produce any change of type in an isolated portion of +the species, while a highly variable species (such as the Ruff) would +rapidly change in any portion that might be indiscriminately isolated. +It was in order to recognize this additional and very important factor +that I chose the name Independent _Variability_ whereby to designate the +diversifying influence of merely indiscriminate isolation, or apogamy. +Later on Mr. Gulick published his elaborate papers upon the divergence +of type under all kinds of isolation; and retained my term Independent, +but changed Variability into Generation. I point this out merely for the +sake of remarking that his Independent Generation is exactly the same +principle as my Independent Variability, and DelbÅ“uf's Mathematical +Law. + +Now, while I fully agree with Mons. Giard where he says, in the +introductory lecture of his course on _The Factors of Evolution_[7], +that sufficient attention has not been hitherto given by naturalists to +this important factor of organic evolution (apogamy), I think I have +shown that among those naturalists who have considered it there is a +sufficient amount of agreement. _Per contra_, I have to note the opinion +of Mr. Wallace, who steadily maintains the impossibility of any cause +other than natural selection (i.e. one of the forms of homogamy) having +been concerned in the evolution of species. But at present it is enough +to remark that even Professor Ray Lankester--whose leanings of late +years have been to the side of ultra-Darwinism, and who is therefore +disposed to agree with Mr. Wallace wherever this is logically +possible--even Professor Ray Lankester observes:-- + + [7] _Revue Scientifique_, Nov. 23, 1889. + + Mr. Wallace does not, in my judgement, give sufficient grounds for + rejecting the proposition which he indicates as the main point of + Mr. Gulick's valuable essay on _Divergent Evolution through + Cumulative Segregation_. Mr. Gulick's idea is that ... no two + portions of a species possess exactly the same average character, + and the initial differences will, if the individuals of the two + groups are kept from intercrossing, assert themselves continuously + by heredity in such a way as to ensure an increasing divergence of + the forms belonging to the two groups, amounting to what is + recognized as specific distinction. Mr. Gulick's idea is simply the + recognition of a permanence or persistency in heredity, which, + _caeteris paribus_, gives a twist or direction to the variations of + the descendants of one individual as compared with the descendants + of another[8]. + + [8] _Nature_, Oct. 10, 1889, p. 568. + +Now we have seen that "Mr. Gulick's idea," although independently +conceived by him, had been several times propounded before; and it is +partly implicated in more than one passage of the _Origin of Species_, +where free intercrossing, or the _absence_ of isolation, is alluded to +as maintaining the _constancy_ of a specific type[9]. Moreover, it is +still more fully recognized in the last edition of the _Variation of +Animals and Plants_, where a paragraph is added for the purpose of +sanctioning the principle in the imperfect form that it was stated by +Weismann[10]. Nevertheless, to Mr. Gulick belongs the credit, not only of +having been the first to conceive (though the last to publish) the +"idea" in question, and of having stated it with greater fullness than +anybody else; but still more of having verified its importance as a +factor of organic evolution. + + [9] e. g. p. 81. + + [10] See Chapter xxiii. vol. ii. p. 262. (Edition of 1888.) + +For, in point of fact, Mr. Gulick was led to his recognition of the +principle in question, not by any deductive reasoning from general +principles, but by his own particular and detailed observations of the +land mollusca of the Sandwich Islands. Here there are an immense number +of varieties belonging to several genera; but every variety is +restricted, not merely to the same island, but actually to the same +valley. Moreover, on tracing this fauna from valley to valley, it is +apparent that a slight variation in the occupants of valley 2 as +compared with those of the adjacent valley 1, becomes more pronounced in +the next--valley 3, still more so in 4, &c., &c. Thus it was possible, +as Mr. Gulick says, roughly to estimate the amount of divergence between +the occupants of any two given valleys by measuring the number of miles +between them. + +As already stated, I have myself examined his wonderful collection of +shells, together with a topographical map of the district; and therefore +I am in a position to testify to the great value of Mr. Gulick's work in +this connexion, as in that of the utility question previously +considered. The variations, which affect scores of species, and +themselves eventually run into fully specific distinctions, are all more +or less finely graduated as they pass from one isolated region to the +next; and they have reference to changes of form and colour, which in no +one case presents any appearance of utility. Therefore--and especially +in view of the fact that, as far as he could ascertain, the environment +in the different valleys was essentially the same--no one who examines +this collection can wonder that Mr. Gulick attributes the results which +he has observed to the influence of apogamy alone, without any reference +to utility or natural selection. + +To this solid array of remarkable facts Mr. Wallace has nothing further +to oppose than his customary appeal to the argument from ignorance, +grounded on the usual assumption that no principle other than natural +selection _can_ be responsible for even the minutest changes of form or +colour. For my own part, I must confess that I have never been so deeply +impressed by the dominating influence of the _a priori_ method as I was +on reading Mr. Wallace's criticism of Mr. Gulick's paper, after having +seen the material on which this paper is founded. To argue that every +one of some twenty contiguous valleys in the area of the same small +island must necessarily present such differences of environment that all +the shells in each are differently modified thereby, while in no one out +of the hundreds of cases of modification in minute respects of form and +colour can any human being suggest an adaptive reason therefor--to argue +thus is merely to affirm an intrinsically improbable dogma in the +presence of a great and consistent array of opposing facts. + +I have laid special stress on this particular case of the Sandwich +Islands' mollusca, because the fifteen years of labour which Mr. Gulick +has devoted to their exhaustive working out have yielded results more +complete and suggestive than any which so far have been forthcoming with +regard to the effects of isolation in divergent evolution. But, if space +permitted, it would be easy to present abundance of additional facts +from other sources, all bearing to the same conclusion--namely, that as +a matter of direct observation, no less than of general reasoning, any +unprejudiced mind will concede to the principle of indiscriminate +isolation an important share in the origination of organic types. For as +indiscriminate isolation is thus seen sooner or later to become +discriminate, and as we have already seen that discriminate isolation is +a necessary condition to all or any modification, we can only conclude +that isolation in both its kinds takes rank with heredity and +variability as one of the three basal principles of organic evolution. + + * * * * * + +Having got thus far in the way of generalities, we must next observe +sundry further matters of comparative detail. + +1. In any case of indiscriminate isolation, or apogamy, the larger the +bulk of the isolated section the more nearly must its average qualities +resemble those of its parent stock; and, therefore, the less divergence +of character will ensue in a given time from this cause alone. For +instance, if one-fourth of a large species were to be separated from the +other three-fourths (say, by subsidence causing a discontinuity of +area), it would continue the specific characters unchanged for an +indefinitely long time, so far as the influence of such an +indiscriminate isolation is concerned. But, on the other hand, if only +half a dozen individuals were to be thus separated from the rest of +their species, a comparatively short time would be needed for their +descendants to undergo some varietal modification at the hands of +apogamy. For, in this case, the chances would be infinitely against the +average characters of the original half-dozen individuals exactly +coinciding with those of all the rest of their species. + +2. In any case of homogamy, however, it is immaterial what proportional +number of individuals are isolated in the first instance. For the +isolation is here discriminate, or effected by the initial difference of +the average qualities themselves--a difference, therefore, which +presupposes divergence as having already commenced, and equally bound to +proceed whether the number of intergenerants be large or small. + +It may here be remarked that, in his essay on the _Influence of +Isolation_, Professor Weismann fails to distinguish between the two +kinds of isolation. This essay deals only with one of the many different +forms of isolation--the geographical--and is therefore throughout +concerned with a consideration of diversity as arising from apogamy +alone. But in dealing with this side of the matter Weismann anticipated +both Gulick and myself in pointing out the law of inverse proportion, +which I have stated in the preceding paragraph in what appears to me its +strictly accurate form. + +3. Segregate Breeding, or homogamy, which arises under any of the many +forms of discriminate isolation, must always tend to be _cumulative_. +For, again to quote Mr. Gulick, who has constituted this fact the most +prominent as it is the most original feature of his essay, "In the first +place, every new form of Segregation[11] that now appears depends on, and +is superimposed upon, forms of Segregation that have been previously +induced; for when Negative Segregation arises [i. e. isolation due to +mutual sterility], and the varieties of a species become less and less +fertile with one another, the complete infertility that has existed +between them and some other species does not disappear, nor does the +Positive Segregation cease [i. e. any other form of isolation previously +existing].... In the second place, whenever Segregation is directly +produced by some quality of the organism, variations that possess the +endowment in a superior degree will have a larger share in producing the +segregated forms of the next generation, and accordingly the segregative +endowment of the next generation will be greater than that of the +present generation; and so with each successive generation the +segregation will become increasingly complete." And to this it may be +added, in the third place, that where the segregation (isolation) is due +to the external conditions of life under which the organism is placed, +or where it is due to natural selection simultaneously operating in +divergent lines of evolution, the same remarks apply. Hence it follows +that discriminate isolation is, in all its forms, cumulative. + + [11] This term may here be taken as equivalent to Isolation. + +4. The next point to be noted is, that the cumulative divergence of type +thus induced can take place only in as many different lines as there are +different _cases_ of isolation. This is a point which Mr. Gulick has not +expressly noticed; but it is one that ought to be clearly recognized. +Seeing that isolation secures the breeding of similar forms by exclusion +(immediate or eventual) of those which are dissimilar, and that only in +as far as it does this can it be a factor in organic evolution, it +follows that the resulting segregation, even though cumulative, can only +lead to divergence of organic types in as many directions as there are +cases of isolation. For any one group of intergenerants only _serial_ +transformation is possible, even though the transformation be cumulative +through successive generations in the single line of change. But there +is always a probability that during the course of such _serial +transformation in time_, some other case of isolation may supervene, so +as to divide the previously isolated group of intergenerants into two or +more further isolated groups. Then, of course, opportunity will be +furnished for _divergent transformation in space_--and this in as many +different lines as there are now different homogamous groups. + +That this must be so is further evident, if we reflect that the +evolutionary power of isolation depends, not only on the _preventing_ of +intercrossing between the isolated portion of a species and the rest of +that species, but also upon the _permitting_ of intercrossing between +all individuals of the isolated portion, whereby the peculiar average of +qualities which they as a whole present may be allowed to assert itself +in their progeny--or, if the isolation has been from the first +discriminate, whereby the resulting homogamy may thus be allowed to +assert itself. Hence any one case of either species of isolation, +discriminate or indiscriminate, can only give rise to what Mr. Gulick +has aptly called "monotypic evolution," or a chain-like series of types +arising successively in time, as distinguished from what he has called +"polytypic evolution," or an arborescent multiplication of types +arising simultaneously in space. + +For example, let us again take the geographical form of isolation. Where +a single small intergenerant group of individuals is separated from the +rest of its species--say, on an oceanic island--_monotypic_ evolution +may take place through a continuous and cumulative course of independent +variation in a single line of change: all the _individuals_ composing +any one given generation will closely resemble one another, although the +_type_ may be progressively altering through a long series of +generations. But if the original species had had two small colonies +separated from itself (one on each of two different islands, so giving +rise to two cases of isolation), then _polytypic_ evolution would have +ensued to the extent of there having been two different lines of +evolution going on simultaneously (one upon each of the two islands +concerned). Similarly, of course, if there had been three or four such +colonies, there would have been three or four divergent lines of +evolution, and so on. + +5. In the _cases_ of isolation just supposed there is only one _form_ of +isolation; and it is thus shown that under one form of isolation there +may be as many lines of divergence as there are separate cases of such +isolation. But now suppose that there are two or more forms of +isolation--for instance, that on the same oceanic island the original +colony has begun to segregate into secondary groups under the influence +of natural selection, sexual selection, physiological selection, or any +of the other forms of isolation--then there will be as many lines of +divergent evolution going on at the same time (and here on the same +area) as there are forms of isolation affecting the oceanic colony. And +this because each of the _forms_ of isolation has given rise to a +different _case_ of isolation. + +Now, inasmuch as different forms of isolation, when thus superadded one +to another, constitute different cases of isolation, we may lay down the +following general law as applying to all the forms of isolation--namely, +_The number of possible directions in which divergent evolution can +occur, is never greater than, though it may be equal to, the number of +cases of efficient isolation--or the number of efficiently separated +groups of intergenerants._ + +6. We have now to consider with some care the particular and highly +important form of isolation that is presented by natural selection. For +while this form of isolation resembles all the other forms of the +discriminate kind in that it secures homogamy, there are two points in +which it differs from all of them, and one point in which it differs +from most of them. + +Natural selection differs from _all_ the other known forms of isolation +(whether discriminate or indiscriminate) in that it has exclusive +reference to _adaptations_ on the one hand, and, on the other hand, +necessitates not only the elimination, but the destruction of the +excluded individuals. Again, natural selection differs from _most_ of +the other forms of isolation in that, unless assisted by some other +form, it can never lead to polytypic, but only to monotypic evolution. +The first two points of difference are here immaterial; but the last is +one of the highest importance, as we shall immediately perceive. + +In nearly all the other forms of isolation, polytypic or divergent +evolution may arise under the influence of that form alone, or without +the necessary co-operation of any other form. This we have already seen, +for example, in regard to geographical isolation, under which there may +be as many different lines of transmutation going on simultaneously as +there are different cases of isolation--say, in so many different +oceanic islands. Again, in regard to physiological isolation the same +remark obviously applies; for it is evident that even upon the same +geographical area there may be as many different lines of transmutation +going on simultaneously as there are cases of this form of isolation. +The bar of mutual sterility, whenever and wherever it occurs, must +always render polytypic evolution possible. And so it is with almost all +the other forms of isolation: that is to say, one _form_ does not +necessarily require the assistance of another _form_ in order to create +an additional _case_ of isolation. But it is a peculiarity of natural +selection, considered as a form of isolation, that it does necessarily +require the assistance of some other form before it can give rise to an +additional case of isolation; and therefore before it can give rise to +any _divergence_ of character in ramifying lines, as distinguished from +_transformation_ of characters in a single line. Or, in other words, +natural selection, when acting alone, can never induce polytypic +evolution, but only monotypic. + +That this important conclusion is a necessary deduction from the theory +of natural selection itself, a very few words will be enough to show. +For, according to the theory, survival of the fittest is a form of +isolation which acts through utility, by _destroying_ all the +individuals whom it fails to isolate. Hence it follows that survival of +the fittest is a form of isolation which, if acting alone, cannot +_possibly_ effect divergent evolution. For, in the first place, there is +nothing in this form of isolation to ensure that the fitter individuals +should fail to interbreed with the less fit which are able to survive; +and, in the second place, in all cases where the less fit are not +sufficiently fit to be suffered to breed, they are exterminated--i. e. +not permitted to form a distinct variety of their own. If it be said +that survival of the fittest may develop simultaneously two or more +lines of _useful_ change, the answer is that it can only do this if each +of the developing varieties is isolated from the others by some +_additional form_ of isolation; for, if not, there can be no +commencement of utilitarian _divergence_, since whatever number of +utilitarian changes may be in course of simultaneous development, they +must in this case be all blended together in a single line of specific +transmutation. Nay, even if specific divergence has actually been +commenced by natural selection when associated with some other form of +homogamy, if the latter should afterwards be withdrawn, natural +selection would then be unable to maintain even so much divergence of +character as may already have been attained: free intercrossing between +the two collateral, and no longer isolated branches, would ensure their +eventual blending into a common stock. Therefore, I repeat, natural +selection, when acting alone, can never induce polytypic evolution, but +only monotypic. + +Now I regret to say that here, for the first and only time throughout +the whole course of the present treatise, I find myself in seeming +opposition to the views of Darwin. For it was the decidedly expressed +opinion of Darwin that natural selection _is_ competent to effect +polytypic, or divergent, evolution. Nevertheless, I believe that the +opposition is to a large extent only apparent, or due merely to the fact +that Darwin did not explicitly state certain considerations which +throughout his discussion on "divergence of character" are seemingly +implied. But, be this as it may, I have not even appeared to desert his +leadership on a matter of such high importance without having duly +considered the question in all its bearings, and to the utmost limit of +my ability. Moreover, about two years after the publication of my first +paper[12] upon the subject, Mr. Gulick followed, at somewhat greater +length, in the same line of dissent. Like all the rest of his work, this +is so severely logical in statement, as well as profoundly thought out +in substance, that I do not see how it is possible for any one to read +impartially what he has written, and then continue to hold that natural +selection, if unassisted by any other form of isolation, can possibly +effect divergence of character--or polytypic as distinguished from +monotypic evolution[13]. + + [12] _Zool. Journal Lin. Soc._, vol. xix. pp. 337-411. + + [13] _Ibid._, vol. xx. pp. 202-212. + +I may here quote from Mr. Gulick's paper three propositions, serving to +state three large and general bodies of observable fact, which +severally and collectively go to verify, with an overwhelming mass of +evidence, the conclusion previously reached on grounds of general +reasoning. + + The facts of geographical distribution seem to me to justify the + following statements:-- + + (1) A species exposed to different conditions in the different + parts of the area over which it is distributed, is not represented + by divergent forms when free interbreeding exists between the + inhabitants of the different districts. In other words, Diversity + of Natural Selection without Separation does not produce divergent + evolution. + + (2) We find many cases in which areas, corresponding in the + character of the environment, but separated from each other by + important barriers, are the homes of divergent forms of the same or + allied species. + + (3) In cases where the separation has been long continued, and the + external conditions are the most diverse in points that involve + diversity of adaptation, there we find the most decided divergences + in the organic forms. That is, where Separation and Divergent + Selection have long acted, the results are found to be the + greatest. + + The 1st and 3rd of these propositions will probably be disputed by + few, if by any. The proof of the 2nd is found wherever a set of + closely allied organisms is so distributed over a territory that + each species and variety occupies its own narrow district, within + which it is shut by barriers that restrain its distribution while + each species of the environing types is distributed over the whole + territory. The distribution of terrestrial molluscs on the Sandwich + Islands presents a great body of facts of this kind. + + + + +CHAPTER II. + +ISOLATION (_continued_). + + +I will now recapitulate the main doctrines which have been set forth in +the foregoing chapter, and then proceed to consider the objections which +have been advanced against them. + +It must be remembered that by isolation I mean exactly what Mr. Gulick +does by "Segregation," and approximately what Professor Weismann does by +"Amixia "--i. e. the prevention of intercrossing. + +Isolation occurs in very many forms besides the geographical, as will be +more fully shown at the end of this chapter; and in all its forms it +admits of degrees. + +It also occurs in two very different species or kinds--namely, +discriminate and indiscriminate. These I have called respectively +Homogamy and Apogamy. This all-important distinction has been clearly +recognized by Mr. Gulick, as a result of his own thought and +observation, independently of anything that I have published upon the +subject. + +In view of this distinction Isolation takes rank with Heredity and +Variability as one of the most fundamental principles of organic +evolution. For, if these other two principles be granted, the whole +theory of descent resolves itself into an inquiry touching the causes, +forms, and degrees of Homogamy. + +Save in cases where very large populations are concerned, apogamy must +sooner or later give rise _per se_ to homogamy, owing to the Law of +DelbÅ“uf, which is the principle that I have called Independent +Variability, and Gulick has called Independent Generation. But of course +this does not hinder that under apogamy various other causes of homogamy +are likely to arise--in particular natural selection. + +That natural selection differs from most of the other forms of isolation +in not being capable of causing _divergent_ or _polytypic_ evolution +must at once become evident, if we remember that the only way in which +isolation of any form can cause such evolution is by partitioning a +given group of intergenerants into two or more groups, each of which is +able to survive as thus separated from the other, and so to carry on the +evolution in divergent lines. But the distinguishing peculiarity of +natural selection, considered as a form of isolation, is that it effects +the isolation _by killing off all the individuals which it fails to +isolate_: consequently, this form of isolation differs from other forms +in prohibiting the possibility of any ramification of a single group of +intergenerants into two or more groups, for the purpose of carrying on +the evolution in divergent lines. Therefore, under this form of +isolation alone, evolution must proceed, palm-like, in a single line of +growth. So to speak, the successive generations continuously ascend to +higher things on the steps supplied by their own "dead selves"; but in +doing so they must climb a single ladder, no rung of which can be +allowed to bifurcate in the presence of the uniformity secured _for that +generation_ by the free intercrossing of the most fit. Even though +beneficial variations may arise in two or more directions +simultaneously, and all be simultaneously selected by survival of the +fittest, the effect of free intercrossing (in the absence of any other +form of isolation) will be to fuse all these beneficial variations into +one common type, and so to end in _monotypic_ evolution as before. In +order to secure _polytypic_ evolution, intercrossing between the +different beneficial variants which may arise must be prevented; and +there is nothing to prevent such intercrossing in the process of natural +selection _per se_. In order that the original group of intergenerants +should be divided and sub-divided into two or more groups of +intergenerants, some additional form of isolation must necessarily +supervene--when, of course, polytypic evolution will result. And, as Mr. +Gulick has shown, the conclusion thus established by deductive reasoning +is verified inductively by the facts of geographical distribution. + +How, then, are we to account for the fact that Darwin attributed to +natural selection the power to cause divergence of character? The answer +is sufficiently simple. _He does so by tacitly invoking the aid of some +other form of homogamy in every case._ If we carefully read pp. 86-97 of +the _Origin of Species_, where this subject is under consideration, we +shall find that in every one of the arguments and illustrations which +are adduced to prove the power of natural selection to effect +"divergence of character," he either pre-supposes or actually names some +other form of homogamy as the originating cause of the diversity that +is afterwards presented to natural selection for further +intensification. To give only one example. At the starting-point of the +whole discussion the priority of such other forms of homogamy is assumed +in the following words:-- + + But how, it may be asked, can any analogous principle [to that of + diversity caused by artificial selection] apply in nature? I + believe it can and does apply most efficiently (though it was a + long time before I saw how), from the simple circumstance that the + more diversified the descendants from any one species become in + structure, constitution, and habits, by so much will they be better + enabled to seize on many and widely diversified places in the + polity of nature, and so be enabled to increase in numbers. + +Now, without question, so soon as segregate breeding in two or more +lines of homogamy has been in any sufficient degree determined by some +"change of structure, constitution, or habits," natural selection will +forthwith proceed to increase the divergence in as many different lines +as there are thus yielded discriminately isolated sections of the +species. And this fact it must have been that Darwin really had before +his mind when he argued that diversification of character is caused by +natural selection, through the benefit gained by the diversified forms +being thus "enabled to increase in number." Nevertheless he does not +expressly state the essential point, that although diversification of +character, _when once begun_, is thus _promoted_ by natural selection, +which forthwith proceeds to cultivate each of the resulting branches, +yet diversification of character can never be _originated_ by natural +selection. The change of "structure," of "constitution," of "habits," of +"station," of geographical area, of reciprocal fertility, and so +on--this change, _whatever_ it may have been, must clearly have been +antecedent to any operation of natural selection through the benefit +which arose from the change. Therefore the change must in all cases have +been due, in the first instance, to some other form of isolation than +the superadded form which afterwards arose from superior fitness in the +possession of superior benefit--although, so long as the prior form of +isolation endured, or continued to furnish the necessary condition to +the co-operation of survival of the fittest, survival of the fittest +would have continued to increase the divergence of character in as many +ramifying lines as there were thus given to its action separate cases of +isolation by other means. + +In short, as divergence of character must in all cases be due to a +prevention of intercrossing, and as in the process of natural selection +there is, _ex hypothesi_, nothing to prevent the intercrossing until the +divergence has already arisen, to suppose that natural selection alone +can have caused the divergence, is to suppose that natural selection can +have caused the conditions of its own activity, which is absurd. + +Seeing, then, that even in cases where any "benefit" arises from +divergence of character, such benefit can arise only after the +divergence has already commenced, and seeing that on this as on other +accounts previously mentioned it is plainly impossible to attribute the +origin of such divergence to natural selection, we find that natural +selection must be in all cases assisted by some other form of isolation, +if it is to be concerned in polytypic as distinguished from monotypic +evolution. But this does not hinder that, when it is so assisted, +natural selection may become--and, I believe, does become--the most +efficient of all the forms of isolation in promoting divergence of +character. For, in the first place, of all the forms of isolation +natural selection is probably the most energetic in promoting monotypic +evolution; so that under the influence of such isolation monotypic +evolution probably advances more rapidly than it does under any other +form of isolation. In the second place, when polytypic evolution has +been begun by any of these other forms of isolation, and natural +selection then sets to work on each of the resulting branches, although +natural selection is thus engaged in as many different acts of monotypic +evolution as there are thus separate cases supplied to it by these other +forms of isolation, the joint result of all these different acts is to +hurry on the polytypic evolution which was originally started by the +other forms of isolation. So to speak, natural selection is the forcing +heat, acting simultaneously on each of the separate branches which has +been induced to sprout by other means; and in thus rapidly advancing the +growth of all the branches, it is still entitled to be regarded as the +most important _single_ cause of diversification in organic nature, +although we must henceforth cease to regard it as in any instance the +_originating_ cause--or even so much as the _sustaining_ cause. + +So much by way of summary and recapitulation. I will now briefly +consider the only objections which, so far as I can see, admit of being +brought against the foregoing doctrine of Isolation as held by Mr. +Gulick and myself. These possible objections are but two in +number--although but one of them has been hitherto adduced. This, +therefore, I will take first. + +Mr. Wallace, with his customary desire to show that natural selection is +everywhere of itself capable of causing organic evolution, seeks to +minimize the swamping effects of free intercrossing, and the consequent +importance of other forms of isolation. His argument is as follows. + +Alluding to the researches of Mr. J. A. Allen, and others, on the amount +of variation presented by individuals of a species in a state of nature, +Mr. Wallace shows that, as regards any given part of the animal under +consideration, there is always to be found a considerable range of +individual variation round the average mean which goes to constitute the +specific character of the type. Thus, for example, Mr. Allen says of +American birds, "that a variation of from fifteen to twenty per cent. in +general size, and an equal degree of variation in the relative size of +different parts, may be ordinarily expected among specimens from the +same species and sex, taken at the same locality, while in some cases +the variation is even greater than this." Now, Mr. Wallace is under the +impression that these facts obviate the difficulty which arises from the +presence of free intercrossing--the difficulty, that is, against the +theory of natural selection when natural selection is supposed to have +been the exclusive means of modification. For, as he says, "if less size +of body would be beneficial, then, as half the variations in size are +above and half below the mean or existing standard of the species, there +would be ample beneficial variations"; and similarly with regard to +longer or shorter legs, wings, tails, &c., darker or lighter colour, and +so on through all the parts of any given organism. + +Well, although I have no wish at all to disparage the biological value +of these actual measurements of the range of individual variation, I +must point out that they are without any value at all in the connexion +which Mr. Wallace adduces them. We did not require these measurements to +tell us the broad and patent fact that "no being on this earthly ball is +like another all in all"--or, in less Tennysonian words, that as regards +every specific structure there is a certain amount of individual +variability round an average mean. Indeed, in my own paper on +_Physiological Selection_--against which Mr. Wallace is here specially +arguing--I expressly said, as previously remarked, "that a specific type +may be regarded as _the average mean of all individual variations_." The +fact of such individual variability round a specific mean has always +been well known to anatomists; it constitutes one of the basal pillars +of the whole Darwinian theory; and is besides a matter of universal +recognition as regards human stature, features, and so forth. The value +of Mr. Allen's work consists in accurately measuring the _amount_ or +_range_ of individual variation; but the question of its amount or range +is without relevancy in the present connexion. For the desirability of +isolation as an aid to natural selection even where monotypic evolution +is concerned, does not arise with any reference to the amount or range +of variation: it arises with reference to the _number_ of variations +which are--or are not--_similar_ and _simultaneous_. If there be a +sufficient number which are both similar and simultaneous, the +desirability of any co-operating form of isolation is correspondingly +removed, because natural selection may then have sufficient material +wherewith to overcome the adverse influence of free intercrossing, and +so of itself to produce monotypic evolution. Now, variations may be +numerous, similar, and simultaneous, either on account of some common +cause acting on many individuals at the same time, or on account of the +structures in question being more or less variable round a specific +mean. In the latter case--which is the only case that Mr. Allen's +measurements have to do with--the law of averages will of course +determine that half the whole number of variations in any given +structure, in any given generation, will be above the mean line. But, +equally of course, no one has ever denied that where, for either of +these reasons, natural selection is provided with sufficient material, +it is correspondingly capable of improving the specific type without the +assistance of any other form of homogamy; so to speak, they protect +themselves by their very numbers, and their superiority over others +leads to their survival and accumulation. But what is the result? _The +result can only be monotypic evolution._ No matter how great the number, +or how great the range, of variations round an average specific mean, +out of such material natural selection can never produce _polytypic_ +evolution: it may _change_ the type to any extent during successive +generations, and in a single line of change; but it cannot _branch_ the +type, unless some other form of homogamy intervenes. Therefore, when Mr. +Wallace adduces the well-known fact that all structures vary more or +less round a specific mean as proof that natural selection need not be +incommoded by free intercrossing, but can of itself produce all the +known phenomena of specific evolution, he fails to perceive that his +argument refers only to one aspect of such evolution (viz. the +transformation of species in time), and does not apply to the aspect +with which alone my paper on _Physiological Selection_ was concerned +(viz. the multiplication of species in space). + +The same thing may be shown in this way. It is perfectly obvious that +where the improvement of type in a linear series is concerned (monotypic +evolution), free intercrossing, far from being a hindrance to the +process, _is the very means by which the process is accomplished_. +Improvement here ascends by successive steps, in successive generations, +simply _because_ of the general intercrossing of the generally most fit +with the result that the species, _as a whole_, gradually becomes +transformed into another species, _as a whole_. Therefore, it would be +mere fatuity in any one to adduce free intercrossing as a "difficulty" +against natural selection alone being competent to produce evolution of +this kind. But where the kind of evolution is that whereby the species +is _differentiated_--where it is required, for instance, to produce +different structures in different portions of the species, such as the +commencement of a fighting spur on the wing of a duck, or _novel_ +characters of any sort in different groups of the species--free +intercrossing is no longer a condition to, but an absolute preventive +of, the process; and, therefore, unless checked as between each portion +of the species by some form of homogamy other than natural selection, +it must effectually inhibit any _segregation_ of specific types, or +divergence of character. + +Hence it is that, while no Darwinian has ever questioned the power of +unaided selection to cause _improvement of character in successive +generations_, in common now with not a few other Darwinians I have +emphatically denied so much as the abstract possibility of selection +alone causing a _divergence of character in two or more simultaneous +lines of change_. + +And, although these opposite views cannot be reconciled, I am under the +impression that they do admit of being explained. For I take them to +indicate a continued failure to perceive the all-important distinction +between evolution as monotypic and polytypic. Unless one has fully +grasped this distinction, and constantly holds it in mind, he is not in +a position to understand the "difficulty" in question; nor can he avoid +playing fast and loose with natural selection as possibly the sole cause +of evolution, and as necessarily requiring the co-operation of some +other cause. But if he once clearly perceives that "evolution" is a +logical genus, of which the monotypic and the polytypic forms are +species, he will immediately escape from his confusion, and find that +while the monotypic form may be caused by natural selection alone the +polytypic form can never be so caused. + + * * * * * + +The second difficulty which I have to mention as at first sight +attaching to the views of Mr. Gulick and myself on the subject of +Isolation is, that in an isolated section of a species Mr. Francis +Galton's law of regression in the average character of offspring to the +typical character of the group through reversion or atavism (_Natural +Inheritance_, p. 97) must have the effect of neutralizing the +segregative influence of mere apogamy. That such, however, cannot be the +case has been well shown by Mr. Gulick in his paper on _Intensive +Segregation_. Without at all disputing the validity of Mr. Galton's law, +he proves that "it can hold in full force only where there is free +crossing, otherwise no divergent race could ever be formed by any amount +of selection and independent breeding[14]." This is so self-evident that +I need not quote his demonstration of the point. + + [14] _Zool. Journal Lin. Soc._, vol. xxiii. p. 313. + + * * * * * + +In conclusion, then, and having regard to the principle of isolation as +a whole, or in all the many and varied forms in which this principle +obtains, I trust that I have redeemed the promise with which I set +out--viz. to show that in relation to the theory of descent this +principle is of an importance second to no other, not even excepting +heredity, variability, and the struggle for existence. This has now been +fully shown, inasmuch as we have clearly seen that the importance of the +struggle for existence, and consequent survival of the fittest, arises +just because survival of the fittest is a form, and a very stringent +form, of isolation; while, as regards both heredity and variability, we +are now in a position to see that the more fully we recognize their +supreme importance as principles concerned in organic evolution, the +more must we also recognize that any rational theory of such evolution +becomes, in the last resort, a theory of the different modes in which +efficient isolation can be secured. For, in whatever degree the process +of organic evolution has been dependent upon heredity with variability, +in that degree must it also have been dependent upon the means of +securing homogamy, whereby alone the force of heredity can be made to +expend itself in the innumerable directions of progressive change, +instead of continually neutralizing the force of variability by +promiscuous intercrossing. + + + + +CHAPTER III. + +PHYSIOLOGICAL SELECTION. + + +So far we have been concerned with the principle of Isolation in +general. We have now to consider that form of isolation which arises in +consequence of mutual infertility between the members of any group of +organisms and those of all other similarly isolated groups occupying +simultaneously the same area. + + * * * * * + +Against the view that natural selection is a sufficient explanation of +the origin of species, there are two fatal difficulties: one, the +contrast between natural species and domesticated varieties in respect +of cross-sterility; the other, the fact that natural selection cannot +possibly give rise to polytypic as distinguished from monotypic +evolution. Now it is my belief that the theory of physiological +selection fully meets both these difficulties. Indeed I hold this to be +undeniable in a formal or logical sense: the only question is as to the +evidence which can be adduced for the theory in a practical or +biological sense. Therefore in this chapter, where the theory has first +of all to be stated, I shall restrict the exposition as much as possible +to the former, leaving for subsequent consideration the biological +side. + +The following is a brief outline sketch of this theory[15]. + + [15] _See Nineteenth Century_, January, 1887, pp. 61, 62. + +Of all parts of those variable objects which we call organisms, the most +variable is the reproductive system; and the variations may carry with +them functional changes, which may be either in the direction of +increased or of diminished fertility. Consequently variations in the way +of greater or less fertility frequently take place, both in plants and +animals; and probably, if we had adequate means of observing this point, +we should find that there is no one variation more common. But of course +where infertility arises--whether as a result of changed conditions of +life, or, as we say, spontaneously--it immediately becomes extinguished, +seeing that the individuals which it affects are less able (if able at +all) to propagate and to hand on the variation. If, however, the +variant, while showing some degree of infertility with the parent form, +continues to be as fertile as before when mated with similar variants, +under these circumstances there is no reason why such differential +fertility should not be perpetuated. + +Stated in another form this suggestion enables us to regard many, if not +most, species as the records of variations in the reproductive systems +of their ancestors. When variations of a non-useful kind occur in any of +the other systems or parts of organisms, they are, as a rule, +immediately extinguished by intercrossing. But whenever they arise in +the reproductive system in the way here suggested, they tend to be +preserved as new natural varieties, or incipient species. At first the +difference would only be in respect of the reproductive systems; but +eventually, on account of independent variation, other differences would +supervene, and the variety would take rank as a true species. + +Now we must remember that physiological isolation is not like those +other forms of isolation (e.g. geographical) which depend for their +occurrence on accidents of the environment, and which may therefore take +place suddenly in a full degree of completeness throughout a large +section of a species. Physiological isolation depends upon distinctive +characters belonging to organisms themselves; and it would be opposed to +the whole theory of descent with progressive modification to imagine +that absolute sterility usually arises, in a single generation between +two sections of a perfectly fertile species. Therefore evolutionists +must believe that in most, if not in all cases--could we trace the +history, say of any two species, which having sprung from a single +parent stock on a common area, are now absolutely sterile with one +another--we should find that this mutual sterility had been itself a +product of gradual evolution. Starting from complete fertility within +the limits of a single parent species, the infertility between +derivative or divergent species, _at whatever stage in their evolution +this began to occur_, must usually at first have been well-nigh +imperceptible, and thenceforth have proceeded to increase stage by +stage. + +But, if it be true that physiological isolation between genetically +allied groups must usually itself have been the product of a gradual +evolution; and if, when fully evolved, it constitutes a condition of the +first importance to any further differentiation of these groups (by +preventing fusion again into one group, more or less resembling the +original parent form), do we not perceive at least a strong probability +that in the lower stages of its evolution such mutual infertility must +have acted as a segregating influence between the diverging types, in a +degree proportional to its own development? The importance of mutual +sterility as a condition to divergent evolution is not denied, _when +this sterility is already present in an absolute degree_; and we have +just seen that, before it can have attained to this absolute degree _it +must presumably, and as a rule, itself have been the subject of a +gradual development_. Does it not therefore become, on merely antecedent +grounds, in a high degree probable, that from the moment of its +inception this isolating agency must have played the part of a +segregating cause, in a degree proportional to that of its completeness +as a physiological character? + +Whoever answers this question in the affirmative will have gone most of +the way towards accepting, on merely antecedent grounds, the theory of +physiological selection. And therefore it is that I have begun this +statement of the theory by introducing it upon these grounds, thereby +hoping to show how extremely simple--how almost self-evident--is the +theory which it will now be my endeavour to substantiate. I may here add +that the theory was foreshadowed by Mr. Belt in 1874[16], clearly +enunciated in its main features by Mr. Catchpool in 1884[17], and very +fully thought out by Mr. Gulick during a period of about fifteen years, +although he did not publish until a year after the appearance of my own +paper in 1886[18]. + + [16] _Nicaragua_, p. 207. + + [17] _Nature_, vol. xxxi. p. 4. + + [18] _Zool. Journal, Lin. Soc._, vol. xix. pp. 337-411 (1886); and + for Mr. Gulick's papers, _ibid._, vol. xx. pp. 189-274 (1887), vol. + xxiii. pp. 312-380 (1889). Mr. Gulick has recently drawn my + attention, in a private letter, to the fact that as early as 1872 a + paper of his was read at the British Association, bearing the title + _Diversity of Evolution under one set of External Conditions_, and + that here the principle of physiological segregation is stated. + Although it does not appear that Mr. Gulick then appreciated the + great importance of this principle, it entitles him to claim + priority. + +I must next proceed to state some of the leading features of +physiological selection in further detail. + +It has already been shown that Darwin clearly perceived that the very +general occurrence of some degree of infertility between allied species +cannot possibly be attributed to the _direct_ agency of natural +selection. His explanation was that the slight structural modifications +entailed by the transformation of one specific type into another, so +react upon the highly delicate reproductive system of the changing type +as to render it in some degree infertile with its parent type. Now the +theory of physiological selection begins by traversing this view. It +does not, however, deny that in _some_ cases the morphological may be +the prior change; but it strenuously denies that this must be so in +_all_ cases. Indeed, according to my statement in 1886, the theory +inclines to the view that, _as a rule_, the physiological change is +prior. At the same time, the theory, as I have always stated it, +maintains that it is immaterial whether, "in the majority of instances," +the physiological change has been prior to the morphological, or vice +versa; since in either case the physiological change will equally make +for divergence of character. + +To show this clearly the best way will be to consider the two cases +separately, taking first that in which the physiological change has +priority. In this case our theory regards any morphological changes +which afterwards supervene as due to the independent variability which +will sooner or later arise under the physiological isolation thus +secured. But to whatever causes the subsequent morphological changes may +be due, the point to notice is that they are as a general rule, +consequent upon the physiological change. For in whatever _degree_ such +infertility arises between two sections of a species occupying the same +area, in that _degree_ is their interbreeding prevented, and, therefore, +opportunity is given for a subsequent divergence of type, whether by the +influence of independent variability alone, or also by that of natural +selection, as now acting more or less independently on each of the +partially separated groups. In short, all that was said in the foregoing +chapters with respect to isolation in general, here applies to +physiological isolation in particular; and by supposing such isolation +to have been the prior change, we can as well understand the subsequent +appearance of morphological divergence on continuous areas, as in other +forms of isolation we can understand such divergence on discontinuous +areas, seeing that even a moderate degree of cross-infertility may be as +effectual for purposes of isolation as a high mountain-chain, or a +thousand miles of ocean. + +Here, then, are two sharply-defined theories to explain the very general +fact of there being some greater or less degree of cross-infertility +between allied species. The older, and hitherto current theory, +supposes the cross-infertility to be but an _accident_ of specific +divergence, which, therefore, has nothing to do with _causing_ the +divergence. The newer theory, on the other hand, supposes the +cross-infertility to have often been a necessary _condition_ to the +divergence having begun at all. Let us now consider which theory has +most evidence in its favour. + +First of all we have to notice the very general occurrence of the fact +in question. For when we include the infertility of hybrids, as well as +first crosses, the occurrence of some degree of infertility between +allied species is so usual that Mr. Wallace recommends experiments to +ascertain whether careful observation might not prove, even of species +which hybridize, "that such species, when crossed with their near +allies, do always produce offspring which are more or less sterile +_inter se_[19]." This seems going too far, but nevertheless it is the +testimony of a highly competent naturalist to the very general +occurrence of an association between the morphological differentiation +of species and the fact of a physiological isolation. Now I regard it as +little short of self-evident that this general association between +mutual infertility and innumerable secondary, or relatively variable +morphological distinctions, is due to the former having been an original +and a necessary condition to the occurrence of the latter, in cases +where intercrossing has not been otherwise prevented. + + [19] _Darwinism_, p. 169. + +The importance of physiological isolation, _when once fully developed_, +cannot be denied, for it is evident that if such isolation could be +suddenly destroyed between two allied species occupying a common area, +they would sooner or later become fused into a common type--supposing, +of course, no other form of isolation to be present. The necessity then +for this physiological form of isolation in _maintaining_ a specific +differentiation which has been already _attained_ cannot be disputed. +Yet it has been regarded as "Darwinian heresy" to suggest that it can +have been of any important service _during the process of attainment_, +or while the specific differentiation is being advanced, and this +notwithstanding that the physiological change must presumably have +developed _pari passu_ with the morphological, and notwithstanding that +in countless cases the former is associated with every conceivable +variety of the latter. + +Again, why should the physiological change be thus associated with +_every conceivable variety_ of morphological change? Throughout the +length and breadth of both vegetable and animal kingdoms we find this +association, in the great majority of cases, where new species arise. +Therefore, on the supposition that in all such cases the physiological +change has been adventitiously induced by the morphological changes, we +have to face an apparently unanswerable question--Why should the +reproductive mechanism of all organic beings have been thus arranged, as +it were, to change in immediate response to the very slightest +alteration in the complex harmony of "somatic" processes, which now more +than ever is recognized as exercising so comparatively little influence +on the _hereditary_ endowments of this mechanism? Consider the +difference between a worm and the bird that is eating it, an oak tree +and the gall-insect that is piercing it: are we to suppose that in all +cases, no matter how greatly the types differ, they must agree in this, +that when any parts of these complex structures change, ever so +slightly, the reproductive system is almost certain to be adventitiously +affected, yet always thus affected in the same peculiar way? + +If it be answered that the reproductive system is known to be very +sensitive to slight changes in the external conditions of life, the +answer proves too much. For though this is true, yet our opponents must +acknowledge that the reproductive system is not so sensitive, _in this +particular respect_, as their interpretation of the origin of specific +infertility requires. The proof of this point is overwhelming, for there +is the evidence from the entire range of our domesticated productions, +both vegetable and animal. Here the amount of structural change, which +has been slowly accumulated by artificial selection, is often much +greater in amount, and incomparably more rapid, than that which has been +induced between allied species by natural selection; and yet there is +scarcely any indication of the reproductive system having been affected +in the particular way that our opponents' theory requires. There are +many instances of its having been affected in sundry other ways +(chiefly, however, without any accompanying morphological change); but +among all the thousands of our more or less enormously modified +artificial types, there is scarcely one instance of such a peculiar +sexual relation between the modified descendants of a common type as so +usually obtains between allied species in nature. Yet in all other +respects evolutionists are bound to believe that the process of +modification has been in both cases strictly analogous. Why then this +conspicuous difference with respect to the reproductive system? + +The answer is simple. It has never been the object of breeders or of +horticulturists to select variations in the direction of +cross-infertility, for the swamping effects of intercrossing are much +more easily and rapidly prevented by artificial isolation. Consequently, +although they have been able to modify natural types in so many +directions and in such high degrees with regard to _morphology_, there +has been no accompanying physiological modification of the kind +required. But in nature there is no such thing as artificial, i.e. +intentional, isolation. Consequently, on common areas it must usually +happen that those changes of morphology which are associated with +cross-infertility are the only ones which can arise. Hence the very +remarkable contrast between our domesticated varieties and natural +species with regard to cross-infertility is just what the present theory +would expect, or, indeed, require. But on any other theory it has +hitherto remained inexplicable. + +In particular, the contrast in question has constituted one of the main +difficulties with which the theory of natural selection has hitherto had +to contend, not only in the popular mind, but also in the judgement of +naturalists, including the joint-authors of the theory themselves. Thus +Darwin says:-- + + The fertility of varieties is, with reference to my theory, of + equal importance with the sterility of species, for it seems to + make a broad and clear distinction between varieties and + species[20]. + + [20] _Origin of Species_, p. 136. + +And Mr. Wallace says:-- + + One of the greatest, or perhaps we may say the greatest, of all the + difficulties in the way of accepting the theory of natural + selection as a complete explanation of the origin of species, has + been the remarkable difference between varieties and species in + respect of fertility when crossed[21]. + + [21] _Darwinism_, p. 152. + +Now, in view of this conspicuous contrast, Darwin suggested that species +in a state of nature "will have been exposed during long periods of time +to more uniform conditions than have domesticated varieties, and [that] +this may well make a wide difference in the result." Now we have to +remember that species, living and extinct, are numbered by millions, and +represent every variety of type, constitution, and habits; is it +probable, then, that this one peculiarity of the reproductive system +should be due, in so many cases, to some merely incidental effect +produced on that system by uniform conditions of life? Again, _ex +hypothesi_, at the time when a variety is first forming, the influence +exercised by uniform conditions of life (whatever in different cases +this may happen to be) cannot be present as regards that variety: yet +this is just the time when its infertility with the parent (or allied) +form is most likely to have arisen; for it is just then that the nascent +variety would otherwise have been most liable to extinction by free +intercrossing--even supposing that in the presence of such intercrossing +the variety could ever have come into existence at all. + +Mr. Wallace meets the difficulty by arguing that sterility between +allied species may have been brought about by the direct influence of +natural selection. But, as previously remarked, this view is expressly +opposed to that of Darwin, who held that Wallace's contention is +erroneous. + +It will be seen, then, that both Darwin, and Wallace, fully recognize +the necessity of finding some explanation of the infertility of allied +species, over and above the mere reaction of morphological +differentiation on the physiology of the reproductive system, and they +both agree in suggesting additional causes, though they entirely +disagree as to what these causes are. Now, the theory of physiological +selection likewise suggests an additional cause--or, rather, a new +explanation--and one which is surely the most probable. For what is to +be explained? The very general association of a certain physiological +peculiarity with that amount of morphological change which distinguishes +species from species, of whatever kind the change may be, and in +whatever family of the animal or vegetable kingdom it may occur. Well, +the theory of physiological selection explains this very general +association by the simple supposition that, at least in a large number +of cases, it was the physiological peculiarity which first of all led to +the morphological divergence, by interposing the bar of sterility +between two sections of a previously uniform species; and by thus +isolating the two sections one from another, started each upon a +subsequently independent course of divergent evolution. + +Or, to put it in another way, if the occurrence of this physiological +peculiarity has been often the only possible means of isolating two +sections of a species occupying a common area, and thus giving rise to a +divergence of specific type (as obviously _must_ have been the case +wherever there was an absence of any other form of isolation), it is +nothing less than a necessary consequence that many allied species +should now present the physiological peculiarity in question. Thus the +association between the physiological peculiarity and the morphological +divergence is explained by the simple hypothesis, that the former has +acted as a necessary condition to the occurrence of the latter. In the +absence of other forms of isolation, the morphological divergence could +not have taken place at all, had not the physiological peculiarity +arisen; and hence it is that we now meet with so many cases where such +divergence is associated with this peculiarity. + + * * * * * + +So far we have been considering the physiological change as historically +the prior one. Here, at first sight, it may seem that the segregative +power of physiological selection must end; for it may well seem +impossible that the physiological change can ever be necessary for the +divergence of morphological varieties into true species in cases where +it has _not_ been the prior change, but has only set in after +morphological changes have proceeded far enough to have already +constituted definite varieties. A little thought, however, will show +that physiological selection is quite as potent a condition to the +differentiation of species when it occurs after varietal divergence has +begun, as it is when it occurs before the divergence--and hence that it +really makes no difference to the theory of physiological selection +whether, in particular cases, the cross-infertility arises before or +after any structural or other modifications with which it is +associated. + +For the theory does not assert that all varieties have been due to +physiological selection. There are doubtless many other causes of the +origin of varieties besides cross-infertility with parent forms; but, as +a general rule, it does not appear that they are by themselves capable +of carrying divergence beyond a merely varietal stage. In order to carry +divergence to the stage of producing _species_, it appears to be a +general condition that, sooner or later, cross-infertility should +arise--seeing that, when varieties do succeed in becoming species, we +almost invariably find that, as a matter of fact, cross-infertility has +arisen. Hence, if cross-infertility has thus usually been a necessary +condition to a varietal divergence becoming specific, it can make no +material difference when the incipient infertility arose. + +It may be asked, however, whether I suppose that, when the physiological +change is subsequent, it is directly _caused_ by change of structure, +size, colour, &c., or that it arises, so to speak, accidentally, from +other causes which may have affected the sexual system in the required +way. To this question I may briefly reply, that, looking to the absence +of any influence exercised on the reproductive systems of our +domesticated plants and animals by the great and varied changes which so +many of these forms present, it would seem that among natural varieties +such closely analogous changes are presumably not the usual causes of +the physiological change, even where the latter are subsequent to the +former. Nevertheless, I do not deny that in some of these cases changes +of structure, size, colour, &c., may be the causes of the physiological +change by reacting on the sexual system in the required way. But in +such cases free intercrossing will have prevented the perpetuation of +any morphological changes, save those which have the power of so +reacting on the reproductive system as to produce the physiological +change, and thus to protect themselves against the full and adverse +power of free intercrossing. We know that slight or initial changes of +structure, colour, &c., frequently occur as varieties, and yet that on +common areas very few of these varieties become distinct species: free +intercrossing prevents any such further divergence of character. But if +in the course of many such abortive attempts, as it were, to produce a +new species, nature happens to hit upon a structural or a colour +variation which is capable of reacting on the sexual system in the +particular way required, then this variation will be enabled to protect +itself against free intercrossing in proportion to its own development. +Or, in other words, the more it develops as a morphological change, the +more will it increase the physiological change; while the more the +physiological change is thus increased, the more will it in turn promote +the morphological. By such action and reaction the development of each +furthers the development of the other, till from an almost imperceptible +variety, apparently quite fertile with its parent form, there arises a +distinct species absolutely sterile with its parent form. In such cases, +therefore, it is still the physiological conditions which have +_selected_ the particular morphological changes capable of so reacting +on the reproductive system as to produce cross-infertility, and thus to +protect themselves against the destructive power of free intercrossing. +So to speak, free intercrossing is always on the watch to level down +any changes which natural selection, or any other cause of varietal +divergence, may attempt to produce; and therefore, in order to +produce--or to increase--such divergence in the absence of any other +form of isolation, natural selection must hit upon such changes of +structure, form, or colour, as are so correlated with the reproductive +system as to create the physiological isolation that is required. + +To show how the principle of selective fertility may be combined with +what apparently is the most improbable form of isolation for this +purpose--the geographical--I quote the following suggestion made by +Professor Lloyd Morgan in his _Animal Life and Intelligence_:-- + + Suppose two divergent local varieties were to arise in adjacent + areas, and were subsequently (by stress of competition or by + geographical changes) driven together into a single area.... If + their unions be fertile, the isolation will be annulled by + intercrossing--the two varieties will form one mean or average + variety. But if the unions be infertile, the isolation will be + preserved, and the two varieties will continue separate. Suppose + now, and the supposition is by no means an improbable one, that + this has taken place again and again in the evolution of species; + then it is clear that those varietal forms which had continued to + be fertile together would be swamped by intercrossing; while those + varietal forms which had become infertile would remain isolated. + Hence, in the long run, isolated forms occupying a common area + would be infertile, (p. 107.) + +If then cross-sterility may thus arise even in association with +geographical isolation, may it not also arise in its absence? And may it +not thus give rise to the differentiation of varieties on account of +this physiological isolation alone? + +Only two further points need be mentioned to make this statement of +physiological selection as complete as the present _résumé_ of its main +principles requires. + +The first is, that, as Mr. Wallace remarks, "every species has come into +existence coincident both in space and time with a pre-existing and +closely allied species." I regard this as important evidence that +physiological selection is one of the natural causes concerned. For the +general fact implied is that every species has come into existence on an +area occupied by its parent type, and therefore under circumstances +which render it imperative that intercrossing with that type should be +prevented. In the case of monotypic evolution by natural selection +alone, intercrossing with the parent type is prevented through the +gradual extinction of that type by successive generations of the +developing type. But in the case of polytypic evolution, intercrossing +with the parent type can only be prevented by some form of isolation +other than natural selection; and here it is evident that +cross-infertility with the parent type must be as efficient to that end +as any other form of isolation that can be imagined. Consequently we +might almost have expected beforehand that in a large proportional +number of cases cross-infertility should have been the means employed. +And the fact that this is actually the case so far corroborates the only +theory which is able to explain it. + +The second point is this. + +It appears to be comparatively rare for any cause of specific divergence +to prove effectual on common areas, unless it sooner or later becomes +associated with some degree of cross-infertility. But through this +association, the segregating influence of both the causes concerned is, +as Mr. Gulick has shown, greatly increased. For instance, if the +segregating influence of some degree of cross-infertility be associated +with that of any other form of isolation, then, not only will the two +segregating influences be added, but multiplied together. And thus, by +their mutual action and reaction, divergent evolution is promoted at a +rapidly increasing rate. + +I will now summarize the main points of the theory of physiological +isolation in a categorical form. + +1. If no other form of isolation be present, specific divergence can +only take place when some degree of cross-infertility has previously +arisen between two or more sections of a species. + +2. When such cross-infertility has arisen it may cause specific +divergence, either (_a_) by allowing independent variability in each of +the physiologically isolated groups; (_b_) by becoming associated with +any other cause of differentiation already operating; or (_c_) by both +these means combined. + +3. As some degree of cross-infertility generally obtains between allied +species, we are justified in concluding that this has been the most +frequent--or, at any rate, the most effective--kind of isolation where +the origin of species is concerned; and therefore the kind with which, +in the case of species-formation, natural selection, or any other cause +of specific divergence, has been most usually associated. + +4. Where varietal divergence has begun in the absence of +cross-infertility, such divergence seems, as a general rule, to have +been incapable of attaining to a specific value. + +5. Therefore, in the vast majority of such cases, it must have been +those varietal changes of structure, size, colour, &c., which happened +to have afterwards been assisted by the reproductive change that were on +this account _selected_ as successful candidates for specific +differentiation. + +6. It follows, that it makes no difference to the general theory of +physiological selection in what proportion of cases the physiological +change has been the initial change; for, whether prior or subsequent to +the varietal changes with which it becomes associated, its presence has +been equally important as a condition to specific divergence. + +7. When physiological isolation becomes associated with natural +selection, or any other form of homogamy, the segregative power of both +is augmented. Moreover, so great is the augmentation that even very +moderate degrees of physiological isolation--themselves capable of +effecting little or nothing--become very powerful when associated with +moderate degrees of any other kind of homogamy, and vice versa. + +8. The theory of physiological selection effectually explains the +divergent evolution of specific types and the cross-infertility of such +types when evolved. + + * * * * * + +To prevent, if possible, the continuance of certain misunderstandings +with regard to my original statement of the new theory, let me here +disclaim some views which have been assigned to me. They are: + +1. That the theory of physiological selection is opposed to the theory +of natural selection. Far from this being so, it is--at all events in my +own opinion--a very important aid to it, in preventing free +intercrossing on a common area, and thus allowing divergent evolution to +occur within that area. + +2. That, in advancing the theory of physiological selection as "an +additional suggestion on the origin of species," I wish to represent it +as being the originating cause of _all_ species. What I hold is, that +all species must have owed their origin to _isolation_, in some form or +other; but that as physiological selection is only one among many other +forms of isolation (including natural selection), and as it can only act +on common areas, a large number of species must have been formed without +its aid. + +3. That I imagine physiological varieties always to arise +"sporadically," or as merely individual "sports" of the reproductive +system. On the contrary, I expressly stated that this is _not_ the way +in which I suppose the "physiological variation" to arise, when giving +origin to a new species; but that it arises, whenever it is effectual, +as a "collective variation" affecting a number of individuals +simultaneously, and therefore characterizing "a whole race, or strain." + +4. That I suppose physiological selection always to act alone. This I +have never supposed. The essential point is, not that the physiological +isolation is unassociated with other forms of isolation, but that unless +associated with some degree of physiological isolation, no one of the +other forms is capable of originating species on common areas with any +approach to frequency. This proposition is the essence of the new +theory, and I take it to be proved, not only by general deductive +reasoning which shows that it _must_ be so, but also by the fact of an +otherwise inexplicable association between specific divergence on common +areas and some more or less considerable degree of mutual infertility. + + + + +CHAPTER IV. + +EVIDENCES OF PHYSIOLOGICAL SELECTION. + + +I will now give an outline sketch of the evidences in favour of the +theory which has been set forth in the preceding chapter, stating first +what is the nature of the verification which it requires. + +The theory is deduced from a highly general association between +distinctive specific characters of _any_ kind and a relatively constant +specific character of a _particular_ kind--namely, sexual exclusiveness. +For it is from this highly general association that the theory infers +that this relatively constant specific character has been at least one +of the needful conditions to the development of the other specific +characters with which it is found associated. Hence the necessary +verification must begin by showing the strength of the theory on these +merely deductive, or antecedent, grounds. It may then proceed to show +how far the facts of organic nature corroborate the theory in other and +independent ways. + +First, let it be carefully observed that here we have to do only with +the _fact_ of selective fertility, and with its _consequences_ as +supposed by the theory: we have nothing to do either with its _causes_ +or its _degrees_. Not with its causes, because in this respect the +theory of physiological selection is in just the same position as that +of natural selection: it is enough for both if the needful variations +are provided, without its being incumbent on either to explain the +causes which produce them. Not with its degrees, because, in the first +place, it can only be those degrees of variation which in particular +cases are supposed adequate to induce specific divergence, that fall +within the scope of the theory; and because, in the second place, +degrees which are adequate only to induce--or to assist in inducing, +_varietal_ divergence, must always tend to increase, or pass into higher +degrees. + + +_Antecedent Standing of the Theory._ + +The antecedent standing or logical basis of the theory has already been +in large measure displayed in the preceding chapter; for it was +impossible to state the theory without thereby showing in how +considerable a degree it is self-evident. A brief recapitulation is +therefore all that is here necessary. + +It has been shown that divergent or polytypic evolution on common areas +is inexplicable by natural selection alone. Hence the question arises: +What form of isolation has, under such circumstances, rendered possible +divergent evolution? In answer to this question the theory of +physiological selection suggests that variations in the reproductive +function occur in such a way as to isolate more or less perfectly from +each other different sections of a species. While cross-fertility +remains unimpaired among the members of each section, there is more or +less cross-infertility when members of either section mate with those of +the other. Thus a physiological barrier is interposed between the two +sections; and any divergences of structure, colouring, or instinct +arising in the members of either section will not in any way be affected +by such divergences as arise among the members of the other. + +In support of this suggestion, it has been shown in the preceding +chapter that the very general association of cross-infertility with +specific differentiation points most strongly to the inference that the +former has usually been an indispensable condition to the occurrence of +the latter. It cannot be denied that in many cases the specific +distinction is now maintained by means of that sexual isolation which +cross-infertility confers: it is therefore probable that such isolation +has been instrumental in securing its initial attainment. + +This probability is strengthened by the observed fact that the general +association in question is conspicuously absent in the case of +domesticated varieties, notwithstanding that their multitudinous and +diverse varietal characters usually equal, and frequently surpass, +specific characters in their degrees of divergence. + +Since, then, it would seem to be impossible for divergent evolution on +common areas to take place in the absence of some mode of isolation; +since cross-infertility appears to be the only possible mode under the +given circumstances; and since among domesticated varieties, where +isolation is otherwise secured by artificial means, cross-infertility is +usually absent, the logical foundations of the theory of physiological +selection would seem to be securely laid. + +We may therefore pass to more special lines of evidence. + + +_Evidence from Geographical Distribution._ + +Darwin has adduced very good evidence to show that large areas, +notwithstanding the disadvantages which (on his theory) must arise from +free intercrossing, are what he terms better manufactories of species +than smaller areas, such as oceanic islands. On the other hand, as a +matter of fact, oceanic islands are comparatively rich in peculiar +species. These two statements, however, are not incompatible. Smaller +areas are, as a rule, rich in peculiar species relatively to the number +of their inhabitants; but it does not follow that they are rich in +species as contrasted with larger areas containing very many more +inhabitants. Therefore, the rules are that large areas turn out an +absolutely greater number of specific types than small areas; although, +relatively to the number of individuals or amount of population, the +small areas turn out a larger number of species than the large areas. + +Now, these two complementary rules admit of being explained as Darwin +explains them. Small and isolated areas are rich in species relatively +to the amount of population, because, as we have before seen, this +population has been permitted to develop an independent history of its +own, shielded from intercrossing with parent forms, and from competition +with exotic forms; while, at the same time, the homogamy thus secured, +combined with change of environment, will give natural selection an +improved chance of finding new points of departure for its operation. On +the other hand, large and continuous areas are favourable to the +production of numerous species, first, because they contain a large +population, thus favouring the occurrence of numerous variations; and, +secondly, because the large area furnishes a diversity of conditions in +its different parts, as to food, climate, attitude, &c., and thus so +many different opportunities for the occurrence of sundry forms of +homogamy. Now, it is obvious that of all these sundry forms of homogamy, +physiological selection must have what may be termed a first-rate +opportunity of assisting in the manufacture of species on large areas. +For not only is it upon large and continuous areas that the antagonistic +effects of intercrossing are most pronounced (and, therefore, that the +influence of physiological selection must be most useful in the work of +species-making); but here also the diversity in the external conditions +of life, which the large area supplies to different parts of the +extensive population, cannot fail to furnish physiological selection +with a greater abundance of that particular variation in the +reproductive system on which its action depends. Again, and of still +more importance, on large areas there are a greater _number_ of species +already differentiated from one another as such; thus a greater number +of already sexually differentiated forms are presented for further +differentiation at the hands of physiological selection. For all these +reasons, therefore, we might have expected, upon the new theory, that +large and continuous areas would be good manufactories of species. + +Again, Darwin has shown that not only large areas, but likewise +"dominant" genera within those areas, are rich in species. By dominant +genera he meant those which are represented by numerous individuals, as +compared with other genera inhabiting the same area. This general rule +he explains by the consideration that the qualities which first led to +the form being dominant must have been useful; that these would be +transmitted to the otherwise varying offspring; and, therefore, that +when these offspring had varied sufficiently to become new species, they +would still enjoy their ancestral advantages in the struggle for +existence. And this, doubtless, is in part a true explanation; but I +also think that the reason why dominant genera are rich in species, is +chiefly because they everywhere present a great number of individuals +exposed to relatively great differences in their conditions of life: or, +in other words, that they furnish the best raw material for the +manufacture of species by physiological selection, as explained in the +last paragraph. For, if the fact of dominant genera being rich in +species is to be explained _only_ by natural selection, it appears to me +that the useful qualities which have already led to the dominance of the +ancestral type ought rather to have proved inimical to its splitting up +into a number of subordinate types. If already so far "in harmony with +its environment" as to have become for this reason dominant, one would +suppose that there is all the more reason for its not undergoing change +by the process of natural selection. Or, at least, I do not see why the +fact of its being in an unusual degree of harmony with its environment +should in itself constitute any unusual reason for its modification by +survival of the fittest. On the other hand, as just observed, I do very +plainly see why such a reason is furnished for the modifying influence +of physiological selection. + +Let us next turn to another of Darwin's general rules with reference to +distribution. He took a great deal of trouble to collect evidence of the +two following facts, namely, (1) that "species of the larger genera in +each country vary more frequently than the species of the smaller +genera"; and (2) that "many of the species included within the larger +genera resemble varieties in being very closely, but unequally, related +to each other, and in having restricted ranges[22]." By larger genera he +means genera containing many species; and he accounts for these general +facts by the principle, "that where many species of a genus have been +formed, on an average many are still forming." But _how_ forming? If we +say by natural selection alone, we should expect to find the +multitudinous species differing from one another in respect of features +presenting well-marked adaptive meanings; yet this is precisely what we +do not find. For Darwin's argument here is that "in large genera the +amount of difference between the species is often exceedingly small, so +that in this respect the species of the larger genera resemble varieties +more than do the species of the smaller genera." Therefore the argument, +while undoubtedly a very forcible one in favour of the fact of +_evolution_, appears to me scarcely consistent with the view of this +evolution being due solely to natural selection. On the other hand, the +argument tells strongly (though unconsciously) in favour of +physiological selection. For the larger a genus, or the greater the +number of its species, the greater must be the opportunity for the +occurrence of that particular kind of variation on which the principle +of physiological selection depends. The species of a genus may be +regarded as so many varieties which have already been separated from one +another physiologically; therefore each of them may now constitute a new +starting-point for a further and similar separation--particularly as, in +virtue of their previous segregation, many are now exposed to different +conditions of life. Thus, it seems to me, we can well understand why it +is that genera already rich in species tend to grow richer; while such +is not the case in so great a degree with genera that are poor in +species. Moreover, we can well understand that, multiplication of +species being as a rule, and in the first instance, determined by +changes in the reproductive system, wherever a large number of new +species are being turned out, the secondary differences between them +should be "often exceedingly small"--a general correlation which, so far +as I can see, we are not able to understand on the theory of natural +selection. + + [22] _Origin of Species_, pp. 44, 45. + +The two subsidiary facts, that very closely allied species have +restricted ranges, and that dominant species are rich in varieties, both +seem to tell more in favour of physiological than of natural selection. +For "very closely allied species" is but another name for species which +scarcely differ from one another at all except in their reproductive +systems; and, therefore, the more restricted their ranges, the more +certainly would they have become fused by intercrossing with one +another, had it not been for the barrier of sterility imposed by the +primary distinction. Or rather, I should say, had it not been for the +original occurrence of this barrier, these now closely-allied species +could never have become species. Again, that dominant species should be +rich in varieties is what might have been expected; for the greater the +number of individuals in a species, the greater is the chance of +variations taking place in all parts of the organic type, and +particularly in the reproductive system, seeing that this system is the +most sensitive to small changes in the conditions of life, and that the +greater the number of individuals composing a specific type, the more +certainty there is of some of them encountering such changes. Hence, the +richness of dominant species in varieties is, I believe, mainly due to +the greater opportunity which such species afford of some degree of +cross-infertility arising between their constituent members. + +Here is another general fact, also first noticed by Darwin, and one +which he experiences some difficulty an explaining on the theory of +natural selection. He says:-- + + In travelling from north to south over a continent, we generally + meet at successive intervals with closely-allied or representative + species, evidently filling the same place in the economy of the + land. These representative species often meet and interlock, and as + one becomes rarer and rarer, the other becomes more and more + frequent, till the one replaces the other. But if we compare these + species where they intermingle, they are generally as absolutely + distinct from each other in every detail of structure as are + specimens taken from the metropolis of each.... In the + intermediate region, having intermediate conditions of life, why do + we not now find closely-linking intermediate varieties? This + difficulty for a long time quite confounded me. But I think it can + in large part be explained[23]. + + [23] _Origin of Species_, ed. 6, pp. 134, 135. + +[Illustration:] + +His explanation is that, "as the neutral territory between two +representative species is generally narrow in comparison with the +territory proper to each, ... and as varieties do not essentially differ +from species, the same rule will probably apply to both; and, therefore, +if we take a varying species inhabiting a very large area, we shall have +to adapt two varieties to two large areas, and a third variety to a +narrow intermediate zone." It is hence argued that this third or +intermediate variety, on account of its existing in lesser numbers, will +probably be soon overrun and exterminated by the larger populations on +either side of it. But how is it possible "to adapt two varieties to two +large areas, and a third [transitional] variety to a narrow intermediate +zone," in the face of free intercrossing on a continuous area? Let _A_, +_B_, and _C_ represent the three areas in question. According to the +argument, variety _A_ passes first into variety _B_, and then into +variety _C_, while variety _B_ eventually becomes exterminated by the +inroads both from _A_ and _C_. But how can all this have taken place +with nothing to prevent intercrossing throughout the entire area _A_, +_B_, _C_? I confess that to me it seems this argument can only hold on +the supposition that the analogy between varieties and species extends +to the reproductive system; or, in a sense more absolute than the +argument has in view, that "varieties do not essentially differ from the +species" which they afterwards form, but from the first show some degree +of infertility towards one another. And, if so, we have of course to do +with the principles of physiological selection. + +That in all such cases of species-distribution these principles have +played an important part in the species-formation, appears to be +rendered further probable from the suddenness of transition on the area +occupied by contiguous species, as well as from the completeness of +it--i. e. the absence of connecting forms. For these facts combine to +testify that the transition was originally due to that particular change +in the reproductive systems of the forms concerned, which still enables +those forms to "interlock" without intercrossing. On the other hand, +neither of these facts appears to me compatible with the theory of +species-formation by natural selection alone. + +But this leads us to another general fact, also mentioned by Darwin, and +well recognized by all naturalists, namely, that closely allied species, +or species differing from one another in trivial details, usually occupy +contiguous areas; or, conversely stated, that contiguity of geographical +position is favourable to the appearance of species closely allied to +one another. Now, the large body of facts to which I here allude, but +need not at present specify, appear to me to constitute one of the +strongest of all my arguments in favour of physiological selection. +Take, for instance, a large continental area, and follow across it a +chain of species, each link of which differs from those on either side +of it by the minute and trivial distinctions of a secondary kind, but +all the links of which differ from one another in respect of the primary +distinction, so that no one member of the series is perfectly fertile +with any other member. Can it be supposed that in every case this +constant primary distinction has been superinduced by the secondary +distinctions, distributed as they are over different parts of all these +kindred organisms, and yet nowhere presenting any but a trifling amount +of morphological change? + +For my own part, I cannot believe--any more than Darwin could +believe--that all these numerous, diverse, and trivial changes have +always had the accidental effect of inducing the same peculiar change in +the reproductive system, and so producing it without any reference to +the process of specific divergence. Nor can I believe, as Darwin +incidentally and provisionally suggested, that prolonged exposure to +uniform conditions of life have so generally induced an equally +meaningless result. I can only believe that all the closely allied +species inhabiting our supposed continent, and differing from one +another in so many and such divers points of small detail, are merely so +many records of the fact that selective fertility has arisen among their +ancestry, and has thus given as many opportunities for the occurrence of +morphological differentiations as it has furnished cases of efficient +isolation. Of course, I do not deny that many, or probably most, of +these trivial morphological differentiations have been produced by +natural selection on account of their utility: I merely deny that they +could have been so produced on this common area, but for the sexual +isolation with which every distinct set of them is now found to be +associated. + + +_Evidence from Topographical Distribution of Species._ + +By topographical distribution I mean the distribution of organisms with +reference to comparatively small areas, as distinguished from larger +regions with reference to which the term geographical distribution is +appropriate. + +It will be at once apparent that a study of the topographical +distribution of organic types is of even more importance for us than a +study of their geographical distribution. For while the former study is +conducted, as it were, with a low power of our observing microscope, the +latter is conducted with a high power. The larger facts of geographical +distribution yield, indeed, all the general characters which we might +expect them to yield, on the theory that divergence of specific types on +common areas has been in chief part determined by physiological +conditions. But for the purpose of testing this theory in a still more +exacting manner, it is of the first importance to consider the more +detailed facts of topographical distribution, since we here come to +closer quarters with the problem of specific differentiation. Therefore, +as we have already considered this problem under the most general points +of view, we will now consider it under more special points of view. + + * * * * * + +It is self-evident, as we have seen in the preceding section, that the +greater the number of individuals of the same species on a given area, +the less must be the power of natural selection to split that species +into two or more allied types; because, the more crowded the population, +the greater must be the uniformitarian effect of free intercrossing. +This obvious fact has been insisted upon by several previous writers on +Darwinism; and the only reason why it has not been recognized by all +naturalists is that so few of them have observed the all-important +distinction between monotypic and polytypic evolution. The denser the +population, and therefore the greater the intercrossing and the severer +the struggle for existence within the species, the better will it be for +_transmutation_ of the species by natural selection; but the worse it +will be for _differentiation_ of the species by this form of homogamy. +On the other hand, if physiological selection be entertained as a form +of homogamy, the denser the population, the better opportunity it will +have of differentiating the species, first, because a greater number of +individuals will be present in which the physiological change may arise, +and, secondly, because, if it does arise, the severity of the struggle +for existence will _then_ give natural selection a better chance of +acting rapidly and effectually on each of the isolated sections. + +Hence, where the question is whether selective fertility has played any +large or general part in the differentiation of specific types, the best +criterion we can apply is to ascertain whether it is a general rule that +closely allied species occur in intimate association, so that their +individual members constitute, as it were, a single population, or, on +the other hand, whether they occur rather on different sides of +physical barriers. If they occur intimately associated, the form of +homogamy to which their differentiation was due must have presumably +been the physiological form; whereas, if they are proved to be +correlated with physical barriers, the form of homogamy which was +concerned in their differentiation must presumably have been the +geographical form. + +Now, at first this consideration was a trouble to me, because Moritz +Wagner had strenuously argued--and supported his argument by a +considerable wealth of illustration--that allied species are always +found correlated with physical barriers or discontinuous areas. +Weismann's answer, indeed, had shown that Wagner's statement was much +too general: nevertheless, I was disappointed to find that so much could +be said in favour of the geographical (or topographical) form of +isolation where closely allied species are concerned. Subsequently, +however, I read the writings of Nägeli on this subject, and in them I +find a very different state of matters represented. + +Seeing as clearly as Wagner that it is impossible under any +circumstances for natural selection to cause specific _differentiation_ +unless assisted by some other forms of homogamy, but committing the same +oversight as Wagner and Weismann in supposing that the only other form +of homogamy in nature is geographical isolation, Nägeli, with great +force of reasoning, and by many examples, founded his argument against +the theory of natural selection on the ground that in the vegetable +kingdom closely allied species are most frequently found in intimate +association with one another, not, that is to say, in any way isolated +by means of physical barriers. This argument is everywhere logically +intact; and, as he sustains it by a large knowledge of topographical +botany, his indictment against natural selection as a cause of specific +_differentiation_ appeared to be insurmountable. And, in point of fact, +it _was_ insurmountable; so that the whole problem of the origin of +species by _differentiation on common areas_ has hitherto been left in +utter obscurity. Nor is there now any escape from this obscurity, unless +we entertain the "supplementary factor" of selective fertility. And, +apparently, the only reason why this has not been universally +recognized, is because Darwinians have hitherto failed to perceive the +greatness of the distinction between the _differentiation_ and the +_transmutation_ of species; and hence have habitually met such +overwhelming difficulties as Nägeli presented by an illogical +confounding of these two totally distinct things. + +But if the idea of selective fertility had ever occurred to Nägeli as a +form of segregation which gives rise to specific differentiation, I can +have no doubt that so astute and logical a thinker would have perceived +that his whole indictment against natural selection was answered. For it +is incredible that he should not have perceived how this physiological +form of homogamy (supposing it to arise _before_ or _during_, and not +_after_ the specific differentiation) would perform exactly the same +function on a continuous area, as he allowed that "isolation" does on a +discontinuous one. + +However, be this as it may, there cannot be any question touching the +immense value of his facts and arguments as evidence in favour of +physiological selection--albeit this evidence was given unconsciously, +or, as it were, prophetically. Therefore I will here quote a few +examples of both, from his paper _Du Développement des Espèces +Sociales_[24]. + + [24] _Archives des Sciences physiques et naturelles_ (Genève), vol. + liii. (1875), pp. 211-236. + +After stating the theory of natural selection, he says that if the +theory is (of itself) a true explanation of the origin (or divergence) +of specific forms, it ought to follow that + + two closely allied forms, derived the one from the other, would + necessarily occupy two different geographical areas [or + topographical stations], since otherwise they would soon become + blended. Until they had already become sufficiently consolidated as + distinct species to render mutual intercrossing highly improbable, + they could not be intermingled without disadvantage [to + differentiation]. Had Darwin endeavoured to support his hypothesis + by facts, he would, at least in the vegetable kingdom, have found + little to favour his cause. I can cite many hundreds of cases, in + which species in every stage of development have been found closely + mingling with one another, and not in any way isolated. Therefore, + I do not think that one can rightly speak of natural selection in + the Darwinian sense in the vegetable kingdom; and, in my + estimation, there is a great difference between the formation of + species by nature and the production of stock by a breeder.... (p. + 212). + + Of the two kinds of distribution (i. e. growing apart and growing + together), Synoicy (or growing together) is by far the most usual + in nature. I reckon that out of a hundred allied vegetable forms, + at least ninety-five would be found to be synoical (p. 219). + +This is a most important point. That so enormous a proportion of +vegetable species should have originated in intimate association with +their parent or sister types, is clearly unintelligible on the theory of +natural selection alone; there obviously _must_ be some other form of +homogamy which, whether or not in all places _associated_ with natural +selection, is the primary condition to the differentiation. Such I hold +with Nägeli, is a logical necessity; and this whether or not I am right +in believing the other form of homogamy in question to be selective +fertility. But I go further and say, Surely there can be no rational +question that this other form of homogamy must have been, at any rate as +a highly general rule, the one which I have assigned. For how is it that +in these ninety-five per cent. of cases, where vegetable species are +growing intimately associated with their nearest allies, there is no +hybridizing, or blending and relapsing to the original undifferentiated +types? We know well the answer. These are fully differentiated species, +and, as such, are protected from mutual intercrossing by the barrier of +mutual sterility. But now, if this bar is thus necessary for preserving +the specific distinctions when they have been fully developed, much more +must it have been so to admit of their development; or, otherwise +stated, since we know that this barrier is associated with "synoical" +species, and since we clearly perceive that were it withdrawn these +species would soon cease to exist, can we reasonably doubt that their +existence (or origin) is due to the previous erection of this barrier? +If synoical species were comparatively rare, the validity of such +reasoning might be open to question; or, even if we should not doubt it +in such cases, at any rate we might well doubt the importance or extent +of selective fertility as a factor in the origination of species. But +the value of Nägeli's writings on the present subject consists in +showing that synoical species constitute so overwhelming a majority of +the vegetable kingdom, that here, at all events, it appears impossible +to rate too highly the importance of the principle I have called +physiological selection. + + + + +CHAPTER V. + +FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION. + + +_Evidence from Topographical Distribution of Varieties._ + +In the last section we have considered the topographical distribution of +closely allied _species_. I now propose to go still further into matters +of detail, by considering the case of natural _varieties_. And here we +come upon a branch of our inquiry where we may well expect to meet with +the most crucial tests of our theory. For if it should appear that these +nascent species more or less resemble fully developed species in +presenting the feature of cross-infertility, the theory would be +verified in the most direct and conclusive manner possible. These +nascent species may be called embryo species, which are actually in +course of differentiation from their parent-type; and therefore, if they +do not exhibit the feature in relation to that type which the present +theory infers to be necessary for the purposes of differentiation, the +theory must be abandoned. On the other hand, if they do exhibit this +feature, it is just the feature which the theory predicted as one that +would be found highly characteristic of such embryo types. +Contrariwise, the theory of natural selection can have no reason to form +any such anticipation; or rather its anticipation would necessarily +require to be the exact opposite. For, according to this theory, the +cross-infertility of allied species is due, either to correlation with +morphological changes which are being produced by the selection, or +else, as Darwin supposed, to "prolonged exposure to uniform conditions +of life"; and thus, in either case, the sterility variation ought to be, +as a general rule at all events, subsequent to the specific +differentiation, and, according to Darwin's view, _long_ subsequent. +Thus we ought not to find that the physiological change is ever, on any +large or general scale, the initial change; nor ought we to find that it +is, on any such scale, even so much as a contemporary change: there +ought, in fact, to be no constant or habitual association between +divergence of embryo-types and the concurrence of cross-infertility. + +Now, it will be my endeavour to prove that there is an extraordinarily +general association between _varietal_ divergence and cross-infertility, +_wherever common areas are concerned_; and in as far as this can be +proved, I take it that the evidence will make wholly in favour of +physiological selection as the prime condition to specific divergence, +while at the same time they will make no less wholly, _and quite +independently_, against natural selection as the unaided cause of such +divergence. + +I shall begin with some further quotations from Nägeli. + + Species may be synoical at all stages of relationship. We come + across varieties, scarcely distinguishable from one another, + growing in the same locality (as, for example, the _Cirsium + heterophyllum_, with smooth or jagged leaves, the _Hieracium + sylvaticum_, with or without caulinary leaves); again, we meet + other varieties more accentuated (as the _H. hoppeanum_, with under + ligules of white or red, the _Campanula_, with white or lilac + flowers, &c.), other varieties even more marked, which might almost + be elevated to the rank of species (_Hieracium alpinum_, with hairs + and glands, and the new form _H. holadenium_, which has only + glands, _Campanula rotundifolia_ with smooth and hairy leaves), or + forms still more distinct, up to well-defined species. I could + enumerate endless examples at all stages. + + It will be seen that in my definition of synoicy I do not mean to + assert that _all_ allied forms are invariably found together, but + that they are much more often seen in groups than singly. Take, for + instance, nine forms closely related (_A_ to _I_). _A_, _E_, _H_ + will be found side by side at one point, _B_, _D_ at another, _C_, + _F_ at a third, &c. These facts are plainly opposed to the theory + of isolation and amixia, and make, on the contrary, in favour of + the social development of species (_loc. cit._, p. 221). + +Not to multiply quotations to the same general effect, I will supply but +one other, referring to a particular case. + + At one spot (_Rothwand_) much exposed to the sun, and difficult of + access, I remarked two closely allied forms, so nearly related to + _H. villosum_ that this would seem to be an intermediary form + between the two. One of these (_H. villosissimum_) is distinguished + by its tongue and thick pubescence, its tolerably large capitula, + and by the lengthened and separated scales of the involucrum; the + other, on the contrary (_H. elongatum_), is less pubescent, has + smaller capitula, and more compact scales on the involucrum than + _H. villosum_. Both are finally distinguishable from the type by + their longer stalks, which are more decidedly aphyllous, and by + their later flowering. At the spot where I found them the two forms + were closely intermingled, and each was represented by a + considerable number of plants. I did not find them anywhere else on + the mountain, nor could I find at the spot where these were growing + a single specimen of the true _H. villosum_, nor a single hybrid + from these two. + + I concluded that these two new forms had, by joining their forces, + expelled the _H. villosum_ from its primitive abode, but had not + succeeded in displacing one another. As to their origin, they had + evidently developed in two different directions from a common point + of departure, namely _H. villosum_. They had succeeded, not only in + separating themselves from the original form, but also in + preventing any intermediary form from interposing. I thought myself + therefore justified in considering this as a case of varieties + which have come into existence subsequently to the Glacial epoch. + The morphological characteristics of the three forms are + sufficiently distinct for them to be designated as species by a + good many writers. They are better defined than some of MM. Frolich + and Fries' weaker species, and as well defined as some of MM. Koch + and Grisebach's (p. 222). + +Now it is clear, without comment, that all this is exactly as it ought +to be, if allied species have been differentiated on common areas by +selective fertility. For if, as Nägeli elsewhere says, "one meets forms +in nature associated with one another, and severally distinguished by +every possible degree of differentiation," not only as Nägeli adds, does +this general fact lead to the inference that species are (usually) +developed when plants grow intimately associated together; but as +certainly it leads to the further inference that such development must +be due to a prior development of cross-infertility between the diverging +varietal forms, cross-infertility which is therefore afterwards so +characteristic of the allied species, when these are found, in their +fully differentiated condition, still occupying the same area in large +and intimately mingled populations. + +To my mind there could not be any inference more strongly grounded than +this, because, with the one exception of the physiological form, no +other form of homogamy can be conceived which shall account for the +origin and permanence of these synoical varieties, in all degrees of +differentiation up to well-defined synoical species. Least of all, as we +have seen, can natural selection alone have had anything to do with such +a state of matters; while, as we have likewise seen, in all its details +it is exactly the state of matters which the theory of physiological +selection requires. + +Nevertheless, although this inference is so strongly grounded, we ought +to remember that it is only an inference. In order fully to verify the +theory of physiological selection, we ought to prove by experiment the +fact of cross-infertility between these synoical varieties, as we learn +that it afterwards obtains between synoical species. It is to be +regretted that the theory of physiological selection did not occur to +the mind of Nägeli, because he would then, no doubt, have ascertained +this by actual experiment. As it is, the great value of his observations +goes no further than establishing a strong presumption, that it _must_ +be selective fertility which causes the progressive differentiation of +synoical varieties; and also that, if so, this _must_ be the principal +factor in the differentiation of vegetable species, seeing that some +ninety-five per cent. are of synoical origin. + + +_Evidence from Experimental Research._ + +My paper on _Physiological Selection_ pointed out that the whole theory +would have to stand or fall with the experimental proof of the presence +or the absence of cross-infertility between varieties of the same +species growing on common areas. From the facts and considerations which +we have hitherto been dealing with, it did indeed appear to me that +there was the strongest conceivable ground for inferring that +cross-infertility between such varieties would be found by experiment to +be a phenomenon of highly general occurrence--amply sufficient ground to +prove that allied species on common areas for the most part owed their +origin to this character of mutual sterility, and not vice versa as +previously supposed. At that time I was not aware that any experiments +had been made in this direction. Soon after the paper was published, +however, my attention was directed to a laborious research which had +been directed to this very point, and carried on for more than thirty +years, by M. Jordan[25]. This had not attracted the general notice which +it undoubtedly deserved; and I have since ascertained that even Darwin +began to look into it only a few months before his death. + + [25] _Remarques sur le fait de l'existence en société à l'état + sauvage des espèces végétales affines et sur d'autres faits relatifs + à la question de l'espèce_, par Alexis Jordan; lues au congrès de + l'Association Française pour l'Avancemeat des Sciences, 2^me + session, Lyon, séance de 28 Août, 1873. + +Having devoted his life to closely observing in divers stations +multitudes of different species of plants--annuals and perennials, +bulbous and aquatic, trees and shrubs--M. Jordan has been able to +satisfy himself, and the French school of botanists to which this line +of observation has given rise, that in most cases (or "nearly +everywhere"), when a Linnean species is indigenous to a country and is +there of common occurrence, this species within that district is +represented by more or less numerous and perfectly constant varieties. +These varieties are constituted by such minute differences of +morphological character that their very existence eluded the +observation of botanists, until M. Jordan began to search specially for +them as the special objects of his scrutiny. Moreover, these varieties +of a Linnean species occupy common areas, and there grow in intimate +association with one another, or as M. Jordan says, "_pêle-mêle_." So +far, be it noticed, Jordan was proceeding on exactly the same lines as +Nägeli; only he carried his observations over a still wider range of +species on the one hand, and into a still minuter search for varieties +on the other. But the all-important point for us is, that he further +proceeded to test by experiment the physiological relations between +these morphological varieties; and found, in many hundreds of cases, +that they not only came true to seed (i. e. are hereditary and not +merely climatic), but likewise cross-sterile _inter se_. For these +reasons, M. Jordan, who is opposed to the theory of evolution, regards +all such varieties as separately created species; and the inspiring +motive of his prolonged investigations has been a desire to multiply +these proofs of creative energy. But it clearly makes no difference, so +far as evolutionists are concerned with them, whether all this multitude +of sexually isolated forms be denominated species or varieties. + +The points which are of importance to evolutionists--and of the first +order of importance in the present connexion--may be briefly summarized +as follows:-- + +(1) The research embraces large numbers of species, belonging to very +numerous and very varied orders of plants; (2) in the majority of +cases--although not all--indigenous species which are of common +occurrence present constant varieties; (3) these varieties, +nevertheless, may be morphologically so slight as to be almost +imperceptible; (4) they occupy common areas and grow in intimate +association; (5) although many of them have undergone so small an amount +of morphological change, they have undergone a surprising amount of +physiological change; for (6) not only do very many of these varieties +come true to seed; but, (7) when they do, they are always more or less +cross-infertile _inter se_. + +Now, it is self-evident that every one of these seven points is exactly +what the theory of physiological selection requires, while there is not +one of them which it does not require. For if the theory be sound, we +should expect to find large numbers of species belonging to numerous and +varied orders of plants presenting constant varieties on common areas; +we should expect this to be a highly general, though not a universal, +rule; and we should expect it to apply only to species which are +indigenous. Moreover, we should expect these varieties, although but +slightly differentiated morphologically, to present a great +differentiation physiologically--and this in the special direction of +selective fertility, combined, of course, with heredity. + +On the other hand, as I have said, this catalogue of evidences leaves +nothing to be supplied. It gives us all the facts--and no more than all +the facts--which my paper on _Physiological Selection_ anticipated as +the eventual result of a prolonged experimental research. And if I have +to regret my ignorance of these facts when that paper was published, at +any rate it now furnishes the best proof that my anticipations were not +guided by the results of a verification which had already been supplied. +These anticipations were deduced exclusively from the theory itself, as +representing what _ought_ to be the case if the theory were true; and, I +must confess, if I had then been told that they had already been +realized--that it had actually been found to be a general rule that +endemic species present constant and hereditary varieties, intimately +commingled on common areas, morphologically almost indistinguishable, +but physiologically isolated by selective fertility--I should have felt +that the theory had been verified in advance. For there are only two +alternatives: either these things are due to physiological selection, or +else they are due--as M. Jordan himself believes--to special creation. +Which is equivalent to saying that, for evolutionists, the facts must be +held to verify the former theory in as complete a manner as it is +logically possible for the theory to be verified. + + +_Evidence from Prepotency._ + +We have now to consider the bearing of what is called "prepotency" on +the theory of physiological selection. + +Speaking of the vast number of species of Compositae, Darwin says:-- + + There can be no doubt that if the pollen of all these species could + be simultaneously or successively placed on the stigma of any one + species, this one would elect with unerring certainty its own + pollen. This elective capacity is all the more wonderful, as it + must have been acquired since the many species of this great group + of plants branched off from a common progenitor. + +Darwin is here speaking of elective affinity in its fully developed +form, as absolute cross-sterility between fully differentiated species. +But we meet with all lower degrees of cross-infertility--sometimes +between "incipient species," or permanent varieties, and at other times +between closely allied species. It is then known as "prepotency" of the +pollen belonging to the same variety or species over the pollen of the +other variety or species, when both sets of pollen are applied to the +same stigma. Although in the absence of the prepotent pollen the less +potent will fertilize the seed, yet, such is the appetency for the more +appropriate pollen, that even if this be applied to the stigma some +considerable time after the other, it will outstrip or overcome the +other in fertilizing the ovules, and therefore produce the same result +on the next generation as if it had been applied to the mother plant +without any admixture of the less potent pollen, although in some cases +such incipient degrees of cross-infertility are further shown by the +number or quality of the seeds being fewer or inferior. + +Now, in different varieties and in different allied species, all degrees +of such prepotency have been noticed by many observers, from the +faintest perceptible amount up to complete impotency of the alien +pollen--when, of course, there is absolute sterility between the two +varieties or allied species. The inference is obvious. In this graduated +scale of prepotency--beginning with an experimentally almost +imperceptible amount of sexual differentiation between two varieties, +and ending in an absolute partitioning of two allied species--we have +the only remaining fact that is required to complete the case in favour +of the present theory. We are here brought back to the very earliest +stages of physiological differentiation or to the stages which lie +behind Jordan's "Physiological Species"; and therefore, when taken in +conjunction with his results, the phenomena of prepotency may be said to +give us the complete and final demonstration of one continuous +development, which, beginning in an almost imperceptible amount of +cross-infertility, ends in absolute cross-sterility. The "elective +capacity" to which Darwin alludes as having been "acquired" by all the +species of Compositae since they "branched off from a common +progenitor," is thus seen among innumerable other species actually in +process of acquisition; and so we can perfectly well understand, what is +otherwise unintelligible, that closely allied species of plants occur, +in ninety-five per cent. of cases, intimately associated on common +areas, while exhibiting towards one another the character of mutual +sterility. + +But more than this. The importance of the widespread phenomena of +prepotency to the theory of physiological selection does not consist +merely in thus supplying the last link in the chain of evidence touching +the origin of species by selective fertility, or "elective capacity." +These phenomena are of further importance as showing how in plants, at +all events, physiological selection appears to be frequently capable of +differentiating specific types without the necessary assistance of any +other form of homogamy. In my original statement of the theory, I was +careful to insist upon the great value, as differentiating agents, of +even small degrees of other forms of homogamy when co-operating with +physiological selection. But I also stated my belief that in many cases +selective fertility is presumably of itself capable of splitting a +specific type; and the reason why I still believe this is, that I do not +otherwise understand these phenomena of prepotency. I cannot believe +that in all the innumerable cases where they arise, they have been +super-induced by some prior morphological changes going on in some other +part of the organism, or by "prolonged exposure to uniform conditions of +life," on the part of two well-nigh identical forms which have arisen +intimately commingled in exactly the same environment, and under the +operation of a previously universal intercrossing. Even if such a thing +could be imagined as happening occasionally, I feel it difficult to +imagine that it can happen habitually, and yet this view must be held by +those who would attribute prepotency to natural selection. + +It must never be forgotten that the relatively enormous changes as to +size, structure, habit, &c., which are presented by our domesticated +plants as results of artificial selection, do not entail the +physiological character of cross-sterility in any degree, save possibly +in some small number of cases. Although in wild species any +correspondingly small percentage of cases (where natural selection +happens to hit upon parts of the organism modifications of which produce +the physiological change by way of correlation) would doubtless be the +ones to survive on common areas, still it is surely incredible that such +an accidental association between natural selection and +cross-infertility is so habitually the means of specific differentiation +as the facts of prepotency (together with the observations of Jordan +and Nägeli) would necessarily demand. + +Moreover, this view of the matter is still further corroborated by +certain other facts and considerations. For example, the phenomena of +prepotency (whether as between varieties or between closely allied +species) are found to occur when the two forms occupy a common area, +i.e. are growing intermingled with one another. Therefore, but for this +physiological differentiation, there could be absolutely nothing to +prevent free intercrossing. Yet the fact that hybrids are so +comparatively rare in a state of nature--a fact which Sir Joseph Hooker +has pointed out to me as otherwise inexplicable--proves the efficacy of +even a low degree of such differentiation in preventing the +physiologically-differentiated forms from intercrossing. Even in cases +where there is no difficulty in producing artificial hybrids or mongrels +between species or varieties growing on common areas, it is perfectly +astonishing what an extremely small percentage of the hybrid or mongrel +forms are found to occur in nature. And there can be no question that +this is due to the very efficient manner in which prepotency does its +work--efficient, I mean, from the point of view of the new theory; for +upon any other theory prepotency is a meaningless phenomenon, which, +notwithstanding its frequent occurrence, plays no part whatever in the +process of organic evolution. + +I attach considerable importance to the phenomena of prepotency in view +of the contrast which is presented between plants and animals in the +relation of their species to physical barriers. For animals--and +especially the higher animals--appear to depend for their specific +differentiations upon such barriers much more than in the case with +plants. This is no more than we should expect; for, in accordance with +our theory, selective fertility is not so likely to work alone in the +case of the higher animals which mate together, as in plants which are +fertilized through the agency of wind or insects. In the former case +there is no opportunity given for the first rise of cross-infertility, +in the form of prepotency; and even where selective fertility has gained +a footing in other ways, the chances against the suitable mating of +"physiological complements" must be much greater than it is in the +latter case. Hence, among the higher animals, selective fertility ought +much more frequently to be found in association with other forms of +homogamy than it is among plants. And this is exactly what we find. Thus +it seems to me that this contrast between the comparative absence and +presence of physical barriers, where allied species of plants and of +higher animals are respectively concerned, is entitled to be taken as a +further corroboration of our theory. For while it displays exactly such +a general correlation as this theory would expect, the correlation is +one which cannot possibly be explained on any other theory. It is just +where physiological selection can be seen to have the best opportunity +of acting (viz. in the vegetable kingdom) that we find the most +unequivocal evidence of its action; while, on the other hand, it is just +where it can be seen to have the least opportunity of asserting itself +(viz. among the higher animals) that we find it most associated with, +and therefore assisted by, other forms of homogamy, i. e. not only +geographical isolation, but also by sexual preference in pairing, and +the several other forms of homogamy, which Mr. Gulick has shown to arise +in different places as the result of intelligence. + + +_Evidence from Special Cases._ + +Hitherto I have been considering, from the most general point of view, +the most widespread facts and broadest principles which serve to +substantiate the theory of physiological selection. I now pass to the +consideration of one of those special cases in which the theory appears +to have been successfully applied. + +Professor Le Conte has adduced the fossil snails of Steinheim as serving +to corroborate the theory of physiological selection[26]. + + [26] _Evolution and its Relations to Religious Thought_, &c. pp. + 236-7. + +The facts are these. The snail population of this lake remain for a long +time uniform and unchanged. Then a small percentage of individuals +suddenly began to vary as regards the form of their shells, and this in +two or three directions at the same time, each affected individual, +however, only presenting one of the variations. But after all these +variations had begun to affect a proportionally large number of +individuals, some individuals occur in which two or more of the +variations are blended together, evidently, as Weismann says, by +intercrossing of the varieties so blended. Later still, both the +separate varieties and their blended progeny became more and more +numerous, and eventually a single blended type, comprising in itself all +the initial varieties, supplanted the parent form. Then another long +period of stability ensued until another eruption of new variations took +place; and these variations, after having affected a greater and greater +number of individuals, eventually blended together by intercrossing and +supplanted their parent form. So the process went on, comparatively +short periods of variation alternating with comparatively long periods +of stability, the variations, moreover, always occurring suddenly in +crops, then multiplying, blending together, and in their finally blended +type eventually supplanting their parent form. + +Now, the remarkable fact here is that whenever the variations arose, +they only intercrossed between themselves, they did not intercross with +their parent form; for, if they had, not only could they never have +survived (having been at first so few in number and there having been no +geographical barriers in the small lake), but we should have found +evidence of the fact in the half-bred progeny. Moreover, natural +selection can have had nothing to do with the process, because not only +are the variations in the form of the shells of no imaginable use in +themselves; but it would be preposterous to suppose that at each of +these "variation periods" several different variations should always +have occurred simultaneously, all of which were of some hidden use, +although no one of them ever occurred during any of the prolonged +periods of stability. How, then, are we to explain the fact that the +individuals composing each crop of varieties, while able to breed among +themselves, never crossed with their parent form? These varieties, each +time that they arose, were intimately commingled with their parent +form, and would certainly have been reabsorbed into it had intercrossing +in that direction been possible. With Professor Le Conte, therefore, I +conclude that there is only one conceivable answer to this question. +Each crop of varieties must have been _protected from intercrossing with +their parent form_. + +They must have been the result of a variation, which rendered the +affected individuals sterile with their parent form, whilst leaving them +fertile amongst themselves. The progeny of these individuals would then +have dispersed through the lake, physiologically isolated from the +parent population, and especially prone to develop secondary variations +as a direct result of the primary variation. Thus, as we might expect, +two or three variations arose simultaneously, as expressions of so many +different lines of family descent from the original or physiological +variety; these were everywhere prevented from intercrossing with their +parent form, yet capable of blending whenever they or their +ever-increasing progeny happened to meet. Thus, without going into +further details, we are able by the theory of physiological selection to +give an explanation of all these facts, which otherwise remain +inexplicable. + + * * * * * + +In view of the evidence which has now been presented, I will now ask +five questions which must be suitably answered by critics of the theory +of physiological selection. + +1. Can you doubt that the hitherto insoluble problem of inter-specific +sterility would be solved, supposing cross-infertility were proved to +arise before or during the process of specific differentiation, instead +of after that process had been fully completed? + +2. Can you doubt, after duly considering the circumstances under which +allied species of plants have been differentiated--viz. in ninety-five +per cent. of cases intimately commingled on common areas, and therefore +under identical environments--that cross-infertility _must_ have arisen +before or during the specific differentiation? + +3. Can you doubt, after duly considering the facts of prepotency on the +one hand and those of Jordan's physiological varieties on the other, +that cross-infertility _does_ arise before or during the specific +differentiation? + +4. If you cannot express a doubt upon any of these points, can you +explain why you refuse to accept the theory of the origin of species by +means of physiological selection, together with the explanation which +this theory affords of the continued cross-fertility of domesticated +varieties? + +5. Supposing this theory to be true, can you conceive of any other +classes of facts which, either quantitatively or qualitatively, could +more directly or more effectually prove its truth than those which have +now been adduced? + +On these five heads I entertain no doubt. I am convinced that the theory +of physiological selection is the only one that can explain the facts of +inter-specific sterility on the one hand, and, on the other hand, the +contrast which these facts display to the unimpaired fertility of our +domesticated varieties. + +In conclusion, it seems desirable once more to insist that there is no +antagonism or rivalry between the theories of natural and of +physiological selection. For which purpose I will quote the final +paragraph of my original paper. + + So much, then, for the resemblances and the differences between the + two theories. It only remains to add that the two are + complementary. I have already shown some of the respects in which + the newer theory comes to the assistance of the older, and this in + the places where the older has stood most in need of assistance. In + particular, I have shown that segregation of the fit entirely + relieves survival of the fittest from the difficulty under which it + has hitherto laboured of explaining why it is that sterility is so + constantly found between species, while so rarely found between + varieties which differ from one another even more than many + species; why so many features of specific distinction are useless + to the species presenting them; and why it is that incipient + varieties are not obliterated by intercrossing with parent forms. + Again, we have seen that physiological selection, by preventing + such intercrossing, enables natural selection to promote diversity + of character, and thus to evolve species in ramifying branches + instead of in linear series--a work which I cannot see how natural + selection could possibly perform unless thus aided by physiological + selection. Moreover, we have seen that although natural selection + alone could not induce sterility between allied types, yet when + this sterility is given by physiological selection, the forms which + present it would be favoured in the struggle for existence; and + thus again the two principles are found playing, as it were, into + each other's hands. And here, as elsewhere, I believe that the + co-operation enables the two principles to effect very much more in + the way of species-making than either of them could effect if + working separately. On the one hand, without the assistance of + physiological selection, natural selection would, I believe, be all + but overcome by the adverse influences of free + intercrossing--influences all the more potent under the very + conditions which are required for the multiplication of species by + divergence of character. On the other hand, without natural + selection, physiological selection would be powerless to create any + differences of specific type, other than those of mutual sterility + and trivial details of structure, form, and colour--differences + wholly without meaning from a utilitarian point of view. But in + their combination these two principles appear to me able to + accomplish what neither can accomplish alone--namely, a full and + satisfactory explanation of the origin of species. + + + + +CHAPTER VI. + +A BRIEF HISTORY OF OPINIONS ON ISOLATION AS A FACTOR OF ORGANIC +EVOLUTION. + + +This historical sketch must begin with a consideration of Darwin's +opinions on the subject; but as these were considerably modified from +time to time during a period of thirty years by the publications of +other naturalists, it will be impossible to avoid cross-references as +between his writings and theirs. It may also be observed that the _Life +and Letters of Charles Darwin_ was not published until the year 1887, so +that the various opinions which I shall quote from the letters, and +which show some considerable approximation in his later years to the +views which have been put forward by Mr. Gulick and myself, were not +before us at the time when our papers were read. + +The earliest allusion that I can find to geographical isolation in the +writings of Darwin occurs in a correspondence with Sir Joseph Hooker, as +far back as 1844. He there says:-- + + I cannot give my reasons in detail; but the most general conclusion + which the geographical distribution of all organic beings appears + to me to indicate is, that isolation is the chief concomitant or + cause of the appearance of _new_ forms (I well know there are some + staring exceptions)[27]. + + [27] _Life and Letters_, vol. ii. p. 28. + +And again:-- + + With respect to original creation or production of new forms, I + have said that isolation appears the chief element[28]. + + [28] _Ibid._ + +Next, in the earlier editions of the _Origin of Species_ this view is +abandoned, and in its stead we meet with the opinion that geographical +isolation lends a certain amount of assistance to natural selection, by +preventing free intercrossing. But here we must note two things. First, +the distinction between monotypic and polytypic evolution is not +defined. Secondly, the levelling effect of free intercrossing in nature, +and hence its antagonism to divergence of character by natural +selection, is not sufficiently recognized; while, on the other hand, and +in consequence of this, the importance of isolation as a factor of +evolution is underrated--not only in its geographical, but likewise in +all its other forms. + +Taking these two points separately, the only passages in Darwin's +writings, so far at least as I can find, in which any distinction is +drawn between evolution as monotypic and polytypic, are those in which +he deals with a somewhat analogous distinction between artificial +selection as intentional and unconscious. He says, for example:-- + + In the case of methodical selection, a breeder selects for some + definite object, and if the individuals be allowed freely to + intercross, his work will completely fail. But when many men, + without intending to alter the breed, have a nearly common + standard of perfection, and all try to procure and breed from the + best animals, improvement surely but slowly follows from this + unconscious process of selection, notwithstanding that there is no + separation of selected individuals. Thus it will be under + nature[29]. + + [29] _Origin of Species_, p. 80, 6th ed. (1872). + +Here we have what may perhaps be regarded as a glimmering of the +distinction between monotypic and polytypic evolution. But that it is +only a glimmering is proved by the immediately ensuing sentences, which +apply this analogy of unconscious selection _not_ to the case of +monotypic, _but_ to that of polytypic evolution. So likewise, in the +succeeding discussion on "divergence of character," the analogy is again +resorted to for the purpose of showing how polytypic evolution may occur +in nature. + +Thus far, then, it may be said that we have scarcely so much as a +glimmering of the distinction between monotypic and polytypic evolution; +and as the same discussion (with but a few verbal alterations) runs +through all the editions of the _Origin_, it may well be asked why I +should have alluded to such passages in the present connexion. Well, I +have done so because it is apparent that, during the last years of his +life, the distinction between selection as "methodical" and +"unconscious" enabled Darwin much more clearly to perceive that between +evolution as monotypic and polytypic. Thus in 1868 he wrote to Moritz +Wagner (who, as we shall presently see, entirely failed to distinguish +between monotypic and polytypic evolution), expressing his belief-- + + That in many large areas all the individuals of the same species + have been slowly modified, in the same manner, for instance, as the + English racehorse has been improved, that is, by the continued + selection of the fleetest individuals, without any separation. But + I admit that by this process two or more new species could hardly + be formed within the same limited area[30]. + + [30] _Life and Letters_, vol. iii. p. 158. + +Again, in 1876 he wrote another letter to Wagner, in which the following +passage occurs:-- + + I believe that all the individuals of a species can be slowly + modified within the same district, in nearly the same manner as man + effects by what I have called the process of unconscious selection. + I do not believe that one species will give birth to two or more + new species as long as they are mingled together within the same + district[31]. + + [31] _Ibid._ p. 159. + +Two years later he wrote to Professor Semper:-- + + There are two different classes of cases, it appears to me, viz. + those in which species becomes slowly modified in the same country, + and those cases in which a species splits into two, or three, or + more new species; and, in the latter case, I should think nearly + perfect separation would greatly aid in their "specification," to + coin a new word[32]. + + [32] _Ibid._ p. 160. + +Now, these passages show a very much clearer perception of the +all-important distinction between monotypic and polytypic evolution than +any which occur in the _Origin of Species_; and they likewise show that +he was led to this perception through what he supposed to be a somewhat +analogous distinction between "unconscious" and "methodical" selection +by man. The analogy, I need hardly say, is radically unsound; and it is +a curious result of its unsoundness that, whereas in the _Origin of +Species_ it is adduced to illustrate the process of polytypic evolution, +as previously remarked, in the letters above quoted we find it adduced +to illustrate the process of monotypic evolution. But the fact of this +analogy being unsound does not affect the validity of the distinction +between monotypic and polytypic evolution to which it led Darwin, in his +later years, so clearly to express[33]. + + [33] The analogy is radically unsound because unconscious selection + differs from methodical selection only in the _degree_ of + "separation" which it effects. These two forms of selection do not + necessarily differ from one another in regard to the _number_ of + characters which are being simultaneously diversified; for while it + may be the object of methodical selection to breed for modification + of a single character alone, it may, on the other hand, be the + result of unconscious selection to diversify an originally uniform + stock, as Darwin himself observes with regard to horse-breeding. The + real distinction between monotypic and polytypic evolution is, not + at all with reference to the _degree_ of isolation (i. e. _amount_ + of "separation"), but to the _number of cases_ in which any + efficient degree of it occurs (i. e. whether in but a single case, + or in two or more cases). + +Turning next to the second point which we have to notice, it is easy to +show that in the earlier editions of his works Darwin did not +sufficiently recognize the levelling effects of free intercrossing, and +consequently failed to perceive the importance of isolation (in any of +its forms) as a factor of organic evolution. This may be most briefly +shown by quoting his own more matured opinion upon the subject. Thus, +with reference to the swamping effects of intercrossing, he wrote to Mr. +Wallace in 1867 as follows:-- + + I must have expressed myself atrociously: I meant to say exactly + the reverse of what you have understood. F. Jenkin argued in the + _North British Review_ against single variations being perpetuated, + and has convinced me, though not in quite so broad a manner as here + put. I always thought individual differences more important; but I + was blind, and thought that single variations might be preserved + much oftener than I now see is possible or probable. I mentioned + this in my former note merely because I believed that you had come + to a similar conclusion, and I like much to be in accord with you. + I believe I was mainly deceived by single variations offering such + simple illustrations, as when man selects [i.e. isolates][34]. + + [34] _Life and Letters_, vol. iii. pp. 157-8. + +Again, somewhere about the same time, he wrote to Moritz Wagner:-- + + Although I saw the effects of isolation in the case of islands and + mountain-ranges, and knew of a few instances of rivers, yet the + greater number of your facts were quite unknown to me. I now see + that, from the want of knowledge, I did not make nearly sufficient + use of the views which you advocate[35]. + + [35] _Ibid._ pp. 157-8. + +Now it would be easy to show the justice of these self-criticisms by +quoting longer passages from earlier editions of the _Origin of +Species_; but as this, in view of the above passages, is unnecessary, we +may next pass on to another point. + +The greatest oversight that Wagner made in his otherwise valuable essays +on geographical isolation, was in not perceiving that geographical +isolation is only one among a number of other forms of isolation: and, +therefore, that although it is perfectly true, as he insisted, that +polytypic evolution cannot be effected by natural selection alone, it is +very far from true, as he further insisted, that _geographical_ +isolation is the only means whereby natural selection can be assisted in +this matter. Hence it is that, when Darwin said he had not himself "made +nearly sufficient use" of geographical isolation as a factor of specific +divergence, he quite reasonably added that he could not go so far as +Wagner did in regarding such isolation as a condition, _sine qua non_, +to divergent evolution in all cases. Nevertheless, he adds the +important words, "I almost wish I could believe in its importance to the +same extent with you; for you well show, in a manner which never +occurred to me, that it removes many difficulties and objections." These +words are important, because they show that Darwin had come to feel the +force of the "difficulties and objections" with regard to divergent +evolution being possible by means of natural selection alone, and how +readily they could be removed by assuming the assistance of isolation. +Hence, it is much to be deplored that Wagner presented a single kind of +isolation (geographical) as equivalent to the principle of isolation in +general. For he thus failed to present the complete--and, therefore, the +true--philosophy of the subject to Darwin's mind; and in this, as in +certain other respects which I shall notice later on, served rather to +confuse than to elucidate the matter as a whole. + +To sum up. Although in his later years, as shown by his correspondence, +Darwin came to recognize more fully the swamping effects of free +intercrossing, and the consequent importance of "separation" for the +prevention of these effects, and although in this connexion he likewise +came more clearly to distinguish between the "two cases" of monotypic +and polytypic evolution, it is evident that he never worked out any of +these matters--"thinking it prudent," as he wrote with reference to them +in 1878, "now I am growing old, to work at easier subjects[36]." +Therefore he never clearly saw, on the one hand, that free +intercrossing, far from constituting a "difficulty" to _monotypic_ +evolution by natural selection, is the very means whereby natural +selection is in this case enabled to operate; or, on the other hand, +that, in the case of _polytypic_ evolution, the "difficulty" in question +is so absolute as to render such evolution, by natural selection alone, +absolutely impossible. Hence, although in one sentence of the _Origin of +Species_ he mentions three forms of isolation (besides the geographical +form) as serving in some cases to assist natural selection in causing +"divergence of character" (i. e. polytypic evolution[37]), on account of +not perceiving how great and how sharp is the distinction between the +two kinds or "cases" of evolution, he never realized that, where "two or +more new species" are in course of differentiation, _some_ form of +isolation other than natural selection must _necessarily_ be present, +whether or not natural selection be likewise so. The nearest approach +which he ever made to perceiving this necessity was in one of his +letters to Wagner above quoted, where, after again appealing to the +erroneous analogy between monotypic evolution and "unconscious +selection," he says:--"But I admit that by this process (i. e. +unconscious selection) two or more new species could hardly be formed +within the same limited area: some degree of separation, if not +indispensable, would be highly advantageous; and here your facts and +views will be of great value." But even in this passage the context +shows that by "separation" he is thinking exclusively of _geographical_ +separation, which he rightly enough concludes (as against Wagner) need +certainly not be "indispensable." Had he gone a step further, he must +have seen that separation, _in some form or another, is_ "indispensable" +to polytypic evolution. Instead of taking this further step, however, +two years later he wrote to Semper as follows:-- + + [36] _Life and Letters_, vol. iii. p. 161. + + [37] Page 81. The three forms of isolation mentioned are, "from + haunting different stations, from breeding at slightly different + seasons, or from the individuals of each variety preferring to pair + together." + + I went as far as I could, perhaps too far, in agreement with Wagner + [i. e. in the last edition of the _Origin of Species_]; since that + time I have seen no reason to change my mind; but then I must add + that my attention has been absorbed on other subjects[38]. + + [38] _Life and Letters_, vol. iii. p. 159. + +And he seems to have ended by still failing to perceive that the +explanation which he gives of "divergence of character" in the _Origin +of Species_, can only hold on the unexpressed assumption that free +intercrossing is in some way prevented at the commencement, and +throughout the development, of each diverging type. + +Lastly, we have to consider Darwin's opinion touching the important +principle of "Independent Variability." This, it will be remembered, is +the principle which ensures that when a portion (not too large) of a +species is prevented from interbreeding with the rest of the species, +sooner or later a divergence of type will result, owing to the fact that +the average qualities of the separated portion at the time of its +separation cannot have been exactly the same as the average qualities of +the specific type as a whole. Thus the state of Amixia, being a state of +what Mr. Gulick calls Independent Generation, will of itself--i.e. even +if unassisted by natural selection--induce divergence of type, in a +ratio that has been mathematically calculated by DelbÅ“uf. + +Darwin wrote thus to Professor Weismann in 1872:-- + + I have now read your essay with very great interest. Your view of + the origin of local races through "Amixia" is altogether new to me, + and seems to throw an important light on an obscure question[39]. + + [39] _Life and Letters_, vol. iii. p. 155. + +And in the last edition of the _Variation of Animals and Plants_ he adds +the following paragraph:-- + + This view may throw some light on the fact that the domestic + animals which formerly inhabited the several districts in Great + Britain, and the half-wild cattle lately kept in several British + parks, differed slightly from one another; for these animals were + prevented from wandering over the whole country and intercrossing, + but would have crossed freely within each district or park[40]. + + [40] _Variation_, &c., vol. ii. p. 262. + +Now, although I allow that Darwin never attributed to this principle of +Amixia, or Independent Variability, anything like the degree of +importance to which, in the opinion of DelbÅ“uf, Gulick, Giard, and +myself, it is entitled, the above passage appears to show that, as soon +as the "view" was clearly "suggested" to his mind, he was so far from +being unfavourably disposed towards it, that he added a paragraph to the +last edition of his _Variation_ for the express purpose of countenancing +it. Nevertheless, later on the matter appears to have entirely escaped +his memory; for in 1878 he wrote to Semper, that he did "not see at all +more clearly than I did before, from the numerous cases which he +[Wagner] has brought forward, how and why it is that a long isolated +form should almost always become slightly modified[41]." I think this +shows entire forgetfulness of the principle in question, because, if +the latter is good for explaining the _initial_ divergence of type as +between separated stocks of "domesticated animals," much more must it be +competent to explain the _further_ divergence of type which is "almost +always" observable in the case of "a long isolated form" under nature. +The very essence of the principle being that, when divergence of type +has once begun, this divergence must _ipso facto_ proceed at an +ever-accelerating pace, it is manifestly inconsistent to entertain the +principle as explaining the first commencement of divergence, and then +to ignore it as explaining the further progress of divergence. Hence, I +can only conclude that Darwin had forgotten this principle altogether +when he wrote his letter to Semper in 1878--owing, no doubt, as he says +in the sentence which immediately follows, to his having "not attended +much of late years to such questions." + + [41] _Life and Letters_, vol. iii. p. 161. + + * * * * * + +So much, then, for Darwin's opinions. Next in order of time we must +consider Moritz Wagner's essays on what he called the "Law of +Migration[42]." The merit of these essays was, first, the firm expression +of opinion upon the swamping effects of free intercrossing; and, second, +the production of a large body of facts showing the importance of +geographical isolation in the prevention of these effects, and in the +consequent differentiation of specific types. On the other hand, the +defect of these essays was, first, not distinguishing between evolution +as monotypic and polytypic; and, second, not perceiving that +geographical isolation is only one among a number of other forms of +isolation. From these two radical oversights--which, however, were +shared by all other writers of the time, with the partial exception of +Darwin himself, as previously shown--there arose the following and most +lamentable errors. + + [42] _Die Darwin'sche Theorie und das Migrationsgesetz_ (1868): + _Ueber den Einfluss der geographischen Isolirung_, &c. (1870). + +Over and over again Moritz Wagner insists, as constituting the +fundamental doctrine of his attempted reform of Darwinism, that +evolution by natural selection is impossible, unless natural selection +be assisted by geographical isolation, in order to prevent the swamping +effects of intercrossing[43]. Now, if instead of "evolution" he had said +"divergence of type," and if instead of "geographical isolation" he had +said "prevention of intercrossing," he would have enunciated the general +doctrine which it has been the joint endeavour of Mr. Gulick and myself +to set forth. But by not perceiving that "evolution" is of two radically +different kinds--polytypic and monotypic--he entirely failed to perceive +that, while for one of its kinds the _prevention_ of intercrossing is an +absolute necessity, for the other of its kinds the _permission_ of +intercrossing is a necessity no less absolute. And, again, in missing +the fact that geographical isolation is but one of the many ways +whereby intercrossing may be prevented, he failed to perceive that, even +as regards the case of polytypic evolution, he greatly erred in +representing this one form of isolation as being universally a necessary +condition to the process. The necessary condition to this process is, +indeed, the prevention of intercrossing _by some means or another_; but +his unfortunate insistence on geographical separation as the only +possible means to this end--especially when coupled with his no less +unfortunate disregard of monotypic evolution--caused him to hinder +rather than to advance a generalization which he had only grasped in +part. And this generalization is, as now so repeatedly stated, that +while the form of isolation which we know as natural selection depends +for its action upon the intercrossing of all the individuals which it +isolates (i. e. selects), when acting alone it can produce only +monotypic evolution; but that when it is supplemented by any of the +other numerous forms of isolation, it is furnished with the necessary +condition to producing polytypic evolution--and this in as many lines of +divergent change as there may be cases of this efficient separation. + + [43] For instance, speaking of common, or continuous areas, he + says:--"In this case a constant variety, or new species, cannot be + produced, because the free crossing of a new variety with the old + unaltered stock will always cause it to revert to the original type; + in other words, will destroy the new form. The formation of a real + variety, which Darwin, as we know, regards as the commencement of a + new species, will only succeed when a few individuals, having + crossed the barrier of their habitat, are able to separate + themselves for a long time from the old stock." And the last + sentence, given as a summary of his whole doctrine, is--"The + geographical isolation of the form, a necessary consequence of + migration, is the cause of its typical character." + +Nevertheless, while we must lament these shortcomings on the part of +Wagner, we ought to remember that he rendered important services in the +way of calling attention to the swamping effects of free intercrossing, +and, still more, in that of showing the high importance of geographical +isolation as a factor of organic evolution. Therefore, although in an +elaborate criticism of his views Weismann was easily able to dispose of +his generalizations in the imperfect form that they presented, I do not +think it was just in Weismann to remark, "if Wagner had confined himself +to the statement that geographical isolation materially assists the +process of natural selection, and thus also promotes the origination of +new species, he would have met with little or no opposition; but then, +of course, in saying this much, he would not have been saying anything +new." No doubt, as I have just shown, he _ought_ thus (as well as in +other and still more important respects not perceived by Prof. Weismann) +to have limited his statement; but, had he done so, it does not follow +that he would not have been saying anything new. For, in point of fact, +in as far as he said what was true, he did say a great deal that was +also new. Thus, most of what he said of the _principle of separation_ +(apogamy) was as new as it was true, although, as we have seen, he said +it to very little purpose on account of his identifying this principle +as a whole with that of but one of its forms. Again, notwithstanding +this great error, or oversight, he certainly showed of the particular +form in question--viz. geographical isolation--that it was of +considerably _more_ importance than had previously been acknowledged. +And this was so far a valuable contribution to the general theory of +descent. + + * * * * * + +Prof. Weismann's essay, to which allusion has just been made[44], was, +however, in all respects a great advance upon those of Wagner. It was +not only more comprehensive in its view of the whole subject of +geographical isolation, but likewise much more adequate in its general +treatment thereof. Its principal defects, in my judgement, were, first, +the inordinately speculative character of some of its parts, and, +second, the restriction of its analysis to but one form of isolation--a +defect which it shares with the essays of Wagner, and in quite as high a +degree. Furthermore, although this essay had the great merit of +enunciating the principle of Amixia, it did so in a very inefficient +manner. For not only was this principle adduced with exclusive reference +to _geographical_ isolation, but even in regard to this one kind of +isolation it was presented in a highly inconsistent manner, as I will +now endeavour to show. + + [44] _Ueber den Einfluss der Isolirung auf die Artbildung_ (1872). + +Weismann was led to perceive the principle in question by the +consideration that new specific characters, when they first appear, do +not all appear together in the same individuals: they appear one in one +individual, another in another, a third in a third, &c.; and it is only +in the course of successive generations that they all become blended in +the same individuals by free intercrossing. Hence, the eventually +emerging constant or specific type is the resultant of all the +transitory or varietal types, when these have been fused together by +intercrossing. From which Weismann deduces what he considers a general +law--namely, that "the constancy of a specific type does not arise +suddenly, but gradually; and it is established by the promiscuous +crossing of all individuals[45]." From which again it follows, that this +constancy must cease so soon as the condition which maintains it +ceases--i. e. so soon as free intercrossing is prevented by the +geographical isolation of a portion of the species from its parent +stock. + + [45] _Loc. cit._, p. 43. + +Now, to begin with, this statement of the principle in question is not a +good statement of it. There was no need while stating the doctrine that +separation induces differentiation, to found the doctrine on any such +highly speculative basis. In point of fact, there is no real evidence +that specific types do attain their constancy in the way supposed; nor, +for the purposes of the doctrine in question, is it necessary that there +should be. For this doctrine does not need to show how the constancy has +been _attained_; it only has to show that the constancy is _maintained_ +by free intercrossing, with the result that when free intercrossing is +_by any means_ prevented, divergence of character ensues. In short, the +correct way of stating the principle is that which has been adopted by +DelbÅ“uf and Gulick--namely, the average characters of a separated +portion of a species are not likely to be the same as those of the whole +species; with the result that divergence of type will be set up in the +separated portion by intercrossing within that portion. Or the principle +may be presented as I presented it under the designation of "Independent +Variability"--namely, "a specific type may be regarded as the average +mean of all individual variations, any considerable departure from this +average mean being, however, checked by intercrossing," with the result +that when intercrossing is prevented between a portion of a species and +the rest of the species, "this population is permitted to develop an +independent history of its own, shielded from intercrossing with its +parent form[46]." + + [46] _Physiological Selection_, pp. 348, 389. + +Not only, however, is Weismann's principle of "Amixia" thus very +differently stated from that of my "Independent Variability" (apogamy), +or Gulick's "Independent Generation"; but, apparently owing to this +difference of statement, the principle itself is not the same. In +particular, while Weismann holds with us that when new characters arise +in virtue of the mere prevention of intercrossing with parent forms +these new characters will be of non-utilitarian kind[47], he appears to +think that divergence of character under such circumstances is not +likely to go on to a _specific_ value. Now, it is of importance to +observe why he arrives at this conclusion, which is not only so +different from that of DelbÅ“uf, Gulick, and myself, but apparently so +inconsistent with his own recognition of the diversifying effect of +"Amixia" as regards the formation of _permanent varieties_. For, as we +have already seen while considering Darwin's views on this same +principle of "Amixia," it is highly inconsistent to recognize its +diversifying effect up to the stage of constituting fixed varieties, and +then not to recognize that, so much divergence of character having been +already secured by the isolation alone, much more must further +divergence continue, and continue at an ever accelerating pace--as +DelbÅ“uf and Gulick have so well shown. What, then, is the explanation +of this apparent inconsistency on Weismann's part? The explanation +evidently is that, owing to his erroneous statement of the principle, he +misses the real essence of it. For, in the first place, he does not +perceive that this essence consists in an initial difference of average +characters on the part of the isolated colony as compared with the rest +of their species. On the contrary, he loses himself in a maze +of speculation about all species having had what he calls +"variation-periods," or eruptions of general variability alternating +with periods of repose--both being as unaccountable in respect of their +causation as they are hypothetical in respect of their occurrence. From +these speculations he concludes, that isolation of a portion of a +species will then only lead to divergence of character when the +isolation happens to coincide with a "variation-period" on the part of +the species as a whole, and that the divergence will cease so soon as +the "variation-period" ceases. Again, in the second place as previously +remarked, equally with Wagner whom he is criticizing, he fails to +perceive that _geographical_ isolation is not the only kind of +isolation, or the only possible means to the prevention of free +intercrossing. And the result of this oversight is, that he thinks +amixia can act but comparatively seldom upon sufficiently small +populations to become a factor of much importance in the differentiation +of species. Lastly, in the third place, owing to his favourite +hypothesis that all species pass through a "variation-period," he +eventually concludes that the total amount of divergence of type +producible by isolation alone (even in a small population) can never be +greater than that between the extremes of variation which occur within +the whole species at the date of its partition (p. 75). In other words, +the possibility of change due to amixia alone is taken to be limited by +the range of deviation from the general specific average, as manifested +by different individual variations, before the species was divided. Thus +the doctrine of amixia fails to recognize the law of DelbÅ“uf, or the +_cumulative_ nature of divergence of type when once such divergence +begins in a separated section. Therefore, in this all-important--and, +indeed, essential--respect, amixia differs entirely from the principle +which has been severally stated by DelbÅ“uf, Gulick, and myself. + + [47] _Loc. cit._, p. 54. + +Upon the whole, then, we must say that although Professor Weismann was +the first to recognize the diversifying influence of merely +indiscriminate isolation _per se_ (apogamy), he did so only in part. He +failed to distinguish the true essence of the principle, and by +overlaying it with a mass of hypothetical speculation, concealed even +more of it than he revealed. + + * * * * * + +The general theory of Isolation, as independently worked out by Mr. +Gulick and myself, has already been so fully explained, that it will +here be sufficient merely to enumerate its more distinguishing features. +These are, first, drawing the sharpest possible line between evolution +as monotypic and polytypic; second, showing that while for the former +the peculiar kind of isolation which is presented by natural selection +suffices of itself to _transform_ a specific type, in order to work for +the latter, or to _branch_ a specific type, natural selection must +necessarily be assisted by some other kind of isolation; third, that +even in the absence of natural selection, other kinds of isolation may +be sufficient to effect specific divergence through independent +generation alone; fourth, that, nevertheless, natural selection, where +present, will always accelerate the process of divergence; fifth, that +monotypic evolution by natural selection depends upon the _presence_ of +intercrossing, quite as much as polytypic evolution (whether with or +without natural selection) depends upon the _absence_ of it; sixth, +that, having regard to the process of evolution throughout all taxonomic +divisions of organic nature, we must deem the physiological form of +isolation as the most important, with the exception only of natural +selection. + +The only difference between Mr. Gulick's essays and my own is, that, on +the one hand, he has analyzed much more fully than I have the various +forms of isolation; while, on the other hand, I have considered much +more fully than he has the particular form of physiological isolation +which so frequently obtains between allied _species_. This particular +form of physiological isolation I have called "physiological selection," +and claim for it so large a share in the differentiation of specific +types as to find in it a satisfactory explanation of the contrast +between natural species and artificial varieties in respect of +cross-infertility. + + * * * * * + +Mr. Wallace, in his _Darwinism_, has done good service by enabling all +other naturalists clearly to perceive how natural selection alone +produces monotypic evolution--namely, through the free intercrossing of +all individuals which have not been eliminated by the isolating process +of natural selection itself. For he very lucidly shows how the law of +averages must always ensure that in respect of any given specific +character, half the individuals living at the same time and place will +present the character above, and half below its mean in the population +as a whole. Consequently, if it should ever be of advantage to a +species that this character should undergo either increase or decrease +of its average size, form, colour, &c., there will always be, in each +succeeding generation, a sufficient number of individuals--i. e. half of +the whole--which present variations in the required direction, and which +will therefore furnish natural selection with abundant material for its +action, without the need of any other form of isolation. It is to be +regretted, however, that while thus so clearly presenting the fact that +free intercrossing is the very means whereby natural selection is +enabled to effect monotypic evolution, he fails to perceive that such +intercrossing must always and necessarily render it impossible for +natural selection to effect polytypic evolution. A little thought might +have shown him that the very proof which he gives of the necessity of +intercrossing where the _transmutation_ of species is concerned, +furnishes, measure for measure, as good a proof of the necessity of its +absence where the _multiplication_ of species is concerned. In justice +to him, however, it may be added, that this distinction between +evolution as monotypic and polytypic (with the important consequence +just mentioned) still continues to be ignored also by other well-known +evolutionists of the "ultra-Darwinian" school. Professor Meldola, for +example, has more recently said that in his opinion the "difficulty from +intercrossing" has been in large part--if not altogether--removed by Mr. +Wallace's proof that natural selection alone is capable of effecting +[monotypic] evolution; while he regards the distinction between +monotypic and polytypic evolution as mere "verbiage[48]." + + [48] _Nature_, vol. xliii. p. 410, and vol. xliv. p. 29. + +It is in relation to my presentment of the impossibility of natural +selection alone causing polytypic evolution, that Mr. Wallace has been +at the pains to show how the permission of intercrossing (panmixia) is +necessary for natural selection in its work of causing monotypic +evolution. And not only has he thus failed to perceive that the +"difficulty" which intercrossing raises against the view of natural +selection being of itself capable of causing polytypic evolution in no +way applies to the case of monotypic; but as regards this "difficulty," +where it does apply, he says:-- + + Professor G. J. Romanes has adduced it as one of the difficulties + which can alone be overcome by his theory of physiological + selection[49]. + + [49] _Darwinism_, p. 143. + +This, however, is a misapprehension. I have by no means represented that +the difficulty in question can alone be overcome by this theory. What I +have represented is, that it can be overcome by any of the numerous +forms of isolation which I named, and of which physiological selection +is but one. And although, _where common areas are concerned_, I believe +that the physiological form of isolation is the most important form, +this is a very different thing from entertaining the supposition which +Mr. Wallace here assigns to me. + + * * * * * + +I may take this opportunity of correcting a somewhat similar +misunderstanding which has been more recently published by Professor W. +A. Herdman, of Liverpool; and as the case which he gives is one of +considerable interest in itself, I will quote his remarks in extenso. In +his _Opening Address to the Liverpool Biological Society_, Professor +Herdman said:-- + + Some of you will doubtless remember that in last year's address, + while discussing Dr. Romanes' theory of physiological selection, I + quoted Professor Flemming Jenkin's imaginary case of a white man + wrecked upon an island inhabited by negroes, given as an + illustration of the supposed swamping effect by free intercrossing + of a marked variety with the parent species. I then went on to say + in criticism of the result at which Jenkin arrived, viz. that the + characteristics of the white man would be stamped out by + intercrossing with the black:-- + + "Two influences have, I think, been ignored, viz. atavism, or + reversion to ancestral characters, and the tendency of the members + of a variety to breed with one another. Keeping to the case + described above, I should imagine that the numbers of intelligent + young mulattoes produced in the second, third, fourth, and few + succeeding generations would to a large extent intermarry, the + result of which would be that a more or less white aristocracy + would be formed on the island, including the king and all the chief + people, the most intelligent men and the bravest warriors. Then + atavism might produce every now and then a much whiter + individual--a reversal to the characteristics of the ancestral + European--who, by being highly thought of in the whitish + aristocracy, would have considerable influence on the colour and + other characteristics of the next generation. Now such a white + aristocracy would be in precisely the same circumstances as a + favourable variety competing with its parent species," &c. + + You may imagine then my pleasure when, a few months after writing + the above, I accidentally found, in a letter[50] written by the + celebrated African traveller Dr. David Livingstone to Lord + Granville, and dated "Unyanyembe, July 1st, 1872," the following + passage:-- + + [50] In Appendix to H. M. Stanley's _How I found Livingstone_, 2nd + ed. London, 1872, p. 715. + + "About five generations ago, a white man came to the highlands of + Basañgo, which are in a line east of the watershed. He had six + attendants, who all died, and eventually their headman, called + Charura, was elected chief by the Basañgo. In the third generation + he had sixty able-bodied spearmen as lineal descendants. This + implies an equal number of the other sex. They are very light in + colour, and easily known, as no one is allowed to wear coral beads + such as Charura brought except the royal family. A book he brought + was lost only lately. The interest of the case lies in its + connexion with Mr. Darwin's celebrated theory on the 'origin of + species,' for it shows that an improved variety, as we whites + modestly call ourselves, is not so liable to be swamped by numbers + as some have thought." + + Here we have a perfect fulfilment of what I last year, in ignorance + of this observation of Livingstone's, predicted as being likely to + occur in such a case. We have the whitish aristocracy in a dominant + condition, and evidently in a fair way to spread their + characteristics over a larger area and give rise to a marked + variety, and it had clearly struck Livingstone fourteen years + before the theory of physiological selection had been heard of, + just as it must strike us now, as an instance telling strongly + against the "swamping" argument as used by Flemming Jenkin and + Romanes. + +Here we have a curious example of one writer supporting the statements +of another, while appearing to be under the impression that he is +controverting those statements. Both Professor Herdman's imaginary case, +and its realization in Livingstone's account, go to show "the tendency +of the members of a variety to breed with one another." This is what I +have called "psychological selection," and, far from "ignoring" it, I +have always laid stress upon it as an obviously important form of +isolation or _prevention_ of free intercrossing. But it is a form of +isolation which can only occur in the higher animals, and, therefore, +the whole of Professor Herdman's criticism is merely a restatement of my +own views as already published in the paper which he is criticizing. +For all that his argument goes to prove is, first, the necessity for +_some_ form of isolation if the overwhelming effects of intercrossing +are to be obviated; and, secondly, the manifest consequence that where +the psychological form is unavailable (as in many of the lower animals +and in all plants), some other form must be present if divergent +evolution is taking place on a common area. + + * * * * * + +Seeing that so much misunderstanding has been shown with reference to my +views on "the swamping effects of intercrossing," and seeing also that +this misunderstanding extends quite as much to Mr. Gulick's views as to +my own, I will here supply brief extracts from both our original papers, +for the double purpose of showing our complete agreement, and of leaving +it to be judged whether we can fairly be held responsible for the +misunderstanding in question. After having supplied these quotations, I +will conclude this historical sketch by considering what Mr. Wallace has +said in reply to the views therein presented. I will transcribe but a +single passage from our papers, beginning with my own. + + Any theory of the origin of species in the way of descent must be + prepared with an answer to the question, Why have species + _multiplied_? How is it that, in the course of evolution, species + have not simply become transmuted in linear series instead of + ramifying into branches? This question Mr. Darwin seeks to answer + "from the simple circumstance that the more diversified the + descendants from any one species becomes in structure, + constitution, and habits, by so much will they be better enabled to + seize on many and widely diversified places in the economy of + nature, and so be enabled to increase in numbers." And he proceeds + to illustrate this principle by means of a diagram, showing the + hypothetical divergence of character undergone by the descendants + of seven species. Thus, he attributes divergence of character + exclusively to the influence of natural selection. + + Now, this argument appears to me unassailable in all save one + particular; but this is a most important particular: the argument + wholly ignores the fact of intercrossing with parent forms. + Granting to the argument that intercrossing with parent forms is + prohibited, and nothing can be more satisfactory. The argument, + however, sets out with showing that it is in limited areas, or in + areas already overstocked with the specific form in question, that + the advantages to be derived from diversification will be most + pronounced. It is where they "jostle each other most closely" that + natural selection will set a premium upon any members of the + species which may depart from the common type. Now, inasmuch as + this jostling or overcrowding of individuals is a needful condition + to the agency of natural selection in the way of diversifying + character, must we not feel that the general difficulty from + intercrossing previously considered is here presented in a special + and aggravated form? At all events, I know that, after having duly + and impartially considered the matter, to me it does appear that + unless the swamping effects of intercrossing with the parent form + on an overcrowded area is in some way prevented to begin with, + natural selection could never have any material supplied by which + to go on with. Let it be observed that I regard Mr. Darwin's + argument as perfectly sound where it treats of the divergence of + _species_, and of their further divergence into _genera_; for in + these cases the physiological barrier is known to be already + present. But in applying the argument to explain the divergence of + individuals into varieties, it seems to me that here, more than + anywhere else, Mr. Darwin has strangely lost sight of the + formidable difficulty in question; for in this particular case so + formidable does the difficulty seem to me, that I cannot believe + that natural selection alone could produce any divergence of + specific character, so long as all the individuals on an + overcrowded area occupy that area together. Yet, if any of them + quit that area, and so escape from the unifying influence of free + intercrossing, these individuals also escape from the conditions + which Mr. Darwin names as those that are needed by natural + selection in order to produce divergence. Therefore, it appears to + me that, under the circumstances supposed, natural selection alone + could not produce divergence; the most it could do would be to + change the whole specific type in some one direction, and thus + induce transmutation of species in a linear series, each succeeding + member of which might supplant its parent form. But in order to + secure _diversity_, _multiplication_, or _ramification_ of species, + it appears to me obvious that the primary condition required is + that of preventing intercrossing with parent forms at the origin of + each branch, whether the prevention be from the first absolute, or + only partial. + +Now for Mr. Gulick, a portion of whose more lengthy discussion of the +subject, however, is all that I need quote:-- + + Having found that the evolution of the fitted is secured through + the prevention of crossing between the better fitted and the less + fitted, can we believe that the evolution of a special race, + regularly transmitting a special kind of fitness, can be realized + without any prevention of crossing with other races that have no + power to transmit that special kind of fitness? Can we suppose that + any advantage, derived from new powers that prevent severe + competition with kindred, can be permanently transmitted through + succeeding generations to one small section of the species while + there is free crossing equally distributed between all the families + of the species? Is it not apparent that the terms of this + supposition are inconsistent with the fundamental laws of heredity? + Does not inheritance follow the lines of consanguinity; and when + consanguinity is widely diffused, can inheritance be closely + limited? When there is free crossing between the families of one + species, will not any peculiarity that appears in one family either + be neutralized by crosses with families possessing the opposite + quality, or, being preserved by natural selection, while the + opposite quality is gradually excluded, will not the new quality + gradually extend to all the branches of the species; so that, in + this way or in that, increasing divergence of form will be + prevented? + + If the advantage of freedom from competition in any given variation + depends on the possession, in some degree, of new adaptations to + unappropriated resources, there must be some cause that favours the + breeding together of those thus specially endowed, and interferes + in some degree with their crossing with other variations, or, + failing this, the special advantage will in succeeding generations + be lost. As some degree of Independent Generation is necessary for + the continuance of the advantage, it is evident that the same + condition is necessary for the accumulation through Natural + Selection of the powers on which the advantage depends. The + advantage of divergence of character cannot be retained by those + that fail to retain the divergent character; and divergent + character cannot be retained by those that are constantly crossing + with other kinds; and the prevention of free crossing between those + that are equally successful is in no way secured by Natural + Selection. + +So much, then, as expressive of Mr. Gulick's opinion upon this subject. +To exactly the same effect Professor Lloyd Morgan has recently published +his judgement upon it thus:-- + + That perfectly free intercrossing, between any or all of the + individuals of a given group of animals, is, so long as the + characters of the parents are blended in the offspring, fatal to + divergence of character, is undeniable. Through the elimination of + less favourable variations, the swiftness, strength, and cunning of + a race may be gradually improved. But no form of elimination can + possibly differentiate the group into swift, strong, and cunning + varieties, distinct from each other, so long as all three varieties + freely interbreed, and the characters of the parents blend in the + offspring. Elimination may and does give rise to progress in any + given group, _as a group_; it does not and cannot give rise to + differentiation and divergence, so long as interbreeding with + consequent interblending of characters be freely permitted. Whence + it inevitably follows, as a matter of simple logic, that where + divergence has occurred, intercrossing and interbreeding must in + some way have been lessened or prevented. Thus a new factor is + introduced, that of _isolation_ or _segregation_. And there is no + questioning the fact that it is of great importance. Its + importance, indeed, can only be denied by denying the swamping + effects of intercrossing, and such denial implies the tacit + assumption that interbreeding and interblending are held in check + by some form of segregation. The isolation explicitly denied is + implicitly assumed[51]. + + [51] _Animal Life and Intelligence_, pp. 98, 99 (1890-1891). + +Similarly, and still more recently, Professor Le Conte writes:-- + + It is evident, then, as Romanes claims, that natural selection + alone tends to _monotypic_ evolution. Isolation of some sort seems + necessary to _polytypic_ evolution. The tree of evolution under the + influence of natural selection alone grows palm-like from its + terminal bud. Isolation was necessary to the starting of lateral + buds, and thus for the profuse ramification which is its most + conspicuous character[52]. + + [52] _The Factors of Evolution_ (1891). + +In order to complete this historical review, it only remains to consider +Mr. Wallace's utterances upon the subject. + +It is needless to say that he stoutly resists the view of Weismann, +DelbÅ“uf, Gulick, and myself, that specific divergence can ever be +due--or, as I understand him, even so much as assisted--by this +principle of indiscriminate isolation (apogamy). It will be remembered, +however, that Mr. Gulick has adduced certain general principles and +certain special facts of geographical distribution, in order to prove +that apogamy eventually leads to divergence of character, provided that +the isolated section of the species does not contain any very large +number of individuals. Now, Mr. Wallace, without making any reference to +this argument of Mr. Gulick, simply states the reverse--namely, that, as +a matter of fact, indiscriminate isolation is not found to be +associated with divergence of character. For, he says, "there is an +entire absence of change, where, if this were a _vera causa_, we should +expect to find it[53]." But the only case which he gives is that of +Ireland. + + [53] _Darwinism_, p. 151. + +This, he says, furnishes "an excellent test case, for we know that it +[Ireland] has been separated from Britain since the end of the glacial +epoch: ... yet hardly one of its mammals, reptiles, or land molluscs has +undergone the slightest change[54]." Here, however, Mr. Wallace shows +that he has failed to understand "the views of those who, like Mr. +Gulick, believe isolation itself to be a cause of modification of +species"; for it belongs to the very essence of these views that the +efficiency of indiscriminate isolation as a "_vera causa_" of organic +evolution varies inversely with the number of individuals (i. e. the +size of the species-section) exposed to its influence. Therefore, far +from being "an excellent test case," the case of Ireland is +unsatisfactory. If we are in search of excellent test cases, in the +sense intended by Mr. Wallace, we ought not to choose a large island, +which from the time of its isolation must have contained large bulks of +each of the geographically separated species concerned: we ought to +choose cases where as small a number as possible of the representatives +of each species were in the first instance concerned. And, when we do +this, the answer yielded by any really "excellent test case" is +unequivocal. + + [54] _Ibid._ + +No better test case of this kind has ever been furnished than that of +Mr. Gulick's land-shells, which Mr. Wallace is specially considering in +the part of his book where the sentence above quoted occurs. How, then, +does he meet this case? He meets it by assuming that in all the numerous +adjacent valleys of a small island there must be as many differences of +environment, each of which is competent to induce slight varietal +changes on the part of its occupants by way of natural selection, +although in no one case can the utility of these slight changes be +surmised. Now, against this explanation there are three overwhelming +considerations. In the first place, it is purely gratuitous, or offered +merely in order to save the hypothesis that there _can_ be no other +cause of even the most trivial change in species than that which is +furnished by natural selection. In the second place, as Mr. Gulick +writes to me in a private letter, "if the divergence of Sandwich Island +land molluscs is wholly due to exposure to different environments, as +Mr. Wallace argues on pages 147-150, then there must be completely +occult influences in the environment that vary progressively with each +successive mile. This is so violent an assumption that it throws doubt +on any theory that requires such support." In the third place, the +assumption that the changes in question must have been due to natural +selection, is wholly incompatible with the facts of isolation +elsewhere--namely, in those cases where (as in that of Ireland) a large +section of species, instead of a small section, has been +indiscriminately isolated. Mr. Wallace, as we have seen, inadvertently +alludes to these "many other cases of isolation" as evidence against +apogamy being _per se_ a cause of specific change. But although, for +the reason above stated, they are without relevancy in this respect, +they appear to me fatal to the explanation which he gives of specific +changes under apogamy where only small sections of species are +concerned. For example, can it be rationally maintained that there are +more differences of environment between every two of the many contiguous +valleys of a small island, such as Mr. Gulick describes, than there are +in the incomparably larger area of the whole of Ireland? But, if not, +and if natural selection is able to work such "occult" wonders in each +successive mile on the Sandwich Islands, why has it so entirely lost +this magic power in the case of Ireland--or in the "many other cases of +isolation" to which Mr. Wallace refers? On his theory there is no +coherent answer to be given to this question, while on our theory the +answer is given in the very terms of the theory itself. The facts are +plainly just what the theory requires that they should be; and +therefore, if they were not as they are, the theory would be deprived of +that confirmation which it now derives from them. + +Thus, in truth, though in an opposite way, the case of Ireland is, as +Mr. Wallace says, "an excellent test case," when once the theory of +apogamy as a "_vera causa_" of specific change is understood; and the +effect of applying the test is fully to corroborate this theory, while +at the same time it as fully negatives the other. For the consideration +whereby Mr. Wallace seeks to explain the inactivity of natural selection +in the case of Ireland is not "coherent." What he says is, "That changes +have not occurred through natural selection, is perhaps due to the less +severe struggle for existence, owing to the smaller number of competing +species[55]." But even with regard to molluscs alone, there is a greatly +larger number of species in Ireland than occurs in any one valley of the +Sandwich Islands; while if we have regard to all the other classes of +animal life, comparison entirely fails. + + [55] _Loc. cit._, p. 151. + +Much more to the point are certain cases which were adduced long ago by +Weismann in his essay previously considered. Nevertheless, although this +essay was published as far back as 1872, and, although it expressly +deals with the question of divergence of character through the mere +prevention of intercrossing (Amixia), Mr. Wallace nowhere alludes to +these cases _per contra_, which are so much more weighty than his own +"test case" of Ireland. Of such are four species of butterflies, +belonging to three genera[56], which are identical in the polar regions +and in the Alps, notwithstanding that the sparse Alpine populations have +been presumably separated from their parent stocks since the glacial +period; or of certain species of fresh water crustaceans (_Apus_), the +representatives of which are compelled habitually to form small isolated +colonies in widely separated ponds, and nevertheless exhibit no +divergence of character, although apogamy has probably lasted for +centuries. These cases are unquestionably of a very cogent nature, and +appear of themselves to prove that apogamy alone is not invariably +capable of inducing divergence--at any rate, so rapidly as we might +expect. There appears, however, to be another factor, the presence or +absence of which makes a great difference. This as stated in the text, +is the degree in which a specific type is stable or unstable--liable or +not liable to vary. Thus, for example, the Goose is what Darwin calls an +"inflexible" type as compared with most other domesticated birds. +Therefore, if a lot of geese were to be indiscriminately isolated from +the rest of their species, the probability is that in a given time their +descendants would not have diverged from the parent type to such an +extent as would a similar lot of ducks under similar circumstances: the +more stable specific type would require a longer time to change under +the influence of apogamy alone. Now, the butterflies and crustaceans +quoted by Weismann may be of a highly stable type, presenting but a +small range of individual variability; and, if so, they would naturally +require a long time to exhibit any change of type under the influence of +apogamy alone. But, be this as it may, Weismann himself adduces these +cases merely for the sake of showing that there are cases which seem to +tell against the general principle of modification as due to apogamy +alone--i.e. the general principle which, under the name amixia, he is +engaged in defending. And the conclusion at which he himself arrives is, +that while it would be wrong to affirm that apogamy _must_ in all cases +produce divergence, we are amply justified in affirming that in many +cases it _may_ have done so; while there is good evidence to prove that +in not a few cases it _has_ done so, and therefore should be accepted +as one of the factors of organic evolution[57]. + + [56] Namely, _Lycaena denzelii_, _L. pheretes_, _Argynnis pales_, + _Erebia mante_. + + [57] Since the above was written, I have heard of some cases which + seem to present greater difficulties to our theory than those above + quoted. These refer to some of the numerous species of land mollusca + which inhabit the isolated rocks near Madeira (Dezertas). My + informant is Dr. Grabham, who has himself investigated the matter, + and reports as follows:-- + + "It is no uncommon thing to meet with examples of the same species, + sub-fossil, recent, and living upon one spot, and presenting no + variation in the long record of descent." Then, after naming these + examples, he adds, "All seem to vary immediately on attaining new + ground, assuming many aspects in different districts." + + Unquestionably these statements support, in a very absolute manner, + Mr. Wallace's opinion, while making directly against my own. It is + but fair, however, to add that the cases are not numerous (some + half-dozen at the most, and all within the limits of a single + genus), and that, even in the opinion of my informant himself, the + facts have not hitherto been sufficiently investigated for any + decisive judgement to be formed upon them. + +My view from the very first has been that variations in the way of +cross-infertility are of frequent occurrence (how, indeed, can they be +otherwise, looking to the complex conditions that have to be satisfied +in every case of full fertility?); and, therefore, however many of such +variations are destined to die out, whenever one arises, "under suitable +conditions," "it must inevitably tend to be preserved as a new natural +variety, or incipient species." Among the higher animals--which are +"comparatively few in number"--I think it probable that some slight +change of form, colour, habit, &c., must be usually needed either to +"superinduce," or, which is quite a different thing, to _coincide_ with +the physiological change But in the case of plants and the lower +invertebrata. I see no reason for any frequent concomitance of this +kind; and therefore believe the physiological change to be, "as a +general rule," the primordial change. At the same time, I have always +been careful to insist that this opinion had nothing to do with "the +essence of physiological selection"; seeing that "it was of no +consequence" to the theory in what proportional number of cases the +cross-sterility had begun _per se_, had been superinduced by +morphological changes, or only enabled to survive by happening to +coincide with any other form of homogamy. In short, "the essence of +physiological selection" consists in _all_ cases of the diversifying +_effect_ of cross-infertility, whensoever and howsoever it may happen in +particular cases to have been _caused_. + +Thus I emphatically reaffirm that "from the first I have always +maintained that it makes no essential difference to the theory _in what +proportional number of cases_ they [the physiological variations] have +arisen 'alone in an otherwise undifferentiated species'"; therefore, +"even if I am wrong in supposing that physiological selection can _ever_ +act alone, the _principle_ of physiological selection, as I have stated +it, is not thereby affected. And this principle is, as Mr. Wallace has +re-stated it, 'that some amount of infertility characterizes the +distinct varieties which are in process of differentiation into +species'--infertility whose absence, 'to obviate the effects of +intercrossing, may be one of the _usual_ causes of their failure to +become developed into distinct species.'" + +These last sentences are quoted from the correspondence in _Nature_[58], +and to them Mr. Wallace replied by saying, "if this is not an absolute +change of front, words have no meaning"; that "if this is 'the whole +essence of physiological selection,' then physiological selection is but +a re-statement and amplification of Darwin's views"; that such a "change +of front" is incompatible, not only with my term "physiological +selection," but also with my having "acknowledged that Mr. Catchpool had +'very clearly put forward the theory of physiological selection'"; and +much more to the same effect. + + [58] Vol. xliii. p. 127. + +Now, to begin with, it is due to Mr. Catchpool to state that his only +publication upon this subject is much too brief to justify Mr. +Wallace's, inference, that he supposes variations in the way of +cross-infertility always to arise "alone in an otherwise +undifferentiated species." What Mr. Catchpool's opinion on this point +may be, I have no knowledge; but, whatever it is, he was unquestionably +the first writer who "clearly stated the leading principles" of +physiological selection, and this fact I am very glad to have +"acknowledged." In my correspondence with Mr. Wallace, however, I not +only named Mr. Catchpool: I also named--and much more prominently--Mr. +Gulick. For even if I were to grant (which I am far indeed from doing) +that there was any want of clearness in my own paper touching the point +in question, I have now repeatedly shown that it is simply impossible +for any reader of Mr. Gulick's papers to misunderstand _his_ views with +regard to it. Accordingly, I replied to Mr. Wallace in _Nature_ by +saying:-- + + Not only have I thus from the first fully recognized the sundry + other causes of specific change with which the physiological + variations may be associated; but Mr. Gulick has gone into this + side of our common theory much more fully, and elaborately + calculated out the high ratio in which the differentiating agency + of any of these other causes must be increased when assisted by--i. + e. associated with--even a moderate degree of the selective + fertility, and vice versa. Therefore, it is simply impossible for + Mr. Wallace to show that "our theory" differs from his in this + respect. Yet it is the only respect in which his reply alleges any + difference. (Vol. xliii. p. 127.) + +I think it is to be regretted that, in his answer to this, Mr. Wallace +alludes only to Mr. Catchpool, and entirely ignores Mr. Gulick--whose +elaborate calculations above alluded to were communicated to the +Linnaean Society by Mr. Wallace himself in 1887. + +The time has now come to prove, by means of quotations, that I have from +the first represented the "principle," or "essence," of physiological +selection to consist in selective fertility furnishing a needful +condition to specific differentiation, in at least a large proportional +number of allied species which afterwards present the reciprocal +character of cross-sterility; that I have never represented variations +in the way of this selective fertility as necessarily constituting the +initial variations, or as always arising "alone, in an otherwise +undifferentiated species"; and that, although I have uniformly given it +as my opinion that these variations do _in some cases_ thus arise +(especially among plants and lower invertebrata), I have as uniformly +stated "that it makes no difference to the theory in what proportional +number of cases they have done so"--or even if, as Mr. Wallace supposes, +they have never done so in any case at all[59]. These statements (all of +which are contradictory of the only points of difference alleged) have +already been published in my article in the _Monist_ of October, 1890. +And although Mr. Wallace, in his reply to that article, ignores my +references to the "original paper," it is scarcely necessary to quote +the actual words of the paper itself, since the reader who is further +interested in this controversy can readily refer to it in the _Journal +of the Linnaean Society_ (vol. xix. pp. 337-411). + + [59] This refers to what I understand Mr. Wallace to say in the + _Nature_ correspondence is the supposition on which his own theory + of the origin of species by cross-infertility is founded. But in the + original statement of that theory itself, it is everywhere + "supposed" that when species are originated by cross-infertility, + the _initial_ change _is_ the physiological change. In his original + statement of that theory, therefore, he literally went further than + I had gone in my "original paper," with reference to supposing the + physiological change to be the initial change. I do not doubt that + this is due to some oversight of expression; but it is curious that, + having made it, he should still continue his endeavour to fix + exactly the same oversight upon me. + +Having arrived at these results with regard to the theory of Isolation +in general and of Physiological Isolation in particular, I arrive also +at the end of this work. And if, while dealing with the post-Darwinian +period, I have imparted to any general reader the impression that there +is still a great diversity of expert opinion; I must ask him to note +that points with reference to which disagreement still exists are but +very subordinate to those with regard to which complete agreement now +prevails. The noise of wrangling disputations which has so filled the +camp of evolutionists since the death of their captain, is apt to hide +from the outside world the solid unanimity that prevails with regard to +all the larger and more fundamental questions, which were similarly the +subjects of warfare in the past generation. Indeed, if we take a fair +and general view of the whole history of Darwinism, what must strike us +as the really significant fact is the astonishing unanimity which has +been so rapidly attained with regard to matters of such immeasurable +importance. It is now but little more than thirty years since the +publication of the _Origin of Species_; and in that period not only have +all naturalists unequivocally embraced the doctrine of descent +considered as a fact; but, in one degree or another, they have all as +unequivocally embraced the theory of natural selection considered as a +method. The only points with regard to which any difference of opinion +still exist, have reference to the precise causation of that mighty +stream of events which, under the name of organic evolution, we have now +all learnt to accept as scientifically demonstrated. But it belongs to +the very nature of scientific demonstration that, where matters of great +intricacy as well as of high generality are concerned, the process of +demonstration must be gradual, even if it be not always slow. It is only +by the labours of many minds working in many directions that, in such +cases, truth admits of being eventually displayed. Line upon line, +precept upon precept, here a little and there a little--such is the +course of a scientific revelation; and the larger the subject-matter, +the more subtle and the more complex the causes, the greater must be the +room for individual differences in our reading of the book of Nature. +Now, if all this be true, must we not feel that in the matter of organic +evolution the measure of agreement which has been attained is out of all +proportion to the differences which still remain--differences which, +although of importance in themselves, are insignificant when compared +with those which once divided the opinions of not a few still living +men? And if we are bound to feel this, are we not bound further to feel +that the very intensity of our disputations over these residual matters +of comparative detail, is really the best earnest that can be given of +the determination of our quest--determination which, like that of our +fathers, cannot fail to be speedily rewarded by the discovery of truth? + +Nevertheless, so long as this noise of conflict is in the Senate, we +cannot wonder if the people are perplexed. Therefore, in conclusion, I +may ask it to be remembered exactly what are the questions--and the only +questions--which still divide the parties. + +Having unanimously agreed that organic evolution is a fact and that +natural selection is a cause, or a factor in the process, the primary +question in debate is whether natural selection is the only cause, or +whether it has been assisted by the co-operation of other causes. The +school of Weismann maintain that it is the only cause; and therefore +deem it worse than useless to search for further causes. With this +doctrine Wallace in effect agrees, excepting as regards the particular +case of the human mind. The school of Darwin, on the other hand--to +which I myself claim to belong--believe that natural selection has been +to a considerable extent supplemented by other factors; and, therefore, +although we further believe that it has been the "main" factor, we agree +with Darwin himself in strongly reprobating all attempts to bar _a +priori_ the progress of scientific investigation touching what, if any, +these other factors may be. Lastly, there are several more or less +struggling schools, chiefly composed of individual members who agree +with each other only to the extent of holding that the causal agency of +natural selection is not so great as Darwin supposed. The Duke of +Argyll, Mr. Mivart and Mr. Geddes may be named in this connexion; +together with the self-styled neo-Lamarckians, who seek to magnify the +Lamarckian principles at the expense of the distinctively Darwinian. + +This primary difference of opinion leads deductively to certain +secondary differences. For if a man starts with the premiss that natural +selection must necessarily be the "exclusive" cause of organic +evolution, he is likely to draw conclusions which another man would not +draw who starts with the premiss that natural selection is but the +"main" cause. Of these subordinate differences the most important are +those which relate to the possible transmission of acquired characters, +to the necessary (or only general) utility of specific characters, and +to the problem touching the inter-sterility of allied species. But we +may well hope that before another ten years shall have passed, even +these still outstanding questions will have been finally settled; and +thus that within the limits of an ordinary lifetime the theory of +organic evolution will have been founded and completed in all its parts, +to stand for ever in the world of men as at once the greatest +achievement in the history of science, and the most splendid monument of +the nineteenth century. + +In the later chapters of the foregoing treatise I have sought to +indicate certain matters of general principle, which many years of study +specially devoted to this great movement of contemporary thought have +led me to regard as almost certainly sound in themselves, and no less +certainly requisite as complements of the Darwinian theory. I will now +conclude by briefly summarizing these matters of general principle in +the form of twelve sequent propositions. And, in doing so, I may ask it +to be noticed that the system which these propositions serve to express +may now claim, at the least, to be a strictly logical system. For the +fact that, not merely in its main outlines, but likewise in its details, +it has been independently constructed by Mr. Gulick, proves at any rate +this much; seeing that, where matters of such intricacy are concerned, +nothing but accurate reasoning from a common foundation of _data_ could +possibly have yielded so exact an agreement. The only difference between +us is, that Mr. Gulick has gone into much further detail than I have +ever attempted in the way of classifying the many and varied forms of +isolation; while I have laid more special stress upon the physiological +form, and found in it what appears to me a satisfactory solution of "the +greatest of all the difficulties in the way of accepting the theory of +natural selection as a complete explanation of the origin of +species"--namely, "the remarkable difference between varieties and +species when crossed." + + + + +GENERAL CONCLUSIONS. + +1. NATURAL SELECTION IS PRIMARILY A THEORY OF THE CUMULATIVE DEVELOPMENT +OF ADAPTATIONS WHEREVER THESE OCCUR; AND THEREFORE IS ONLY INCIDENTALLY, +OR LIKEWISE, A THEORY OF THE ORIGIN OF SPECIES IN CASES WHERE ALLIED +SPECIES DIFFER FROM ONE ANOTHER IN RESPECT OF PECULIAR CHARACTERS, WHICH +ARE ALSO ADAPTIVE CHARACTERS. + +2. HENCE, IT DOES NOT FOLLOW FROM THE THEORY OF NATURAL SELECTION THAT +ALL SPECIES--MUCH LESS ALL SPECIFIC CHARACTERS--MUST NECESSARILY HAVE +OWED THEIR ORIGIN TO NATURAL SELECTION; SINCE IT CANNOT BE PROVED +DEDUCTIVELY FROM THE THEORY THAT NO "MEANS OF MODIFICATION" OTHER THAN +NATURAL SELECTION IS COMPETENT TO PRODUCE SUCH SLIGHT DEGREES OF +MODIFICATION AS GO TO CONSTITUTE DIAGNOSTIC DISTINCTIONS BETWEEN CLOSELY +ALLIED SPECIES; WHILE, ON THE OTHER HAND, THERE IS AN OVERWHELMING MASS +OF EVIDENCE TO PROVE THE ORIGIN OF "A LARGE PROPORTIONAL NUMBER OF +SPECIFIC CHARACTERS" BY CAUSES OF MODIFICATION OTHER THAN NATURAL +SELECTION. + +3. THEREFORE, AND UPON THE WHOLE, AS DARWIN SO EMPHATICALLY HELD, +"NATURAL SELECTION HAS BEEN THE MAIN, BUT NOT THE EXCLUSIVE MEANS OF +MODIFICATION." + +4. EVEN IF IT WERE TRUE THAT ALL SPECIES AND ALL SPECIFIC CHARACTERS +MUST NECESSARILY OWE THEIR ORIGIN TO NATURAL SELECTION, IT WOULD STILL +REMAIN ILLOGICAL TO DEFINE THE THEORY OF NATURAL SELECTION AS +INDIFFERENTLY A THEORY OF SPECIES OR A THEORY OF ADAPTATIONS; FOR, EVEN +UPON THIS ERRONEOUS SUPPOSITION, SPECIFIC CHARACTERS AND ADAPTIVE +CHARACTERS WOULD REMAIN VERY FAR INDEED FROM BEING CONTERMINOUS--MOST OF +THE MORE IMPORTANT ADAPTATIONS WHICH OCCUR IN ORGANIC NATURE BEING THE +COMMON PROPERTY OF MANY SPECIES. + +5. IN NO CASE CAN NATURAL SELECTION HAVE BEEN THE CAUSE OF MUTUAL +INFERTILITY BETWEEN ALLIED, OR ANY OTHER, SPECIES--_I.E._ OF THE MOST +GENERAL OF ALL "SPECIFIC CHARACTERS." + +6. WITHOUT ISOLATION, OR THE PREVENTION OF FREE INTERCROSSING, ORGANIC +EVOLUTION IS IN NO CASE POSSIBLE. THEREFORE, IT IS ISOLATION THAT _HAS_ +BEEN "THE EXCLUSIVE MEANS OF MODIFICATION," OR, MORE CORRECTLY, THE +UNIVERSAL CONDITION TO IT. THEREFORE, ALSO, HEREDITY AND VARIABILITY +BEING GIVEN, THE WHOLE THEORY OF ORGANIC EVOLUTION BECOMES A THEORY OF +THE CAUSES AND CONDITIONS WHICH LEAD TO ISOLATION. + +7. ISOLATION MAY BE EITHER DISCRIMINATE OR INDISCRIMINATE. WHEN +DISCRIMINATE, IT HAS REFERENCE TO RESEMBLANCES BETWEEN INDIVIDUALS +CONSTITUTING THE ISOLATED COLONY OR GROUP; WHEN INDISCRIMINATE, IT HAS +NO SUCH REFERENCE. IN THE FORMER CASE THERE ARISES HOMOGAMY, AND IN THE +LATTER CASE THERE ARISES APOGAMY. + +8. EXCEPT WHERE VERY LARGE POPULATIONS ARE CONCERNED, INDISCRIMINATE +ISOLATION ALWAYS TENDS TO BECOME INCREASINGLY DISCRIMINATE; AND, IN THE +MEASURE THAT IT DOES SO, APOGAMY PASSES INTO HOMOGAMY, BY VIRTUE OF +INDEPENDENT VARIABILITY. + +9. NATURAL SELECTION IS ONE AMONG MANY OTHER FORMS OF DISCRIMINATE +ISOLATION, AND PRESENTS IN THIS RELATION THE FOLLOWING PECULIARITIES:-- +(_A_) THE ISOLATION IS WITH REFERENCE TO SUPERIORITY OF FITNESS; (_B_) +IS EFFECTED BY DEATH OF THE EXCLUDED INDIVIDUALS; AND (_C_) UNLESS +ASSISTED BY SOME OTHER FORM OF ISOLATION, CAN ONLY EFFECT MONOTYPIC AS +DISTINGUISHED FROM POLYTYPIC EVOLUTION. + +10. IT IS A GENERAL LAW OF ORGANIC EVOLUTION THAT THE NUMBER OF +POSSIBLE DIRECTIONS IN WHICH DIVERGENCE MAY OCCUR CAN NEVER BE MORE THAN +EQUAL TO THE NUMBER OF CASES OF EFFICIENT ISOLATION; BUT, EXCEPTING +NATURAL SELECTION, ANY ONE FORM OF ISOLATION NEED NOT NECESSARILY +REQUIRE THE CO-OPERATION OF ANOTHER FORM IN ORDER TO CREATE AN +ADDITIONAL CASE OF ISOLATION, OR TO CAUSE POLYTYPIC AS DISTINGUISHED +FROM MONOTYPIC EVOLUTION. + +11. WHERE COMMON AREAS AND POLYTYPIC EVOLUTION ARE CONCERNED, THE MOST +GENERAL AND MOST EFFICIENT FORM OF ISOLATION HAS BEEN THE PHYSIOLOGICAL, +AND THIS WHETHER THE MUTUAL INFERTILITY HAS BEEN THE ANTECEDENT OR THE +CONSEQUENT OF MORPHOLOGICAL CHANGES ON THE PART OF THE ORGANISMS +CONCERNED, AND WHETHER OR NOT THESE CHANGES ARE OF AN ADAPTIVE +CHARACTER. + +12. THIS FORM OF ISOLATION--WHICH, IN REGARD TO INCIPIENT SPECIES, I +HAVE CALLED PHYSIOLOGICAL SELECTION--MAY ACT EITHER ALONE OR IN +CONJUNCTION WITH OTHER FORMS OF ISOLATION ON COMMON AREAS: IN THE FORMER +CASE ITS AGENCY IS OF MOST IMPORTANCE AMONG PLANTS AND THE LOWER CLASSES +OF ANIMALS; IN THE LATTER CASE ITS IMPORTANCE CONSISTS IN ITS GREATLY +INTENSIFYING THE SEGREGATIVE POWER OF WHATEVER OTHER FORM OF ISOLATION +IT MAY BE WITH WHICH IT IS ASSOCIATED. + + + + +_APPENDICES_ + + + + + +APPENDIX A. + +MR. GULICK'S CRITICISM OF MR. WALLACE'S VIEWS ON PHYSIOLOGICAL +SELECTION. + + +I have received from Mr. Gulick the results of his consideration of Mr. +Wallace's criticism. As these results closely resemble those which I +have myself reached, and as they were independently worked out on the +other side of the globe, I deem it desirable to publish them here for +the sake of comparison. + +In his covering letter Mr. Gulick writes:-- + + Mr. Wallace has most certainly adopted the fundamental principles + of our theory, and in an arbitrary way attempted to claim the + results produced by these principles as the effects of natural + selection. He takes our principles, which in the previous chapter + he has combated; but he makes such disjointed use of them that I am + not willing to recognize his statement as an intelligible + exposition of our theory.... I have endeavoured to indicate at what + points Mr. Wallace has deserted his own principles, and at what + points he has failed to make the best use of ours. To bring out + these points distinctly has been no easy task; but if you regard + this paper on _The Preservation and Accumulation of + Cross-infertility_ as giving any help in elucidating the true + principles, and in showing Mr. Wallace's position in regard to + them, I shall be satisfied. Please make any use of it that may seem + desirable, and then forward it to Professor Dana. + +The following is a general summary of Mr. Gulick's results:-- + + Mr. Wallace's criticism of the theory of Physiological Selection is + unsatisfactory; (l) because he has accepted the fundamental + principle of that theory on pages 173-9, in that he maintains that + without the cross-infertility the incipient species there + considered would be swamped; (2) because he assumes that + physiological selection pertains simply to the infertility of first + crosses, and has nothing to do with the infertility of mongrels and + hybrids; (3) because he assumes that infertility between first + crosses is of rare occurrence between species of the same genus, + ignoring the fact that in many species of plants the pollen of the + species is pre-potent on the stigma of the same species when it has + to compete with the pollen of other species of the same genus; (4) + because he not only ignores Mr. Romanes' statement that + cross-infertility often affects "a whole race or strain," but he + gratuitously assumes that the theory of Physiological Selection + excludes this "racial incompatibility" (which Mr. Romanes maintains + is the more probable form), and bases his computation on the + assumption that the cross-infertility is not associated with any + other form of segregation; (5) because he claims to show that "all + infertility not correlated with some _useful_ variation has a + constant tendency to effect its own elimination," while his + computation only shows that, if the cross-infertility is not + associated with some form of _positive_ segregation, it will + disappear[60]; and (6) because he does not observe that the positive + segregation may be secured by the very form of the physiological + incompatibility.... Without here entering into any computation, it + is evident that, e.g. the prepotency of pollen of each kind with + its own kind, if only very slight, will prevent cross-fertilization + as effectually as a moderate degree of instinctive preference in + the case of an animal. + + [60] "Positive segregation" is Mr. Gulick's term for forms of + homogamy other than that which is due to selective fertility. Of + these other, or "positive" forms, natural selection is one; but as + it is far from being the _only_ one, the criticism points out that + utility is not the _only_ conserving principle with which selective + fertility may be associated. + +The paper likewise indicates a point which, in studying Mr. Wallace's +theory, I have missed. It will be remembered that the only apparent +difference between his theory and mine has been shown to consist in +this--that while I was satisfied to state, in a general way, that +natural selection is probably able to increase a selective fertility +which has already been begun by other causes, Mr. Wallace has sought to +exhibit more in detail the precise conditions under which it can do so. +Now, Mr. Gulick shows that the particular conditions which Mr. Wallace +describes, even if they do serve to promote an increase of +cross-infertility, are conditions which preclude the possibility of +natural selection coming into play at all. So that if, under these +particular conditions, a further increase of cross-infertility does take +place, it does not take place in virtue of natural selection. To me it +appears that this criticism is sound; and, if so, it disposes of even +the one very subordinate addition to our theory which Mr. Wallace +"claims" as the most "distinctive" part of his. + +The following is the criticism in question:-- + + On pages 173-186 Mr. Wallace maintains that "Natural selection is, + in some probable cases at all events, able to accumulate variations + in infertility between incipient species" (p. 174); but his + reasoning does not seem to me conclusive. Even if we grant that the + increase of this character [cross-infertility] occurs by the steps + which he describes, _it is not a process of accumulation by natural + selection_. In order to be a means of cumulative modification of + varieties, races, or species, selection, whether artificial or + adaptational [i.e. natural], must preserve certain forms of an + intergenerating stock, to the exclusion of other forms of the same + stock. Progressive change in the size of the occupants of a + poultry-yard may be secured by raising only bantams the first, only + common fowls the second, and only Shanghai fowls the third year; + but this is not the form of selection that has produced the + different races of fowls. So in nature, rats may drive out and + supplant mice; but this kind of selection modifies neither rats + nor mice. On the other hand, if certain variations of mice prevail + over others, through their superior success in escaping their + pursuers, then modification begins. Now, turning to page 175, we + find that, in the illustrative case introduced by Mr. Wallace, the + commencement of infertility between the incipient species is in the + relations to each other of two portions of a species that are + locally segregated from the rest of the species, and partially + segregated from each other by different modes of life. These two + local varieties, being by the terms of his supposition better + adapted to the environment than the freely interbreeding forms in + other parts of the general area, increase till they supplant these + original forms. Then, in some limited portion of the general area, + there arise two still more divergent forms, with greater mutual + infertility, and with increased adaptation to the environment, + enabling them to prevail throughout the whole area. The process + here described, if it takes place, is not modification by natural + selection. + +On the other hand, it _is_ modification by physiological selection. For, +among the several other forms of isolation which are called +into requisition, the physiological (i.e. ever accumulating +cross-infertility) is supposed to play an important part. That the +modification is not modification by natural selection may perhaps be +rendered more apparent by observing, that in as far as _any_ other mode +of isolation is involved or supposed, so far is the _possible_ agency of +natural selection eliminated _as between the two or more otherwise +isolated sections of a species_; and yet it is modes of isolation other +than that furnished by natural selection (i.e. perishing of the less +fit), that Mr. Wallace here supposes to have been concerned--including, +as I have before shown, the physiological form, to which, indeed, he +really assigns most importance of all. Or, as Mr. Gulick states the +matter in his independent criticism:-- + + In the supposed case pictured by Mr. Wallace, the principle by + which the two segregating forms are kept from crossing, and so are + eventually preserved as permanently distinct forms, is no other + than that which Mr. Romanes and myself have discussed under the + terms Physiological Selection and Segregate Fecundity. Not only is + Mr. Wallace's exposition of the divergence and the continuance of + the same in accord with these principles which he has elsewhere + rejected, but his whole exposition is at variance with his own + principle, which, in the previous chapter, he vigorously maintains + in opposition to my statement that many varieties and species of + Sandwich Island land molluscs have arisen, while exposed to the + same environment, in the isolated groves of the successive valleys + of the same mountain range. If he adhered to his own theory, "the + greater infertility between the two forms in one portion of the + area" would be attributed to a difference between the _environment_ + presented in that portion and that presented in the other portions; + and the difficulty would be to consistently show how this greater + infertility could continue unabated when the varieties thus + characterized spread beyond the environment on which the character + depends. But, without power to continue, the process which he + describes would not take place. Therefore, in order to solve the + problem of the _origin_ and _increase_ of infertility between + species, he tacitly gives up his own theory, and adopts not only + the theory of Physiological Selection but that of Intensive + Segregation[61] through Isolation, though he still insists on + calling the process natural selection; for on page 183 he says, "No + form of infertility or sterility between the individuals of a + species can be increased by natural selection unless correlated + with some useful variation, while all infertility not so correlated + has a constant tendency to effect its own elimination." Even this + claim he seems to unwittingly abandon when on page 184 he says: + "The moment it [a species] becomes separated either by geographical + or selective isolation, or by diversity of station or of habits, + then, while each portion must be kept fertile _inter se_, there is + nothing to prevent infertility arising between the two separated + portions." + + [61] By Intensive Segregation Mr. Gulick means what I have called + Independent Variability. + +The criticism proceeds to show yet further inconsistencies and +self-contradictions in Mr. Wallace's treatment of this subject; but it +now seems needless to continue. Nor, indeed, should I have quoted this +much but for the sake of so fully justifying my own criticism by showing +the endorsement which it has received from a completely independent +examination. + + + + +APPENDIX B. + +AN EXAMINATION BY MR. FLETCHER MOULTON OF MR. WALLACE'S CALCULATION +TOUCHING THE POSSIBILITY OF PHYSIOLOGICAL SELECTION EVER ACTING ALONE. + + +We have seen that the only important point of difference between Mr. +Wallace's more recent views and my own on the problem of inter-specific +sterility, has reference to the question whether variations in the way +of cross-infertility can _ever_ arise and act "alone, in an otherwise +undifferentiated species," or whether they can _never_ so arise and act. +It is Mr. Wallace's opinion that, even if they ever do arise alone, at +all events they can never act in differentiating a specific type, seeing +that the chances against their suitable mating must be so great: only if +they be from the first associated with some other form of homogamy, +which will have the effect of determining their suitable mating, does he +think that they can act in the way supposed by our theory of "selective +fertility"[62]. On the other hand, as previously and frequently stated, +I have so strong a belief in the segregating power of physiological +selection, or selective fertility, that I do not think it is necessary +for this principle to be _always___ associated with some other form of +homogamy. From the first, indeed, I have laid great stress (as, also, +has Mr. Gulick) on the re-enforcing influence which association with any +other form of homogamy must exercise upon the physiological form, and +vice versa; but I have also said that, in my opinion, the physiological +form may in many cases be able to act entirely alone, or without +assistance derived from any other source. The question here is, as we +have already so fully seen, a question of but secondary importance; +since, whether or not the physiological form of homogamy ever acts +alone, even Mr. Wallace now allows, or rather argues, that it acts in +combination--and this so habitually, as well as with so much effect, +that it constitutes a usual condition to the origination of species. +Nevertheless, although the only relevancy of his numerical computation +of chances--whereby he thinks that he overturns my theory _in toto_--is +such relevancy as it bears to this question of secondary importance, I +have thought it desirable to refer the question, together with Mr. +Wallace's views upon it, to the consideration of a trained +mathematician. + + [62] His sentence, "all fertility not correlated with some _useful_ + variation has a constant tendency to effect its own elimination," + still further restricts the possible action of physiological + selection to cases where at least one of the other forms of homogamy + with which it is associated is natural selection. Or, in other + words, it is represented that physiological selection must always be + associated with natural selection, even if it be likewise associated + with any other form of exclusive breeding. But as this further + limitation appears to me self-evidently unjustifiable (seeing that + utility is not the only possible means of securing effective + isolation) I here neglect it, and take the wider ground marked out + above. It is needless to say that this is giving Mr. Wallace every + possible advantage, by not holding him to his still narrower ground. + +As this "subordinate question" depends entirely on numerical +computations involving the doctrine of chances, I should first of all +like to remark, that in reference to biological problems of the kind now +before us, I do not myself attach much importance to a merely +mathematical analysis. The conditions which such problems involve are so +varied and complex, that it is impossible to be sure about the validity +of the _data_ upon which a mathematical analysis is founded. +Nevertheless, for the sake of meeting these criticisms upon their own +ground, I will endeavour to show that, even as mathematical +calculations, they are quite untrustworthy. And, in order to do this +effectually, I will quote the results of a much more competent, as well +as a much more thorough, inquiry. I applied to Mr. Moulton for this +purpose, not only because he is one of the ablest mathematicians of my +acquaintance; but also because his interest in biology, and his +knowledge of Darwinian literature, render him well fitted to appreciate +exactly, and in all their bearings, the questions which were submitted +to his consideration. I need only add that his examination was +completely independent, and in no way influenced by me. Having +previously read my paper on _Physiological Selection_, Mr. Gulick's +paper on _Divergent Evolution_, and Mr. Wallace's book on _Darwinism_, +he was in possession of all the materials; and I merely requested the +favour of his opinion upon the whole case from a mathematical point of +view. The following is his reply; and I give it _in extenso_, because it +serves to place in another light some of the general considerations +which it has already been my endeavour to present[63]. + + [63] In our _Nature_ correspondence of 1890-1891, Mr. Wallace + remarked: "If Dr. Romanes will carefully work out numerically (as I + have attempted to do) a few cases showing the preservative and + accumulative agency of pure physiological selection within an + otherwise undifferentiated species, he will do more for his theory + than volumes of general disquisition or any number of assertions + that it _does_ possess this power." Several months before this was + written I had already in my hands Mr. Moulton's letter, with its + accompanying calculations. + +After some introductory remarks on Mr. Wallace's "adoption of the theory +of physiological selection pure and simple," and "the pure caricature of +it which he puts forward as" mine, the letter proceeds thus:-- + + The reason why it is so easy to attack your theory is that it is so + easy to confuse the survival of an _individual_ with the survival + of a _peculiarity_ of _type_. No one has ever said that an + _individual_ is _assisted_ by the possession of selective + fertility: that is a matter which cannot affect his chance of + _life_. Nor has any one said that the possession of selective + fertility in an _individual_ will _of itself_ increase the chance + of his having _progeny_ that will survive, and in turn become the + progenitors of others that will survive. Taken by itself, the fact + that an _individual_ is capable of fertility with some only of the + opposite sex lessens the chance of his having progeny. Whether or + not he is more or less favourably situated than his _confreres_ for + the battle of life must be decided by the _total sum_ of his + peculiarities; and the question whether or not this selective + fertility will be a hindrance must be decided by considerations + depending on the other peculiarities associated with it. + + But when we come to consider the survival or permanence of a _type_ + or _peculiarity_, the case is quite different. It then becomes not + only a favourable circumstance, but, in my opinion, almost a + necessary condition, that the peculiarity should be associated with + selective fertility[64]. + + [64] As, for example, in the case of sexuality in general. It is not + to the advantage of such individual male Arthropoda as perish after + the performance of the sexual act that they should perform it; but + its performance is necessary for the perpetuation of their + species.--G. J. R. + + Take the case of the Jews. I don't think that intermarriage with + other nations would lessen their fertility, or diminish the number + of their progeny; nor is there any reason to think that this + progeny would be unequal to the struggle for existence. But no one + doubts that the abandonment of their voluntary isolation (which + operates so far as this is concerned as a selective fertility), + would lead to the disappearance of the familiar Jewish type. All + the world would get some of it; but as a whole it would be + "swamped." + + Now although no doubt Wallace would admit all this, he fails to + give it the weight it ought to have. In discussing the question of + its operation he considers too exclusively the case of the + individual. + + Of course, a type can only be perpetuated through the medium of + individuals, and all that his argument amounts to is, that + selective fertility would be so fatal to individuals that _no_ type + which presents it could be formed or perpetuated--a conclusion + which is not only absurd in itself, but contradicted by his own + subsequent adoption of your theory. Besides, apart from + calculations (with which I will deal when I write next), such + reasoning brings its own refutation. Selective fertility is not in + the same category as some of the other influences to which an + important share has been ascribed in the formation of the existing + types. _It exists as a recognized phenomenon._ Hence all these + numerical proofs that it would lead to extinction, because it is so + disadvantageous to the possessor, prove too much. They would show + that the degree of selective fertility which so frequently + characterizes species is a most onerous gift; and that, were it not + present, there would be a vastly increased chance of fertility, + which would render the races fitter and lead to their increased + survival. Why then has it not been got rid of? + + The two answers which no doubt would be given seem to me to support + rather than to make against your theory. In the first place, + Wallace might say that this infertility is an advantage because it + keeps pure a type which is specially fitted to its surroundings, as + shown by its continued existence. But if this be so, and it is + necessary to protect the _developed_ type, how much more necessary + to protect the _incipient_ type! In the second place, he might say + that this selective fertility is not so disadvantageous when the + species has been formed, because the individual can choose his mate + from his like; whereas, when it is beginning to be formed, he must + mate blindly, or without what you call "psychological selection." + But this seems to me to be wholly inapplicable to at least half the + animal, and to all the vegetable kingdom. Moreover, with regard to + the other half of the animal kingdom, it merely raises the + question,--How soon will such an incipient type recognize itself? + Seeing it is probable that many families [broods] will belong to + the same [incipient] type, I should not be surprised if it were + found that this sexual recognition and preference sets in very + early. + + But this leads me to the question of your letter. I understand you + to want me to examine and criticize the attempted numerical + arguments against or for your theory. Now it seems to me that it + will be best to take, in the first instance, the vegetable kingdom, + and with regard to it I cannot see how there can be any numerical + argument against the theory. For we often have species side by side + with others nearly allied, but much more numerous. The condition of + these is precisely analogous to that of your incipient species. + They are exposed to fertilization from, say, ten times as numerous + individuals of the allied species. They reject this in favour of + that from the relatively few individuals of their own. Yet the two + species are in competition. I could go through the numerical + arguments of your assailant word for word, applying them to such a + case as this, and they would triumphantly show that the specific + fertility of the rarer kind would lead to its certain extinction. + Yet we know that this is not so. + + Indeed, the too triumphant character of the logic used against you + seems to me to be capable of being turned to your use. If + cross-infertility is so intensely disadvantageous to the + individuals presenting it, it cannot have been _that_ which made + these individuals and their progeny survive. It is therefore a + burden which they have carried. But we find that it is more or less + present in all the closely allied types that occur on common areas: + therefore it must be a necessary feature in the formation of such + types; for it cannot be an accident that it is present in so many. + In other words, it must be the price which the individual and his + progeny pay for their formation into a type. And this is your + theory pure and simple. + + The more I consider the matter, the more I feel that it is + impossible to decide as to the sufficiency of selective fertility + to explain the formation of species, if we consider merely the + effect it would have on the number of individuals, as contrasted + with what it would be if no such peculiarity had developed itself. + Indeed, I may say that on pondering over the matter I have come to + the conclusion, that mere fertility is probably a comparatively + unimportant factor in the preservation of the species, after a + certain sufficient degree of fertility is attained. I do not wish + to be misunderstood. To a certain point fertility is not only + advantageous but necessary, in order to secure survival of the + type; but I feel that little reliance can be placed on calculations + based on the numerical co-efficient of fertility (i. e. the ratio + of the number of offspring to the number of parents) in determining + the relative chance of type-survival. + + Take, for instance, the oak tree. It produces thousands of acorns, + almost the whole of which die without producing any progeny. Have + we any reason to believe that if the number of acorns borne by oak + trees were diminished, even so much as to one-tenth, the race of + oaks would perish? It may of course be said that, if all other + things are equal, the probabilities of survival must be increased + by increased fertility of this kind; but I feel convinced that when + numerical fertility has attained to a high point in circumstances + in which actual increase of the race cannot take place to any + substantial extent, the numerical value of this fertility sinks + down into a factor of the second or third order of importance--that + is to say, into the position of a factor whose effects are only to + be considered when we have duly allowed for the full effects of all + the main factors. Until we have done that, we gain little or + nothing in the way of accuracy of conclusion by taking into + consideration the minor factors. It may be very well to neglect the + effect of the attraction of Jupiter in our early researches on the + motion of the Moon; and our doing so will not prevent the results + being approximate and having considerable value, because we are + retaining the two main factors that establish the motion, viz. the + effects of the Earth and the Sun. But if we exclude the effect of + one of these main factors, our results would be worthless; and it + would not be rendered substantially less so by the fact that we had + taken Jupiter into account in arriving at them. + + You must not imagine, however, that I think it wholly profitless to + see whether there would be any substantial effect on numerical + fertility were _selective_ fertility to manifest itself. But if we + want to derive any assistance from calculation, it must be by + applying it with a good deal more precision and definiteness than + anything that Wallace shows. And, in the first place, it is useless + to confuse the vegetable and animal kingdoms. In the former you + have union unaffected by choice; in the latter, so far at all + events as the higher animals are concerned, you have "psychological + selection." In order to give you a specimen of what can safely be + done by calculation if you take a problem of sufficient + definiteness, I have chosen the case of a flowering plant in which + a certain proportion of the race have developed the peculiarity of + being sterile with the remainder, while retaining the normal + fertility of the race in unions among themselves. In order to give + the greatest advantage to your critics, I have assumed that such + flowers as possess the peculiarity are not self-fertilizable; for + it is clear that if we suppose that they are self-fertilizable, the + fertility need be very slightly affected. + + As I have excluded self-fertilization, it is necessary, if we are + to get any trustworthy results, that one should consider the mode + in which fertilization will be produced. I have taken the case of + fertilization by insects, and have assumed that each flower is + visited a certain number of times by insects during the period when + fertilization is possible; and, further, that the insects which + visit it have on the average visited a certain number of flowers of + the same species before they came there. Of course nothing but + observation can fix these latter numbers; but I should not be + surprised at finding that they are of considerable magnitude[65]. In + order to make the results a little more intelligible, I have + grouped them under the numbers which represent the average number + of flowers that an insect visits in a journey. This is a little + more than twice as great as the number which represents the number + of flowers he has on the average visited before coming to the + individual whose fertility we are considering. + + [65] In this anticipation Mr. Moulton is right. The well-known + botanist, Mr. Bennett, read a most interesting paper on the subject + before the British Association in 1881. His results have since been + corroborated by other observers. In particular, Mr. R. M. Christy + has recorded the movements of 76 insects while visiting at least + 2,400 flowers. (_Entomologist_, July 1883, and _Zool. Journal Lin. + Soc._, August 1883.) The following is an analysis of his results. In + the case of butterflies, in twelve observations on nearly as many + species, there are recorded altogether 99 visits to fifteen species + of flowers; and of these 99 visits 94 were constant to the same + species, leaving only 5 visits to any other, or second species. In + the case of the hive-bee, there were 8 individuals observed: these + visited altogether 258 flowers, and all the visits paid by the same + individual were paid to the same species in each of the eight cases. + Lastly, as regards bumble-bees, there were altogether observed 55 + individuals belonging to four species. These paid altogether 1751 + visits to 94 species of flowers. Of these 1751 visits, 1605 were + paid to one species, 131 to two species, 16 to three, 6 to four, and + 1 to five. Adding all these results together, we find that 75 + insects (butterflies and bees) visited 117 species of flowers: of + these visits, 1957 were constant to one species of flower; 136 were + paid also to a second species, 16 also to a third, 6 also to a + fourth, and 1 also to a fifth. Or, otherwise stated, while 1957 were + absolutely constant, from such absolute constancy there were only + 159 deviations. Moreover, if we eliminate three individual humble + bees, which paid nearly an equal number of visits to two species + (and, therefore, would have ministered to the work of physiological + selection almost as well as the others), the 159 deviations become + reduced to 72, or about four per cent. of the whole.--G. J. R. + + I send you the formula and the calculation on which it is based in + an Appendix; but as I know you have a holy horror of algebraical + formulae, I give you here a few numerical results. + + The cases I have worked out are those in which the number of + insects visiting each flower is 5, or 10, or 15; and I have also + taken 5, 10, and 15, to represent the number of flowers which an + insect visits each journey. This makes nine cases in all; and I + have applied these to two instances--viz. one in which one-fifth of + the whole race have developed cross-infertility, and the other in + which one-tenth only have done so. Taking first the instance where + one-fifth have developed the peculiarity, I find that if on the + average five insects visit a flower, and each insect on the average + visits five flowers on a journey, the fertility is diminished by + about one-tenth. If, however, the average number of flowers the + insect visits is ten, the reduction of fertility is less than one + per cent. And it becomes inappreciable if the average number is + fifteen. If on the average ten insects visit each flower, then, if + each insect visits on the average five flowers on a journey, the + reduction of fertility is a little over one per cent.; but if it + visits ten or fifteen the reduction is inappreciable. If fifteen + insects visit the flower on an average, then, if these insects on + the average visit five or more flowers on a journey, the reduction + of fertility is inappreciable. + + By the term inappreciable I mean that it is not substantially + greater than one-tenth of one per cent.--i.e. not more than + one-thousandth. + + Of course, if the proportion of individuals acquiring the + peculiarity is less, the effect on the fertility under the above + hypothesis will be greater; and it will not be counteracted so + fully unless the number of insect visits is larger, or unless the + insects visit more flowers on a journey. Thus if only one-tenth of + the race have developed the peculiarity, then, if each flower is + visited on the average by five insects who visit five flowers on + each trip, the fertility will be reduced about one-third. If, + however, the insects visit on the average ten flowers per trip, it + will be only diminished about one-tenth; and if they visit fifteen + on each trip, it will be only diminished about one-fortieth. If in + the same case we suppose that each flower receives ten insect + visits, then, if the insects visit on an average five flowers per + trip, the fertility will be diminished about one-eighth. If they + visit ten on a trip, it will be diminished about one-hundredth, and + the diminution is inappreciable if they visit fifteen on a trip. + Similarly, if a flower receives fifteen insect visits, the + diminution is about one-twenty-fifth, if insects visit on the + average five flowers on a trip; and is inappreciable if they visit + ten or fifteen. + + These figures will show you that it is exceedingly possible that a + peculiarity like this, the effect of which at first sight would + seem to be so prejudicial to fertility, may in fact have little or + no influence upon it; and if you set against this the overwhelming + importance of such a peculiarity in segregating the type so as to + give it a chance of becoming a fixed species, you will, I think, + feel that your hypothesis has nothing to fear from a numerical + examination. + + I have not examined the case of fertilization by other means; nor + have I examined the case of fertilization in animals, where + psychological selection can come in. To obtain any useful results, + one would have to consider very carefully the circumstances of each + case; and at present, at all events, I do not think it would be + useful to do so. Nor have I attempted to show the converse of the + problem--viz. the effect of swamping where cross-fertilization is + possible. I shall be very glad to examine any one of these cases if + you want me to do so; but I should prefer to leave it until I hear + from you again. + + If you contrast the results that I have given above with those + given on pages 181 to 183 of Wallace's book, you will see the + enormous difference. His calculations can only apply to the animal + kingdom in those cases in which there is only a union between one + individual of each sex; and before you can deal with the question + of such animals, you will have to take into consideration many + elements besides that of mere fertility, if you wish to get any + tolerably accurate result[66]. + + [66] Here follows the Appendix presenting the calculations on which + the above results are founded; but it seems unnecessary to reproduce + it on the present occasion.--G. J. R. + +The above analysis leaves nothing to be added by me. But, in conclusion, +I may once more repeat that the particular point with which it is +concerned is a point of very subordinate importance. For even if Mr. +Wallace's computation of chances had been found by Mr. Moulton to have +been an adequate computation--and, therefore, even if it had been thus +proved that physiological homogamy must always be associated with some +other form of homogamy in order to produce specific divergence--still +the importance of selective fertility as a factor of organic evolution +would not have been at all diminished. For such a result would merely +have shown that, not only "in many cases" (as I originally said), but +actually in all cases, the selective fertility which I hold to have been +so generally concerned in the differentiation of species has required +for this purpose the co-operation of some among the numerous other forms +of homogamy. But inasmuch as, by hypothesis, no one of these other or +co-operating factors would of itself have been capable of effecting +specific divergence in any of the cases where its association with +selective fertility is concerned, the mathematical proof that such an +association is _always_--and not merely _often_--necessary, would not +have materially affected the theory of the origin of species by means of +physiological selection. We have now seen, however, that a competent +mathematical treatment proves the exact opposite; and, therefore, that +Mr. Wallace's criticism fails even as regards the very subordinate point +in question. + + + + +APPENDIX C. + +SOME EXTRACTS FROM THE AUTHOR'S NOTE-BOOKS. + + +_Bearing of Weismannism on Physiological Selection._--If in view of +other considerations I could fully accept Professor Weismann's theory of +heredity, it would appear to me in no small measure to strengthen my own +theory of physiological selection. For Weismann's theory supposes that +all changes of specific type must have their origin in variations of a +continuous germ-plasm. But _the more the origin of species is referred +directly to variations arising in the sexual elements, the greater is +the play given to the principles of physiological selection_[67]; while, +on the other hand, the less standing-ground is furnished to the theory +that cross-infertility between allied species is due to "external +conditions of life," "prolonged exposure to uniform change of +conditions," "structural modifications re-acting on the sexual +functions"; or, in short, that "somatogenetic" changes of any kind can +of themselves induce the "blastogenetic" change of cross-infertility +between progeny of the same parental stock. + + [67] _Doctrine of Descent and Darwinism_, Eng. trans. p. 139. + + +_Cross-infertility and Diversity of Life._--Observe that one great +consequence of duly recognizing the importance of intercrossing is +indefinitely to raise our estimate of the part played by the principle +of cross-infertility in diversifying organic nature. For whenever in any +line of descent the bar of sterility arises, there the condition is +given for a new crop of departures (species of a genus); and when genera +are formed by the occurrence of this bar, there natural selection and +all other equilibrating causes are supplied with new material for +carrying on adaptational changes in new directions. Thus, owing to +cross-infertility, all these causes are enabled to work out numberless +adaptations in many directions (i. e. lines of descent) simultaneously. + + +_Cross-infertility and Stability._--The importance of sterility as a +diagnostic feature is obvious if we consider that more than any other +feature it serves to give _stability_ to the type; and unless a type is +stable or constant, it cannot be ranked as a species. That Darwin +himself attributes the highest importance to this feature as diagnostic, +see _Forms of Flowers_, pp. 58, 64. + + +_Cross-infertility and Specific Differentiation._--In their elaborate +work on the many species of the genus Hieracium, Nägeli and Peter are +led to the general conclusion that the best defined species are always +those which display absolute sterility _inter se_; while the species +which present most difficulty to the systematist are always those which +most easily hybridize. Moreover, they find, as another general rule +applicable to the whole genus, that there is a constant correlation +between inability to hybridize and absence of intermediate varieties, +and, conversely, between ability to hybridize and the presence of such +varieties. + + +_Cross-infertility in Domesticated Cattle._--Mr. J. W. Crompton, who has +had a large experience as a professional cattle-breeder, writes to me +(March 2, 1887)-- + + "That form of barrenness, very common in some districts, which + makes heifers become what are called 'bullers'--that is, + irregularly in 'season,' wild, and failing to conceive--is + certainly produced by excess of iron in their drinking-water, and I + suspect also by a deficiency of potash in the soil." + +He also informs me that pure white beasts of either sex are so well +known by experienced breeders to be comparatively infertile together, +that they are never used for breeding purposes, so that "in some parts +of the country, where a tendency to sterility had become so confirmed in +the white race that they utterly died out," only the coloured breeds are +now to be found. He goes on to say that if "a lot of white heifers were +put to a lot of white bulls, I think you would probably get a fertile +breed of pure white cattle.... I think, in short, that domestication has +produced just what your theory suggests, a new variety inclined to prove +sterile with its parent stock." + +Commenting on the origin of domesticated cattle, Professor Oscar Schmidt +remarks (_Doctrine of Descent_, p. 139)-- + + "Rütimeyer's minute researches on domestic cattle have shown that, + in Europe at least, three well-defined species of the diluvial + period have contributed to their formation--_Bos primigenius_, + _longifrons_, and _frontosus_. These species once lived + geographically separate, but contemporaneously; and they and their + specific peculiarities have perished, to rise again in our domestic + races. These races breed together with unqualified fertility. In + the form of skull and horns they recall one or other of the extinct + species; but collectively they constitute a new main species. That + from their various breeds, the three or any one of the aboriginal + species would ever emerge in a state of pristine purity, would be + an utterly ludicrous assertion." + +Now, seeing that these "aboriginal species," although living +"contemporaneously," were "geographically separate," we can well +understand that their divergence of type from a common ancestor did not +require, as a condition to their divergence, that any cross-sterility +should have arisen between them. The geographical isolation was enough +to secure immunity from mutual intercrossing, and therefore, as our +present theory would have expected as probable, morphological divergence +occurred without any corresponding physiological divergence, as must +almost certainly have been the case if such polytypic evolution had +occurred on a common area. Indeed, one of the two lines of experimental +verification of our theory consists in selecting cases where nearly +allied species are separated by geographical barriers, and proving that, +in such cases, there is no cross-sterility. + + +_Fertility of Domesticated Varieties._--Some writers have sought to +explain the contrast between domesticated varieties and natural species +in respect of fertility when crossed, by the consideration that it is +only those natural species which have proved themselves so far flexible +as to continue fertile under changed conditions of life that can have +ever allowed themselves to become domesticated. But although this +condition may well serve to explain the unimpaired fertility under +domestication of such species as for this very reason have ever become +domesticated, I fail to see how it explains the further and altogether +different fact, that this fertility continues unimpaired between all the +newly differentiated morphological types which have been derived from +the original specific type. It is one thing that this type should +continue fertile after domestication: it is quite another thing that +fertility should continue as between all its modified descendants, even +although the amount of modification may extend much further than that +which usually obtains between different natural species. + + +_Testing for Cross-infertility_ among varieties growing on the same area +is a much more crucial line of verification than testing for unimpaired +fertility between allied species which occupy different areas, because +while in the former case we are dealing with "incipient species" with a +view to ascertaining whether the divergence which they have already +undergone is accompanied by physiological isolation, in the latter case +we can never be sure that two allied species, which are now widely +disconnected geographically, have always been so disconnected. They may +both have originated on the same area; or one may have diverged from the +other before it migrated from that area; or even if, when it migrated, +it was unchanged, and if in its new home it afterwards split into two +species by physiological selection, the newer species would probably +prove infertile, not only with its parent type, but also with its +grand-parent in any other part of the world. + + +_Seebohm on Isolation._--Seebohm is so strongly influenced by the +difficulty from "the swamping effects of free intercrossing," that he is +driven by it to adopt Asa Gray's hypothesis of variations as +teleological. Indeed, he goes as far as Wagner, for he maintains that in +no case can there be divergence or multiplication of species without +isolation. He makes the important statement that "the more the +geographical distribution of birds is studied, the more doubtful it +seems to be that any species of bird has ever been differentiated +without the aid of geographical isolation" (_Charadriidae_, p. 17). If +this is true, it makes in favour of physiological selection by showing +the paramount importance of the swamping effects of intercrossing, and +consequent importance of isolation. But it makes against physiological +selection by showing that the geographical form of isolation is +sufficient to explain all the cases of specific differentiation in +birds. But I must remember that the latter point rests largely on +negative inference, and that birds, owing to their highly locomotive +habits, are the class of animals where physiological selection is likely +to be most handicapped. + + +_Herbert on Hybridization._--Herbert tells us that when he first +astonished the Horticultural Society by laying before them the results +of his experiments on hybridization, his brother botanists took serious +alarm. For it appeared to them that this "intermixture of species would +confuse the labours of botanists, and force them to work their way +through a wilderness of uncertainty." Therefore he was bluntly told by +several of these gentlemen, "I do not thank you for your mules." Now, +although naturalists have travelled far and learnt much since those +days, it appears to me that a modern evolutionist might still turn to +the horticulturist with the same words. For assuredly he has no reason +to thank the horticulturist for his mules, until he has found a +satisfactory answer to the question why it is that natural species +differ so profoundly as regards their capacity for hybridizing. + + +_Advance on Herbert's Position._--- If it be said that all my work +amounts to showing what Herbert said long ago--viz. that the only true +or natural distinction between organic types is the sexual +distinction--I answer that my work does much more than this. For it +shows that the principle of sterility is the main condition to the +differentiation, not merely of species and genera, but also to the +evolution of adaptations everywhere, in higher as well as in lower +taxonomic divisions. Moreover, even though naturalists were everywhere +to consent to abandon specific designations, and, as Herbert advises, to +"entrench themselves behind genera," there would still remain the facts +of what are now called specific differences (of the secondary or +morphological kind), and by whatever name these are called, they alike +demand explanation at the hands of the evolutionist. + + +_Fritz Müller on Cross-infertility._--Fritz Müller writes, "Every plant +requires, for the production of the strongest possible and most prolific +progeny, a certain amount of difference between male and female elements +which unite. Fertility is diminished as well when this degree is too low +(in relatives too closely allied) as when it is too high (in those too +little related)." Then he adds, as a general rule, "Species which are +wholly sterile with pollen of the same stock, and even with pollen of +nearly allied stocks, will generally be fertilized very readily by the +pollen of another species. The self-sterile species of the genus +Abutilon, which are, on the other hand, so much inclined to +hybridization, afford a good example of this theory, which appears to be +confirmed also by Lobelia, Passiflora, and Oncidium" (_American +Naturalist_, vol. viii, pp. 223-4, 1874). + + +_Different groups of plants exhibit remarkable differences in the +capability of their constituent species to hybridize._--In so far as +these differences have reference only to first crosses, they have no +bearing either for or against my theory. Only in so far as the +differences extend to the production of fertile hybrids does any +question arise for me. First of all, therefore, I must ascertain whether +(or how far) there is any correlation between groups whose species +manifest aptitude to form first crosses, and groups where first crosses +manifest aptitude to produce fertile hybrids. Next, whatever the result +of this inquiry should be, if I find that certain natural groups of +plants exhibit comparatively well-marked tendencies to form fertile +hybrids, the question will arise, Are these tendencies correlated with +_paucity_ of species? If they are, the fact would make strongly in +favour of physiological selection. For the fact would mean that in these +natural groups, owing to "the nature of the organisms" included under +them, less opportunity is given to physiological selection in its work +of differentiating specific types than is given by other natural groups +where the nature of the organism renders them more prone to mutual +sterility. But in prosecuting this branch of verification, I must +remember to allow for possibilities of differential degrees of +geographical isolation in the different groups compared. + +On this subject Focke writes me as follows:--"In a natural group +(family, order, genus) showing considerable variability in the structure +of the flower, we may expect to find [or do find] a greater number of +mules than in a group whose species are only distinguished by +differences in the shape of the leaves, or in growth, &c. I do not +know, however, which in this connexion of things is the cause and which +the effect. A useful ancestral structure of the flower may be conserved +by an otherwise varying progeny, on condition that the progress of +diversity be not disturbed by frequent intercrossings. [Therefore, if +this condition be satisfied, the structure of the flower in different +members of the group will continue constant: here the cause of +_constancy_ in the flower (however much variability there may be in the +leaves, &c.) is its original _inability_ to hybridize.] On the other +hand, in species or groups ready to hybridize [or capable of +hybridizing], the fixation of a new specific type will require some +change in the structure of the flower, and a change considerable enough +to alter the conditions of fertilization. [Here the reason of the +_in_constancy of the flower in different members of the group is the +original _aptitude_ of their ancestral forms to hybridize.] Perhaps +there is something in this suggestion, but certainly there are other +efficient physiological relations, which are at present unknown. Your +theory of physiological selection may serve to explain many difficult +facts." + + +_The Importance of Prepotency._--A. Kerner shows by means of his own +observations on sundry species of plants which hybridize in the wild +state, that they do so very much more frequently if both, or even if +only one of the parent forms be rare in the neighbourhood. This fact can +only be explained by supposing that, even in species most prone to +hybridizing under Nature, there is some degree of prepotency of pollen +of the same species over that of the other species; so that where both +species are common, it is correspondingly rare that the foreign pollen +gets a chance. But if there were no prepotency, the two species would +blend; and this Kerner supposes must actually take place wherever two +previously separated species, thus physiologically circumstanced, happen +to be brought together. (Kerner's paper is published in _Oester. Bot. +Zeitschrift_, XXI, 1871, where he alludes to sundry other papers of his +own advocating similar views.) + +The relation of these observations to Jordan's _espèces affines_ is +obvious. We have only to suppose that some such slight and constant +difference characterizes the sexual elements of these allied varieties +as demonstrably characterizes their morphology, and we can understand +how pollen-prepotency would keep the forms distinct--such forms, +therefore, being so many records of such prepotency. + +Both from Kerner's work, and still more from that of Jordan and Nägeli, +I conclude that (at all events in plants) prepotency is the way in which +physiological selection chiefly acts. That is to say, _sudden_ and +_extreme_ variations in the way of sexual incompatibility are probably +rare, as compared with some degree of prepotency. According as this +degree is small or great so will be the amount of the corresponding +separation. This view would show that in plants the principle of +physiological selection is one of immensely widespread influence, +causing (on the same areas) more or less permanent varieties much below +specific rank. And when we remember on how delicate a balance of +physiological conditions complete correspondency of pollen to ovules +depends, we may be prepared to expect that the phenomenon of prepotency +is not of uncommon occurrence. + + +_Self-fertilization and Variability._--It occurred to Count Berg Sagnitz +that, if physiological selection is a true principle in nature, +vegetable species in which self-fertilization obtains ought to be more +rich in constant varieties than are species in which cross-fertilization +rules. For, although even in the latter case physiological isolation may +occasionally arise, it cannot be of such habitual or constant occurrence +as it must be in the former case. Acting on this idea, Count Berg +Sagnitz applied himself to ascertain whether there is any general +correlation between the habit of self-fertilization and the fact of +high variability; and he says that in all the cases which he has +hitherto investigated, the correlation in question is unmistakable. + + +_Additional Hypothesis concerning Physiological Selection._--In +reciprocal crosses _A_ × _B_ is often more fertile than _B_ × _A_. If +hybrid _AB_ is more fertile with _A_, and hybrid _BA_ with _B_, than +vice versa, there would be given a good analogy on which to found the +following hypothesis. + +Let _A_ and _B_ be two intergenerating groups in which segregate +fecundity is first beginning. Of the hybrids, _AB_ will be more fertile +with _A_, and _BA_ with _B_, than vice versa. The interbreeding of _AB_ +with _A_ will eventually modify sexual characters of _A_ by assimilating +it to those of _AB_, while the interbreeding of _BA_ with _B_ will +similarly modify sexual characters of _B_ by assimilating it to those of +_BA_. Consequently, _A_ will become more and more infertile with _B_, +while _B_ becomes more and more infertile with _A_. Fewer and fewer +hybrids will thus be produced till mutual sterility is complete. + +To sustain this hypothesis it would be needful to prove experimentally, +(1) that hybrid forms _AB_ are more fertile with _A_ than with _B_, +while hybrid forms _BA_ are more fertile with _B_ than with _A_ [or, it +may be possible that the opposite relations would be found to obtain, +viz. that _AB_ would be more fertile with _B_, and _BA_ with _A_]; (2) +that, if so, effect of intercrossing _AB_ with _A_ is to make progeny +more fertile with _A_ than with _B_, while effect of intercrossing _BA_ +with _B_ is to make progeny more fertile with _B_ than with _A_. + +Such experiments had best be tried with species where there is already +known to be a difference of fertility between reciprocal crosses (e.g. +Matthiola annua and M. glabra, see _Origin of Species_, p. 244). + + + + +INDEX + + +A. + +ALLEN, Mr. J. A., on variation under nature, 34. + +Amixia, 12-28, 110-115, 117-133. + +Apogamy, 5, 6, 10, 18, 28. + + +B. + +BELT, on physiological selection, 44. + +BERG SAGNITZ, Count, on self-fertilization and variability, 177. + +Breeding, separate and segregate, 5. + +Butterflies of polar regions and Alps, 133. + + +C. + +CATCHPOOL, Mr., on physiological selection, 44, 137. + +Cross-infertility, 46; + and varietal divergence, 82; + and diversity of life, 169; + and stability, 170; + and specific differentiation, 170; + in domesticated cattle, 170; + testing for, 172; + Fritz Müller on, 174. + + +D. + +_Darwin_, Charles, on isolation, 2, 106; + on diversity under nature, 31; + on the fertility of varieties, 50; + on the origin of cross-infertility, 51; + on distribution, 68; + on prepotency, 89; + on geographical isolation, 101, 108; + on methodical selection, 102; + on modification in large areas, 103; + on the swamping effects of intercrossing, 105; + on independent variability, 109; + on domestic animals, 110. + +DELBÅ’UF, law of independent variability, 13. + +Differentiation under natural selection, 37. + +Diversity of life and cross-infertility, 169. + +Domesticated cattle and cross-infertility, 170, 172. + + +E. + +Evidences of physiological selection, 62. + +Evolution, monotypic and polytypic, 21, 75, 102, 107, 112, 129. + +Experimental research in physiological selection, 85. + + +F. + +Fertility of domesticated varieties, 172. + +FOCKE, Herr, on hybridization, 175. + + +G. + +GALTON, Mr. Francis, law of regression, 39. + +General conclusions, 144. + +Geographical distribution and physiological selection, 65. + +GIARD, M., on apogamy, 14. + +GRABHAM, Dr., on mollusca of Madeira, 135. + +GULICK, Rev. J., on natural Selection as a mode of isolation, 9; + on divergence, 11; + on segregate breeding, 19; + on geographical distribution, 27; + on the prevention of intercrossing, 127; + on Mr. Wallace's criticisms, 151. + + +H. + +HERBERT, on hybridization, 173; + advance on his position, 174. + +HERDMAN, Prof., on physiological isolation, 123. + +Historical sketch of opinions on isolation, 101. + +Homogamy, 5, 6; + forms of, 7, 19, 29. + +Hybridization, HERBERT on, 173; + in plants, 175. + +Hypothesis, additional, concerning physiological selection, 178. + + +I. + +Independent variability, 12-29. + +Isolation, defined, 2; + forms of, 3, 6; + geographical, 3; + discriminate and indiscriminate, 5; + physiological, 9, 41, 58; + its importance, 39; + sketch of opinions on, 101; + general conclusions, 144; + SEEBOHM on, 173. + + +J. + +JORDAN, M., on cross sterile varieties of plants, 86; + his researches summarized, 87. + + +K. + +KERNER, Prof. A., on prepotency, 176. + + +L. + +LANKESTER, Prof. Ray, on divergent evolution, 15. + +LE CONTE, Prof., on fossil snails of steinheim, 95; + on isolation, 129. + +LIVINGSTONE, Dr. David, Quoted, 123. + + +M. + +MELDOLA, Prof., on difficulty from intercrossing, 121. + +Misunderstandings of Physiological selection, 59. + +Monotypic evolution, see Evolution. + +MORGAN, Prof. Lloyd, on sterility, 56; + on isolation, 128. + +MOULTON, Mr. Fletcher, an examination of Mr. Wallace's calculations on +physiological selection, 157. + +MÃœLLER, Fritz, on cross-infertility, 174. + + +N. + +NÄGELI, on isolation, 76; + on synoicy, 78, 82. + +Natural selection, a form of discriminate isolation, 9, 10, 23; + leads to monotypic evolution, 24-29; + difficulties of, 41, 51. + + +P. + +Panmixia, 12. + +Physiological selection, 9, 41; + summarized, 58; + misunderstandings of, 59; + evidences of, 81-119; + and Weismannism, 169; + additional hypothesis, 178. + +Polytypic evolution, see Evolution. + +Prepotency, 89; importance of, 176. + + +S. + +SCHMIDT, Prof. Oscar, on domesticated cattle, 171. + +SEEBOHM on isolation, 173. + +Segregation, 28. + +Selection, physiological, see Physiological selection. + +Self-fertilization and variability, 177. + +Snails of Sandwich Islands, 16, 130; + fossil of Steinheim, 95. + +Specific differentiation and cross-infertility, 170. + +Stability and cross-infertility, 170. + +Synoicy, 78. + + +T. + +Topographical distribution and physiological selection, 74; + of varieties, 81. + +Transformation, serial and divergent, 21, 121. + + +V. + +Variability and self-fertilization, 177. + +Variation in birds, 34. + +Varieties, topographical distribution of, 81. + + +W. + +WAGNER, Maritz, 3; + on geographical isolation, 76; + quoted, 103; + law of migration, 111. + +WALLACE, Mr. A. R., 3, 17; + quoted, 34, 47, 51, 57,130-136; + criticized by Gulick, 152. + +WEISMANN, Prof., on geographical isolation, 76, 114-118. + +Weismannism and physiological selection, 169. + + + + +TITLE LIST OF OPEN COURT PUBLICATIONS ARRANGED ALPHABETICALLY BY AUTHORS + + +ANESAKI, M. + +345. BUDDHIST AND CHRISTIAN GOSPELS, Being Gospel Parallels from Pali +Texts. Now first compared from the originals by Albert J. Edmunds. +Edited with parallels and notes from the Chinese Buddhist Triptaka by +_M. Anesaki._ $1.50 net. + + +BAYNE, JULIA TAFT. + +323. HADLEY BALLADS. _Julia Taft Bayne._ 75c net. + + +BERKELEY, GEORGE. + +307. A TREATISE CONCERNING THE PRINCIPLES OF HUMAN KNOWLEDGE. _George +Berkeley._ Cloth, 60c net. (3s. net.) + +308. THREE DIALOGUES BETWEEN HYLAS AND PHILONOUS. _George Berkeley._ +Cloth, 60c net. (3s. net.) + + +BINET, ALFRED. + +201. THE PSYCHIC LIFE OF MICRO-ORGANISMS. _Alfred Binet._ 75c. (3s. 6d.) + +270. 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Thus, we do not necessarily +keep eBooks in compliance with any particular paper edition. + + +Most people start at our Web site which has the main PG search facility: + + https://www.gutenberg.org + +This Web site includes information about Project Gutenberg-tm, +including how to make donations to the Project Gutenberg Literary +Archive Foundation, how to help produce our new eBooks, and how to +subscribe to our email newsletter to hear about new eBooks. diff --git a/37777-0.zip b/37777-0.zip Binary files differnew file mode 100644 index 0000000..c3bcfb7 --- /dev/null +++ b/37777-0.zip diff --git a/37777-8.txt b/37777-8.txt new file mode 100644 index 0000000..52e58c0 --- /dev/null +++ b/37777-8.txt @@ -0,0 +1,6851 @@ +The Project Gutenberg EBook of Darwin, and After Darwin (Vol 3 of 3), by +George John Romanes + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Darwin, and After Darwin (Vol 3 of 3) + Post-Darwinian Questions: Isolation and Physiological Selection + +Author: George John Romanes + +Release Date: October 17, 2011 [EBook #37777] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK DARWIN, AFTER DARWIN (VOL 3 OF 3) *** + + + + +Produced by Marilynda Fraser-Cunliffe, LN Yaddanapudi and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + + + + + + + + +DARWIN, AND AFTER DARWIN + +III + +POST-DARWINIAN QUESTIONS + +ISOLATION AND PHYSIOLOGICAL SELECTION + + +BY THE SAME AUTHOR. + +DARWIN, AND AFTER DARWIN. An Exposition of the Darwinian Theory and a +Discussion of Post-Darwinian Questions. + + 1. THE DARWINIAN THEORY. With portrait of Darwin. 460 pages. 125 + illustrations. Cloth, $2.00. + + 2. POST-DARWINIAN QUESTIONS. Edited by Prof. C. Lloyd Morgan. With + portrait of G. J. Romanes. 338 pages. Cloth, $1.50. + + 3. POST-DARWINIAN QUESTIONS. ISOLATION AND PHYSIOLOGICAL SELECTION. + Edited by Prof. C. Lloyd Morgan. With portrait of Mr. J. T. + Gulick. 181 pages. Cloth, $1.00. + + All three volumes together, $4.00 net. + +AN EXAMINATION OF WEISMANNISM. With portrait of Weismann. 236 pages. +Cloth, $1.00. + +THOUGHTS ON RELIGION. Edited by Charles Gore, M.A., Canon of +Westminster. Third Edition. 184 pages. Cloth, gilt top, $1.25. + +THE OPEN COURT PUBLISHING COMPANY, +324 DEARBORN STREET, CHICAGO. + +[Illustration: Frontispiece--John J. Gulick] + + + + +DARWIN, AND AFTER DARWIN + +_AN EXPOSITION OF THE DARWINIAN THEORY +AND A DISCUSSION OF +POST-DARWINIAN QUESTIONS_ + +BY THE LATE + +GEORGE JOHN ROMANES, M.A., LL.D., F.R.S. + +_Honorary Fellow of Gonville and Caius College, Cambridge_ + + +III + +POST-DARWINIAN QUESTIONS + +ISOLATION +AND PHYSIOLOGICAL SELECTION + + +Chicago +THE OPEN COURT PUBLISHING COMPANY +1906 + +CHAPTER 1. COPYRIGHTED BY +THE OPEN COURT PUBLISHING CO. +1897. + + + +The Lakeside Press +R. R. DONNELLEY & SONS CO., CHICAGO + + + + +PREFACE + + +Of the six chapters which constitute this concluding volume of G. J. +Romanes' _Darwin, and after Darwin_, three, the first two and the last, +were in type at the time of his death. I have not considered myself at +liberty to make any alterations of moment in these chapters. For the +selection and arrangement of all that is contained in the other three +chapters I am wholly responsible. + +Two long controversial Appendices have been omitted. Those marked A and +B remain in accordance with the author's expressed injunctions. In a +third, marked C, a few passages from the author's note-books or MSS. +have been printed. + +The portrait of the Rev. J. Gulick, which forms the frontispiece, was +prepared for this volume before the author's death. Mr. Gulick's chief +contributions to the theory of physiological selection are to be found +in the Linnean Society's _Journal_ (_Zoology_, vols. xx and xxiii), and +in four letters to _Nature_ (vol. xli. p. 536; vol. xlii. pp. 28 and +369; and vol. xliv. p. 29). + +I have to thank Mr. Francis Galton, D.C.L., F.R.S. and Mr. F. Howard +Collins for valuable assistance generously rendered for the sake of one +whom all who knew him held dear. For he was, if I may echo the words of +Huxley, "a friend endeared to me, as to so many others, by his kindly +nature, and justly valued by all his colleagues for his powers of +investigation and his zeal for the advancement of science." + +C. LLOYD MORGAN. + +BRISTOL, _May 1897_. + + + + +CONTENTS + + +CHAPTER I. +ISOLATION 1 + +CHAPTER II. +ISOLATION (_continued_) 28 + +CHAPTER III. +PHYSIOLOGICAL SELECTION 41 + +CHAPTER IV. +EVIDENCES OF PHYSIOLOGICAL SELECTION 62 + +CHAPTER V. +FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION 81 + +CHAPTER VI. +A BRIEF HISTORY OF ISOLATION AS A FACTOR IN +ORGANIC EVOLUTION 101 + +GENERAL CONCLUSIONS 144 + +APPENDIX A. MR. GULICK'S CRITICISM OF MR. WALLACE'S +VIEWS ON PHYSIOLOGICAL SELECTION 151 + +APPENDIX B. AN EXAMINATION BY MR. FLETCHER +MOULTON OF MR. WALLACE'S CALCULATION TOUCHING +THE POSSIBILITY OF PHYSIOLOGICAL SELECTION +EVER ACTING ALONE 157 + +APPENDIX C. SOME EXTRACTS FROM THE AUTHOR'S +NOTE-BOOKS 169 + + + + +_ISOLATION_ + + + + +CHAPTER I. + +ISOLATION. + + +This treatise will now draw to a close by considering what, in my +opinion, is one of the most important principles that are concerned in +the process of organic evolution--namely, Isolation. I say in _my_ +opinion such is the case, because, although the importance of isolation +is more or less recognized by every naturalist, I know of only one other +who has perceived all that the principle involves. This naturalist is +the Rev. J. Gulick, and to his essays on the subject I attribute a +higher value than to any other work in the field of Darwinian thought +since the date of Darwin's death[1]. For it is now my matured conviction +that a new point of departure has here been taken in the philosophy of +Darwinism, and one which opens up new territories for scientific +exploration of an endlessly wide and varied character. Indeed I believe, +with Mr. Gulick, that in the principle of Isolation we have a principle +so fundamental and so universal, that even the great principle of +Natural Selection lies less deep, and pervades a region of smaller +extent. Equalled only in its importance by the two basal principles of +Heredity and Variation, this principle of Isolation constitutes the +third pillar of a tripod on which is reared the whole superstructure of +organic evolution. + + [1] It will be remembered that I regard Weismann's theory of + heredity, with all its deductive consequences, as still _sub + judice_. + +By isolation I mean simply the prevention of intercrossing between a +separated section of a species or kind and the rest of that species or +kind. Whether such a separation be due to geographical barriers, to +migration, or to any other state of matters leading to exclusive +breeding within the separated group, I shall indifferently employ the +term isolation for the purpose of designating what in all cases is the +same result--namely, a prevention of intercrossing between A and B, +where A is the separated portion and B the rest of the species or kind. + +The importance of isolation as against dissimilar forms has always been +fully appreciated by breeders, fanciers, horticulturists, &c., who are +therefore most careful to prevent their pedigree productions from +intercrossing with any other stock. Isolation is indeed, as Darwin has +observed, "the corner-stone of the breeder's art." And similarly with +plants and animals in a state of nature: unless intercrossing with +allied (i.e. dissimilar) forms is prevented, the principle of heredity +is bound to work for uniformity, by blending the dissimilar types in +one: only when there is exclusive breeding of similarly modified forms +can the principle of heredity work in the direction of change--i.e. of +evolution. + +Now, the forms of isolation--or the conditions which may lead to +exclusive breeding--are manifold. One of the most important, as well as +the most obvious, is geographical isolation; and no one questions that +this has been an important factor in the process of evolution, although +opinions still vary greatly as to the degree of its importance in this +respect. At one end of the series we may place the opinion of Mr. +Wallace, who denies that any of what may be termed the evolutionary +effect of geographical isolation is due to "influence exerted by +isolation _per se_." This effect, he says, is to be ascribed exclusively +to the fact that a geographically isolated portion of a species must +always encounter a change of environment, and therefore a new set of +conditions necessitating a new set of adaptations at the hands of +natural selection[2]. At the other end of the series we must place the +opinion of Moritz Wagner, who many years ago published a masterly +essay[3], the object of which was to prove that, in the absence of +geographical isolation (including migration), natural selection would be +powerless to effect any change of specific type. For, he argued, the +initial variations on which the action of this principle depends would +otherwise be inevitably swamped by free intercrossing. Wagner adduced a +large number of interesting facts in support of this opinion; but +although he thus succeeded in enforcing the truth that geographical +isolation is an important aid to organic evolution, he failed to +establish his conclusion that it is an indispensable condition. +Nevertheless he may have been right--and, as I shall presently show, I +believe he was right--in his fundamental premiss, that in the presence +of free intercrossing natural selection would be powerless to effect +divergent evolution. Where he went wrong was in not perceiving that +geographical isolation is not the only form of isolation. Had it +occurred to him that there may be other forms quite as effectual for the +prevention of free intercrossing, his essay could hardly have failed to +mark an epoch in the history of Darwinism. But, on account of this +oversight, he really weakened his main contention, namely, that in the +presence of free intercrossing natural selection must be powerless to +effect divergent evolution. This main contention I am now about to +re-argue. At present, therefore, we have only to observe that Wagner did +it much more harm than good by neglecting to perceive that free +intercrossing may be prevented in many other ways besides by migration, +and by the intervention of geographical barriers. + + [2] _Darwinism_, p. 150. + + [3] _The Darwinian Theory, and the Law of Migration_ (Eng. Trans., + Stanford, London, 1873). + +In order that we may set out with clearer views upon this matter, I will +make one or two preliminary remarks on the more general facts of +isolation as these are found to occur in nature. + +In the first place, it is obvious that isolation admits of degrees: it +may be either total or partial; and, if partial, may occur in numberless +grades of efficiency. This is so manifest that I need not wait to give +illustrations. But now, in the second place, there is another general +fact appertaining to isolation which is not so manifest, and a clear +appreciation of which is so essential to any adequate consideration of +the subject, that I believe the reason why evolutionists have hitherto +failed to perceive the full importance of isolation, is because they +have failed to perceive the distinction which has now to be pointed out. +The distinction is, that isolation may be either discriminate or +indiscriminate. If it be discriminate, the isolation has reference to +the resemblance of the separated individuals to one another; if it be +indiscriminate, it has no such reference. For example, if a shepherd +divides a flock of sheep without regard to their characters, he is +isolating one section from the other indiscriminately; but if he places +all the white sheep in one field, and all the black sheep in another +field, he is isolating one section from the other discriminately. Or, if +geological subsidence divides a species into two parts, the isolation +will be indiscriminate; but if the separation be due to one of the +sections developing, for example, a change of instinct determining +migration to another area, or occupation of a different habitat on the +same area, then the isolation will be discriminate, so far as the +resemblance of instinct is concerned. + +With the exception of Mr. Gulick, I cannot find that any other writer +has hitherto stated this supremely important distinction between +isolation as discriminate and indiscriminate. But he has fully as well +as independently stated it, and shown in a masterly way its far-reaching +consequences. Indiscriminate isolation he calls Separate Breeding, while +discriminate isolation he calls Segregate Breeding. For the sake, +however, of securing more descriptive terms, I will coin the words +Apogamy and Homogamy. Apogamy, of course, answers to indiscriminate +isolation, or separate breeding. Homogamy, on the other hand, answers +to discriminate isolation, or segregate breeding: only individuals +belonging to the same variety or kind are allowed to propagate. +Isolation, then, is a genus, of which Apogamy and Homogamy are +species[4]. + + [4] I may here most conveniently define the senses in which all the + following terms will be used throughout the present + discussion:--_Species_ of isolation are, as above stated, homogamy + and apogamy, or isolation as discriminate and indiscriminate. + _Forms_ of isolation are modes of isolation, such as the + geographical, the sexual, the instinctive, or any other of the + numerous means whereby isolation of either species may be secured. + _Cases_ of isolation are the instances in which any of the forms of + isolation may be at work: thus, if a group of _n_ intergenerants be + segregated into five groups, _a_, _b_, _c_, _d_, _e_, then, before + the segregation there would have been one case of isolation, but + after the segregation there would be five such cases. + +Now, in order to appreciate the unsurpassed importance of isolation as +one of the three basal principles of organic evolution, let us begin by +considering the discriminate species of it, or Homogamy. + +To state the case in the most general terms, we may say that if the +other two basal principles are given in heredity and variability, the +whole theory of organic evolution becomes neither more nor less than a +theory of homogamy--that is, a theory of the causes which lead to +discriminate isolation, or the breeding of like with like to the +exclusion of unlike. For the more we believe in heredity and variability +as basal principles of organic evolution, the stronger must become our +persuasion that discriminate breeding leads to divergence of type, while +indiscriminate breeding leads to uniformity. This, in fact, is securely +based on what we know from the experience supplied by artificial +selection, which consists in the intentional mating of like with like +to the exclusion of unlike. + +The point, then, which in the first instance must be firmly fastened in +our minds is this:--so long as there is free intercrossing, heredity +cancels variability, and makes in favour of fixity of type. Only when +assisted by some form of discriminate isolation, which determines the +exclusive breeding of like with like, can heredity make in favour of +change of type, or lead to what we understand by organic evolution. + +Now the forms of discriminate isolation, or homogamy, are very numerous. +When, for example, any section of a species adopts somewhat different +habits of life, or occupies a somewhat different station in the economy +of nature, homogamy arises within that section. There are forms of +homogamy on which Darwin has laid great stress, as we shall presently +find. Again, when for these or any other reasons a section of a species +becomes in any small degree modified as to form or colour, if the +species happens to be one where any psychological preference in pairing +can be exercised--as is very generally the case among the higher +animals--exclusive breeding is apt to ensue as a result of such +preference; for there is abundant evidence to show that, both in birds +and mammals, sexual selection is usually opposed to the intercrossing of +dissimilar varieties. Once more, in the case of plants, intercrossing of +dissimilar varieties may be prevented by any slight difference in their +seasons of flowering, of topographical stations, or even, in the case of +flowers which depend on insects for their fertilization, by differences +in the instincts and preferences of their visitors. + +But, without at present going into detail with regard to these different +forms of discriminate isolation, there are still two others, both of +which are of much greater importance than any that I have hitherto +named. Indeed, these two forms are of such immeasurable importance, that +were it not for their virtually ubiquitous operation, the process of +organic evolution could never have begun, nor, having begun, continued. + +The first of these two forms is sexual incompatibility--either partial +or absolute--between different taxonomic groups. If all hares and +rabbits, for example, were as fertile with one another as they are +within their own respective species, there can be no doubt that sooner +or later, and on common areas, the two types would fuse into one. And +similarly, if the bar of sterility could be thrown down as between all +the species of a genus, or all the genera of a family, _not otherwise +prevented from intercrossing_, in time all such species, or all such +genera, would become blended into a single type. As a matter of fact, +complete fertility, both of first crosses and of their resulting +hybrids, is rare, even as between species of the same genus; while as +between genera of the same family complete fertility does not appear +ever to occur; and, of course, the same applies to all the higher +taxonomic divisions. On the other hand, some degree of infertility is +not unusual as between different varieties of the same species; and, +wherever this is the case, it must clearly aid the further +differentiation of those varieties. It will be my endeavour to show that +in this latter connexion sexual incompatibility must be held to have +taken an immensely important part in the differentiation of varieties +into species. But meanwhile we have only to observe that _wherever_ such +incompatibility is concerned, it is to be regarded as an isolating +agency of the very first importance. And as it is of a character purely +physiological, I have assigned to it the name Physiological Isolation; +while for the particular case where this general principle is concerned +in the origination of specific types, I have reserved the name +Physiological Selection. + +The other most important form of discriminate isolation to which I have +alluded is Natural Selection. To some evolutionists it has seemed +paradoxical thus to regard natural selection as a form of isolation; but +a little thought will suffice to show that such is really the most +accurate way of regarding it. For, as Mr. Gulick says, "Natural +selection is the exclusive breeding of those better adapted to the +environment: ... it is a process in which the fittest are prevented from +crossing with the less fitted, by the exclusion of the less fitted." +Therefore it is, strictly and accurately, a mode of isolation, where the +isolation has reference to adaptation, and is secured in the most +effectual of possible ways--i.e. by the destruction of all individuals +whose intercrossing would interfere with the isolation. Indeed, the very +term "natural _selection_" shows that the principle is tacitly +understood to be one of isolation, because this name was assigned to the +principle by Darwin for the express purpose of marking the analogy that +obtains between it and the intentional isolation which is practised by +breeders, fanciers, and horticulturists. The only difference between +"natural selection" and "artificial selection" consists in this--that +under the former process the excluded individuals must necessarily +perish, while under the latter they need not do so. But clearly this +difference is accidental: it is in no way essential to the process +considered as a process of discriminate isolation. For, as far as +homogamous breeding is concerned, it can matter nothing whether the +exclusion of the dissimilar individuals is effected by separation or by +death. + +Natural selection, then, is thus unquestionably a form of isolation of +the discriminate kind; and therefore, notwithstanding its unique +importance in certain respects, considered as a principle of organic +evolution it is less fundamental--and also less extensive--than the +principle of isolation in general. In other words, it is but a part of a +much larger whole. It is but a particular form of a general principle, +which, as just shown, presents many other forms, not only of the +discriminate, but likewise of the indiscriminate kind. Or, reverting to +the terminology of logic, it is a sub-species of the species Homogamy, +which in its turn is but a constituent part of the genus Isolation. + +So much then for homogamy, or isolation of the discriminate order. +Passing on now to apogamy, or isolation of the indiscriminate kind, we +may well be disposed, at first sight, to conclude that this kind of +isolation can count for nothing in the process of evolution. For if the +fundamental importance of isolation in the production of organic forms +be due to its segregation of like with like, does it not follow that any +form of isolation which is indiscriminate must fail to supply the very +condition on which all the forms of discriminate isolation depend for +their efficacy in the causing of organic evolution? Or, to return to our +concrete example, is it not self-evident that the farmer who separated +his stock into two or more parts indiscriminately, would not effect any +more change in his stock than if he had left them all to breed together? + +Well, although at first sight this seems self-evident, it is in fact +untrue. For, unless the individuals which are indiscriminately isolated +happen to be a very large number, sooner or later their progeny will +come to differ from that of the parent type, or unisolated portion of +the previous stock. And, of course, as soon as this change of type +begins, the isolation ceases to be indiscriminate: the previous apogamy +has been converted into homogamy, with the usual result of causing a +divergence of type. The reason why progeny of an indiscriminately +isolated section of an originally uniform stock--e.g. of a species--will +eventually deviate from the original type is, to quote Mr. Gulick, as +follows:--"No two portions of a species possess exactly the same average +character, and, therefore, the initial differences are for ever reacting +on the environment and on each other in such a way as to ensure +increasing divergence as long as the individuals of the two groups are +kept from intergenerating[5]." Or, as I stated this principle in my +essay on _Physiological Selection_, published but a short time before +Mr. Gulick's invaluable contributions to these topics:-- + + [5] _Divergent Evolution through Cumulative Segregation_ (_Zool. + Journal, Linn. Soc._, vol. xx. pp. 189-274). + + As a matter of fact, we find that no one individual "is like + another all in all"; which is another way of saying that a specific + type may be regarded as the average mean of all its individual + variations, any considerable departure from this average being, + however, checked by intercrossing.... Consequently, if from any + cause a section of a species is prevented from intercrossing with + the rest of its species, we might expect that new varieties should + arise within that section, and that in time these varieties should + pass into new species. And this is just what we do find[6]. + + [6] The passage proceeds to show that in view of this consideration + we have a strong additional reason for rejecting the _a priori_ + dogma that all specific characters must necessarily be useful + characters. For it is evident that any divergence of specific + character which is brought about in this way need not present any + utilitarian significance--although, of course, natural selection + will ensure that it shall never be deleterious. + +The name which I gave to this cause of specific change was Independent +Variability, or variability in the absence of overwhelming +intercrossing. But it now appears to me that this cause is really +identical with that which was previously enunciated by Delboeuf. +Again, in his important essay on _The Influence of Isolation_, Weismann +concludes, on the basis of a large accumulation of facts, that the +constancy of any given specific type "does not arise suddenly, but +gradually, and is established by the promiscuous intercrossing of all +individuals." From which, he says, it follows, that this constancy must +cease so soon as the condition which maintains it ceases--i. e. so soon +as intercrossing (Panmixia) between all individuals ceases, or so soon +as a portion of a species is isolated from its parent stock. To this +principle he assigns the name of Amixia. But Weismann's Amixia differs +from my Independent Variability in several important particulars; and on +this account I have designedly abstained from adopting his term. Here +it is enough to remark that it answers to the generic term Isolation, +without reference to the _kind_ of isolation as discriminate or +indiscriminate, homogamous or apogamous. On the other hand, my +Independent Variability is merely a re-statement of the so-called "Law +of Delboeuf," which, in his own words, is as follows:-- + + One point, however, is definitely attained. It is that the + proposition, which further back we designated paradoxical, is + rigorously true, A constant cause of variation, however + insignificant it may be, changes the uniformity [of type] little by + little, and diversifies it _ad infinitum_. From the homogeneous, + left to itself, only the homogeneous can proceed; but if there be a + slight disturbance ["léger ferment"] in the homogeneous, the + homogeneity will be invaded at a single point, differentiation will + penetrate the whole, and, after a time--it may be an infinite + time--the differentiation will have disintegrated it altogether. + +In other words, the "Law," which Delboeuf has formulated on +mathematical grounds, and with express reference to the question of +segregate breeding, proves that, no matter how infinitesimally small the +difference may be between the average qualities of an isolated section +of a species compared with the average qualities of the rest of that +species, if the isolation continues sufficiently long, differentiation +of specific type is necessarily bound to ensue. But, to make this +mathematical law biologically complete, it ought to be added that the +time required for the change of type to supervene (supposing apogamy to +be the only agent of change) will be governed by the range of individual +variability which the species in question presents. A highly stable +species (such as the Goose) might require an immensely long time for +apogamy alone to produce any change of type in an isolated portion of +the species, while a highly variable species (such as the Ruff) would +rapidly change in any portion that might be indiscriminately isolated. +It was in order to recognize this additional and very important factor +that I chose the name Independent _Variability_ whereby to designate the +diversifying influence of merely indiscriminate isolation, or apogamy. +Later on Mr. Gulick published his elaborate papers upon the divergence +of type under all kinds of isolation; and retained my term Independent, +but changed Variability into Generation. I point this out merely for the +sake of remarking that his Independent Generation is exactly the same +principle as my Independent Variability, and Delboeuf's Mathematical +Law. + +Now, while I fully agree with Mons. Giard where he says, in the +introductory lecture of his course on _The Factors of Evolution_[7], +that sufficient attention has not been hitherto given by naturalists to +this important factor of organic evolution (apogamy), I think I have +shown that among those naturalists who have considered it there is a +sufficient amount of agreement. _Per contra_, I have to note the opinion +of Mr. Wallace, who steadily maintains the impossibility of any cause +other than natural selection (i.e. one of the forms of homogamy) having +been concerned in the evolution of species. But at present it is enough +to remark that even Professor Ray Lankester--whose leanings of late +years have been to the side of ultra-Darwinism, and who is therefore +disposed to agree with Mr. Wallace wherever this is logically +possible--even Professor Ray Lankester observes:-- + + [7] _Revue Scientifique_, Nov. 23, 1889. + + Mr. Wallace does not, in my judgement, give sufficient grounds for + rejecting the proposition which he indicates as the main point of + Mr. Gulick's valuable essay on _Divergent Evolution through + Cumulative Segregation_. Mr. Gulick's idea is that ... no two + portions of a species possess exactly the same average character, + and the initial differences will, if the individuals of the two + groups are kept from intercrossing, assert themselves continuously + by heredity in such a way as to ensure an increasing divergence of + the forms belonging to the two groups, amounting to what is + recognized as specific distinction. Mr. Gulick's idea is simply the + recognition of a permanence or persistency in heredity, which, + _caeteris paribus_, gives a twist or direction to the variations of + the descendants of one individual as compared with the descendants + of another[8]. + + [8] _Nature_, Oct. 10, 1889, p. 568. + +Now we have seen that "Mr. Gulick's idea," although independently +conceived by him, had been several times propounded before; and it is +partly implicated in more than one passage of the _Origin of Species_, +where free intercrossing, or the _absence_ of isolation, is alluded to +as maintaining the _constancy_ of a specific type[9]. Moreover, it is +still more fully recognized in the last edition of the _Variation of +Animals and Plants_, where a paragraph is added for the purpose of +sanctioning the principle in the imperfect form that it was stated by +Weismann[10]. Nevertheless, to Mr. Gulick belongs the credit, not only of +having been the first to conceive (though the last to publish) the +"idea" in question, and of having stated it with greater fullness than +anybody else; but still more of having verified its importance as a +factor of organic evolution. + + [9] e. g. p. 81. + + [10] See Chapter xxiii. vol. ii. p. 262. (Edition of 1888.) + +For, in point of fact, Mr. Gulick was led to his recognition of the +principle in question, not by any deductive reasoning from general +principles, but by his own particular and detailed observations of the +land mollusca of the Sandwich Islands. Here there are an immense number +of varieties belonging to several genera; but every variety is +restricted, not merely to the same island, but actually to the same +valley. Moreover, on tracing this fauna from valley to valley, it is +apparent that a slight variation in the occupants of valley 2 as +compared with those of the adjacent valley 1, becomes more pronounced in +the next--valley 3, still more so in 4, &c., &c. Thus it was possible, +as Mr. Gulick says, roughly to estimate the amount of divergence between +the occupants of any two given valleys by measuring the number of miles +between them. + +As already stated, I have myself examined his wonderful collection of +shells, together with a topographical map of the district; and therefore +I am in a position to testify to the great value of Mr. Gulick's work in +this connexion, as in that of the utility question previously +considered. The variations, which affect scores of species, and +themselves eventually run into fully specific distinctions, are all more +or less finely graduated as they pass from one isolated region to the +next; and they have reference to changes of form and colour, which in no +one case presents any appearance of utility. Therefore--and especially +in view of the fact that, as far as he could ascertain, the environment +in the different valleys was essentially the same--no one who examines +this collection can wonder that Mr. Gulick attributes the results which +he has observed to the influence of apogamy alone, without any reference +to utility or natural selection. + +To this solid array of remarkable facts Mr. Wallace has nothing further +to oppose than his customary appeal to the argument from ignorance, +grounded on the usual assumption that no principle other than natural +selection _can_ be responsible for even the minutest changes of form or +colour. For my own part, I must confess that I have never been so deeply +impressed by the dominating influence of the _a priori_ method as I was +on reading Mr. Wallace's criticism of Mr. Gulick's paper, after having +seen the material on which this paper is founded. To argue that every +one of some twenty contiguous valleys in the area of the same small +island must necessarily present such differences of environment that all +the shells in each are differently modified thereby, while in no one out +of the hundreds of cases of modification in minute respects of form and +colour can any human being suggest an adaptive reason therefor--to argue +thus is merely to affirm an intrinsically improbable dogma in the +presence of a great and consistent array of opposing facts. + +I have laid special stress on this particular case of the Sandwich +Islands' mollusca, because the fifteen years of labour which Mr. Gulick +has devoted to their exhaustive working out have yielded results more +complete and suggestive than any which so far have been forthcoming with +regard to the effects of isolation in divergent evolution. But, if space +permitted, it would be easy to present abundance of additional facts +from other sources, all bearing to the same conclusion--namely, that as +a matter of direct observation, no less than of general reasoning, any +unprejudiced mind will concede to the principle of indiscriminate +isolation an important share in the origination of organic types. For as +indiscriminate isolation is thus seen sooner or later to become +discriminate, and as we have already seen that discriminate isolation is +a necessary condition to all or any modification, we can only conclude +that isolation in both its kinds takes rank with heredity and +variability as one of the three basal principles of organic evolution. + + * * * * * + +Having got thus far in the way of generalities, we must next observe +sundry further matters of comparative detail. + +1. In any case of indiscriminate isolation, or apogamy, the larger the +bulk of the isolated section the more nearly must its average qualities +resemble those of its parent stock; and, therefore, the less divergence +of character will ensue in a given time from this cause alone. For +instance, if one-fourth of a large species were to be separated from the +other three-fourths (say, by subsidence causing a discontinuity of +area), it would continue the specific characters unchanged for an +indefinitely long time, so far as the influence of such an +indiscriminate isolation is concerned. But, on the other hand, if only +half a dozen individuals were to be thus separated from the rest of +their species, a comparatively short time would be needed for their +descendants to undergo some varietal modification at the hands of +apogamy. For, in this case, the chances would be infinitely against the +average characters of the original half-dozen individuals exactly +coinciding with those of all the rest of their species. + +2. In any case of homogamy, however, it is immaterial what proportional +number of individuals are isolated in the first instance. For the +isolation is here discriminate, or effected by the initial difference of +the average qualities themselves--a difference, therefore, which +presupposes divergence as having already commenced, and equally bound to +proceed whether the number of intergenerants be large or small. + +It may here be remarked that, in his essay on the _Influence of +Isolation_, Professor Weismann fails to distinguish between the two +kinds of isolation. This essay deals only with one of the many different +forms of isolation--the geographical--and is therefore throughout +concerned with a consideration of diversity as arising from apogamy +alone. But in dealing with this side of the matter Weismann anticipated +both Gulick and myself in pointing out the law of inverse proportion, +which I have stated in the preceding paragraph in what appears to me its +strictly accurate form. + +3. Segregate Breeding, or homogamy, which arises under any of the many +forms of discriminate isolation, must always tend to be _cumulative_. +For, again to quote Mr. Gulick, who has constituted this fact the most +prominent as it is the most original feature of his essay, "In the first +place, every new form of Segregation[11] that now appears depends on, and +is superimposed upon, forms of Segregation that have been previously +induced; for when Negative Segregation arises [i. e. isolation due to +mutual sterility], and the varieties of a species become less and less +fertile with one another, the complete infertility that has existed +between them and some other species does not disappear, nor does the +Positive Segregation cease [i. e. any other form of isolation previously +existing].... In the second place, whenever Segregation is directly +produced by some quality of the organism, variations that possess the +endowment in a superior degree will have a larger share in producing the +segregated forms of the next generation, and accordingly the segregative +endowment of the next generation will be greater than that of the +present generation; and so with each successive generation the +segregation will become increasingly complete." And to this it may be +added, in the third place, that where the segregation (isolation) is due +to the external conditions of life under which the organism is placed, +or where it is due to natural selection simultaneously operating in +divergent lines of evolution, the same remarks apply. Hence it follows +that discriminate isolation is, in all its forms, cumulative. + + [11] This term may here be taken as equivalent to Isolation. + +4. The next point to be noted is, that the cumulative divergence of type +thus induced can take place only in as many different lines as there are +different _cases_ of isolation. This is a point which Mr. Gulick has not +expressly noticed; but it is one that ought to be clearly recognized. +Seeing that isolation secures the breeding of similar forms by exclusion +(immediate or eventual) of those which are dissimilar, and that only in +as far as it does this can it be a factor in organic evolution, it +follows that the resulting segregation, even though cumulative, can only +lead to divergence of organic types in as many directions as there are +cases of isolation. For any one group of intergenerants only _serial_ +transformation is possible, even though the transformation be cumulative +through successive generations in the single line of change. But there +is always a probability that during the course of such _serial +transformation in time_, some other case of isolation may supervene, so +as to divide the previously isolated group of intergenerants into two or +more further isolated groups. Then, of course, opportunity will be +furnished for _divergent transformation in space_--and this in as many +different lines as there are now different homogamous groups. + +That this must be so is further evident, if we reflect that the +evolutionary power of isolation depends, not only on the _preventing_ of +intercrossing between the isolated portion of a species and the rest of +that species, but also upon the _permitting_ of intercrossing between +all individuals of the isolated portion, whereby the peculiar average of +qualities which they as a whole present may be allowed to assert itself +in their progeny--or, if the isolation has been from the first +discriminate, whereby the resulting homogamy may thus be allowed to +assert itself. Hence any one case of either species of isolation, +discriminate or indiscriminate, can only give rise to what Mr. Gulick +has aptly called "monotypic evolution," or a chain-like series of types +arising successively in time, as distinguished from what he has called +"polytypic evolution," or an arborescent multiplication of types +arising simultaneously in space. + +For example, let us again take the geographical form of isolation. Where +a single small intergenerant group of individuals is separated from the +rest of its species--say, on an oceanic island--_monotypic_ evolution +may take place through a continuous and cumulative course of independent +variation in a single line of change: all the _individuals_ composing +any one given generation will closely resemble one another, although the +_type_ may be progressively altering through a long series of +generations. But if the original species had had two small colonies +separated from itself (one on each of two different islands, so giving +rise to two cases of isolation), then _polytypic_ evolution would have +ensued to the extent of there having been two different lines of +evolution going on simultaneously (one upon each of the two islands +concerned). Similarly, of course, if there had been three or four such +colonies, there would have been three or four divergent lines of +evolution, and so on. + +5. In the _cases_ of isolation just supposed there is only one _form_ of +isolation; and it is thus shown that under one form of isolation there +may be as many lines of divergence as there are separate cases of such +isolation. But now suppose that there are two or more forms of +isolation--for instance, that on the same oceanic island the original +colony has begun to segregate into secondary groups under the influence +of natural selection, sexual selection, physiological selection, or any +of the other forms of isolation--then there will be as many lines of +divergent evolution going on at the same time (and here on the same +area) as there are forms of isolation affecting the oceanic colony. And +this because each of the _forms_ of isolation has given rise to a +different _case_ of isolation. + +Now, inasmuch as different forms of isolation, when thus superadded one +to another, constitute different cases of isolation, we may lay down the +following general law as applying to all the forms of isolation--namely, +_The number of possible directions in which divergent evolution can +occur, is never greater than, though it may be equal to, the number of +cases of efficient isolation--or the number of efficiently separated +groups of intergenerants._ + +6. We have now to consider with some care the particular and highly +important form of isolation that is presented by natural selection. For +while this form of isolation resembles all the other forms of the +discriminate kind in that it secures homogamy, there are two points in +which it differs from all of them, and one point in which it differs +from most of them. + +Natural selection differs from _all_ the other known forms of isolation +(whether discriminate or indiscriminate) in that it has exclusive +reference to _adaptations_ on the one hand, and, on the other hand, +necessitates not only the elimination, but the destruction of the +excluded individuals. Again, natural selection differs from _most_ of +the other forms of isolation in that, unless assisted by some other +form, it can never lead to polytypic, but only to monotypic evolution. +The first two points of difference are here immaterial; but the last is +one of the highest importance, as we shall immediately perceive. + +In nearly all the other forms of isolation, polytypic or divergent +evolution may arise under the influence of that form alone, or without +the necessary co-operation of any other form. This we have already seen, +for example, in regard to geographical isolation, under which there may +be as many different lines of transmutation going on simultaneously as +there are different cases of isolation--say, in so many different +oceanic islands. Again, in regard to physiological isolation the same +remark obviously applies; for it is evident that even upon the same +geographical area there may be as many different lines of transmutation +going on simultaneously as there are cases of this form of isolation. +The bar of mutual sterility, whenever and wherever it occurs, must +always render polytypic evolution possible. And so it is with almost all +the other forms of isolation: that is to say, one _form_ does not +necessarily require the assistance of another _form_ in order to create +an additional _case_ of isolation. But it is a peculiarity of natural +selection, considered as a form of isolation, that it does necessarily +require the assistance of some other form before it can give rise to an +additional case of isolation; and therefore before it can give rise to +any _divergence_ of character in ramifying lines, as distinguished from +_transformation_ of characters in a single line. Or, in other words, +natural selection, when acting alone, can never induce polytypic +evolution, but only monotypic. + +That this important conclusion is a necessary deduction from the theory +of natural selection itself, a very few words will be enough to show. +For, according to the theory, survival of the fittest is a form of +isolation which acts through utility, by _destroying_ all the +individuals whom it fails to isolate. Hence it follows that survival of +the fittest is a form of isolation which, if acting alone, cannot +_possibly_ effect divergent evolution. For, in the first place, there is +nothing in this form of isolation to ensure that the fitter individuals +should fail to interbreed with the less fit which are able to survive; +and, in the second place, in all cases where the less fit are not +sufficiently fit to be suffered to breed, they are exterminated--i. e. +not permitted to form a distinct variety of their own. If it be said +that survival of the fittest may develop simultaneously two or more +lines of _useful_ change, the answer is that it can only do this if each +of the developing varieties is isolated from the others by some +_additional form_ of isolation; for, if not, there can be no +commencement of utilitarian _divergence_, since whatever number of +utilitarian changes may be in course of simultaneous development, they +must in this case be all blended together in a single line of specific +transmutation. Nay, even if specific divergence has actually been +commenced by natural selection when associated with some other form of +homogamy, if the latter should afterwards be withdrawn, natural +selection would then be unable to maintain even so much divergence of +character as may already have been attained: free intercrossing between +the two collateral, and no longer isolated branches, would ensure their +eventual blending into a common stock. Therefore, I repeat, natural +selection, when acting alone, can never induce polytypic evolution, but +only monotypic. + +Now I regret to say that here, for the first and only time throughout +the whole course of the present treatise, I find myself in seeming +opposition to the views of Darwin. For it was the decidedly expressed +opinion of Darwin that natural selection _is_ competent to effect +polytypic, or divergent, evolution. Nevertheless, I believe that the +opposition is to a large extent only apparent, or due merely to the fact +that Darwin did not explicitly state certain considerations which +throughout his discussion on "divergence of character" are seemingly +implied. But, be this as it may, I have not even appeared to desert his +leadership on a matter of such high importance without having duly +considered the question in all its bearings, and to the utmost limit of +my ability. Moreover, about two years after the publication of my first +paper[12] upon the subject, Mr. Gulick followed, at somewhat greater +length, in the same line of dissent. Like all the rest of his work, this +is so severely logical in statement, as well as profoundly thought out +in substance, that I do not see how it is possible for any one to read +impartially what he has written, and then continue to hold that natural +selection, if unassisted by any other form of isolation, can possibly +effect divergence of character--or polytypic as distinguished from +monotypic evolution[13]. + + [12] _Zool. Journal Lin. Soc._, vol. xix. pp. 337-411. + + [13] _Ibid._, vol. xx. pp. 202-212. + +I may here quote from Mr. Gulick's paper three propositions, serving to +state three large and general bodies of observable fact, which +severally and collectively go to verify, with an overwhelming mass of +evidence, the conclusion previously reached on grounds of general +reasoning. + + The facts of geographical distribution seem to me to justify the + following statements:-- + + (1) A species exposed to different conditions in the different + parts of the area over which it is distributed, is not represented + by divergent forms when free interbreeding exists between the + inhabitants of the different districts. In other words, Diversity + of Natural Selection without Separation does not produce divergent + evolution. + + (2) We find many cases in which areas, corresponding in the + character of the environment, but separated from each other by + important barriers, are the homes of divergent forms of the same or + allied species. + + (3) In cases where the separation has been long continued, and the + external conditions are the most diverse in points that involve + diversity of adaptation, there we find the most decided divergences + in the organic forms. That is, where Separation and Divergent + Selection have long acted, the results are found to be the + greatest. + + The 1st and 3rd of these propositions will probably be disputed by + few, if by any. The proof of the 2nd is found wherever a set of + closely allied organisms is so distributed over a territory that + each species and variety occupies its own narrow district, within + which it is shut by barriers that restrain its distribution while + each species of the environing types is distributed over the whole + territory. The distribution of terrestrial molluscs on the Sandwich + Islands presents a great body of facts of this kind. + + + + +CHAPTER II. + +ISOLATION (_continued_). + + +I will now recapitulate the main doctrines which have been set forth in +the foregoing chapter, and then proceed to consider the objections which +have been advanced against them. + +It must be remembered that by isolation I mean exactly what Mr. Gulick +does by "Segregation," and approximately what Professor Weismann does by +"Amixia "--i. e. the prevention of intercrossing. + +Isolation occurs in very many forms besides the geographical, as will be +more fully shown at the end of this chapter; and in all its forms it +admits of degrees. + +It also occurs in two very different species or kinds--namely, +discriminate and indiscriminate. These I have called respectively +Homogamy and Apogamy. This all-important distinction has been clearly +recognized by Mr. Gulick, as a result of his own thought and +observation, independently of anything that I have published upon the +subject. + +In view of this distinction Isolation takes rank with Heredity and +Variability as one of the most fundamental principles of organic +evolution. For, if these other two principles be granted, the whole +theory of descent resolves itself into an inquiry touching the causes, +forms, and degrees of Homogamy. + +Save in cases where very large populations are concerned, apogamy must +sooner or later give rise _per se_ to homogamy, owing to the Law of +Delboeuf, which is the principle that I have called Independent +Variability, and Gulick has called Independent Generation. But of course +this does not hinder that under apogamy various other causes of homogamy +are likely to arise--in particular natural selection. + +That natural selection differs from most of the other forms of isolation +in not being capable of causing _divergent_ or _polytypic_ evolution +must at once become evident, if we remember that the only way in which +isolation of any form can cause such evolution is by partitioning a +given group of intergenerants into two or more groups, each of which is +able to survive as thus separated from the other, and so to carry on the +evolution in divergent lines. But the distinguishing peculiarity of +natural selection, considered as a form of isolation, is that it effects +the isolation _by killing off all the individuals which it fails to +isolate_: consequently, this form of isolation differs from other forms +in prohibiting the possibility of any ramification of a single group of +intergenerants into two or more groups, for the purpose of carrying on +the evolution in divergent lines. Therefore, under this form of +isolation alone, evolution must proceed, palm-like, in a single line of +growth. So to speak, the successive generations continuously ascend to +higher things on the steps supplied by their own "dead selves"; but in +doing so they must climb a single ladder, no rung of which can be +allowed to bifurcate in the presence of the uniformity secured _for that +generation_ by the free intercrossing of the most fit. Even though +beneficial variations may arise in two or more directions +simultaneously, and all be simultaneously selected by survival of the +fittest, the effect of free intercrossing (in the absence of any other +form of isolation) will be to fuse all these beneficial variations into +one common type, and so to end in _monotypic_ evolution as before. In +order to secure _polytypic_ evolution, intercrossing between the +different beneficial variants which may arise must be prevented; and +there is nothing to prevent such intercrossing in the process of natural +selection _per se_. In order that the original group of intergenerants +should be divided and sub-divided into two or more groups of +intergenerants, some additional form of isolation must necessarily +supervene--when, of course, polytypic evolution will result. And, as Mr. +Gulick has shown, the conclusion thus established by deductive reasoning +is verified inductively by the facts of geographical distribution. + +How, then, are we to account for the fact that Darwin attributed to +natural selection the power to cause divergence of character? The answer +is sufficiently simple. _He does so by tacitly invoking the aid of some +other form of homogamy in every case._ If we carefully read pp. 86-97 of +the _Origin of Species_, where this subject is under consideration, we +shall find that in every one of the arguments and illustrations which +are adduced to prove the power of natural selection to effect +"divergence of character," he either pre-supposes or actually names some +other form of homogamy as the originating cause of the diversity that +is afterwards presented to natural selection for further +intensification. To give only one example. At the starting-point of the +whole discussion the priority of such other forms of homogamy is assumed +in the following words:-- + + But how, it may be asked, can any analogous principle [to that of + diversity caused by artificial selection] apply in nature? I + believe it can and does apply most efficiently (though it was a + long time before I saw how), from the simple circumstance that the + more diversified the descendants from any one species become in + structure, constitution, and habits, by so much will they be better + enabled to seize on many and widely diversified places in the + polity of nature, and so be enabled to increase in numbers. + +Now, without question, so soon as segregate breeding in two or more +lines of homogamy has been in any sufficient degree determined by some +"change of structure, constitution, or habits," natural selection will +forthwith proceed to increase the divergence in as many different lines +as there are thus yielded discriminately isolated sections of the +species. And this fact it must have been that Darwin really had before +his mind when he argued that diversification of character is caused by +natural selection, through the benefit gained by the diversified forms +being thus "enabled to increase in number." Nevertheless he does not +expressly state the essential point, that although diversification of +character, _when once begun_, is thus _promoted_ by natural selection, +which forthwith proceeds to cultivate each of the resulting branches, +yet diversification of character can never be _originated_ by natural +selection. The change of "structure," of "constitution," of "habits," of +"station," of geographical area, of reciprocal fertility, and so +on--this change, _whatever_ it may have been, must clearly have been +antecedent to any operation of natural selection through the benefit +which arose from the change. Therefore the change must in all cases have +been due, in the first instance, to some other form of isolation than +the superadded form which afterwards arose from superior fitness in the +possession of superior benefit--although, so long as the prior form of +isolation endured, or continued to furnish the necessary condition to +the co-operation of survival of the fittest, survival of the fittest +would have continued to increase the divergence of character in as many +ramifying lines as there were thus given to its action separate cases of +isolation by other means. + +In short, as divergence of character must in all cases be due to a +prevention of intercrossing, and as in the process of natural selection +there is, _ex hypothesi_, nothing to prevent the intercrossing until the +divergence has already arisen, to suppose that natural selection alone +can have caused the divergence, is to suppose that natural selection can +have caused the conditions of its own activity, which is absurd. + +Seeing, then, that even in cases where any "benefit" arises from +divergence of character, such benefit can arise only after the +divergence has already commenced, and seeing that on this as on other +accounts previously mentioned it is plainly impossible to attribute the +origin of such divergence to natural selection, we find that natural +selection must be in all cases assisted by some other form of isolation, +if it is to be concerned in polytypic as distinguished from monotypic +evolution. But this does not hinder that, when it is so assisted, +natural selection may become--and, I believe, does become--the most +efficient of all the forms of isolation in promoting divergence of +character. For, in the first place, of all the forms of isolation +natural selection is probably the most energetic in promoting monotypic +evolution; so that under the influence of such isolation monotypic +evolution probably advances more rapidly than it does under any other +form of isolation. In the second place, when polytypic evolution has +been begun by any of these other forms of isolation, and natural +selection then sets to work on each of the resulting branches, although +natural selection is thus engaged in as many different acts of monotypic +evolution as there are thus separate cases supplied to it by these other +forms of isolation, the joint result of all these different acts is to +hurry on the polytypic evolution which was originally started by the +other forms of isolation. So to speak, natural selection is the forcing +heat, acting simultaneously on each of the separate branches which has +been induced to sprout by other means; and in thus rapidly advancing the +growth of all the branches, it is still entitled to be regarded as the +most important _single_ cause of diversification in organic nature, +although we must henceforth cease to regard it as in any instance the +_originating_ cause--or even so much as the _sustaining_ cause. + +So much by way of summary and recapitulation. I will now briefly +consider the only objections which, so far as I can see, admit of being +brought against the foregoing doctrine of Isolation as held by Mr. +Gulick and myself. These possible objections are but two in +number--although but one of them has been hitherto adduced. This, +therefore, I will take first. + +Mr. Wallace, with his customary desire to show that natural selection is +everywhere of itself capable of causing organic evolution, seeks to +minimize the swamping effects of free intercrossing, and the consequent +importance of other forms of isolation. His argument is as follows. + +Alluding to the researches of Mr. J. A. Allen, and others, on the amount +of variation presented by individuals of a species in a state of nature, +Mr. Wallace shows that, as regards any given part of the animal under +consideration, there is always to be found a considerable range of +individual variation round the average mean which goes to constitute the +specific character of the type. Thus, for example, Mr. Allen says of +American birds, "that a variation of from fifteen to twenty per cent. in +general size, and an equal degree of variation in the relative size of +different parts, may be ordinarily expected among specimens from the +same species and sex, taken at the same locality, while in some cases +the variation is even greater than this." Now, Mr. Wallace is under the +impression that these facts obviate the difficulty which arises from the +presence of free intercrossing--the difficulty, that is, against the +theory of natural selection when natural selection is supposed to have +been the exclusive means of modification. For, as he says, "if less size +of body would be beneficial, then, as half the variations in size are +above and half below the mean or existing standard of the species, there +would be ample beneficial variations"; and similarly with regard to +longer or shorter legs, wings, tails, &c., darker or lighter colour, and +so on through all the parts of any given organism. + +Well, although I have no wish at all to disparage the biological value +of these actual measurements of the range of individual variation, I +must point out that they are without any value at all in the connexion +which Mr. Wallace adduces them. We did not require these measurements to +tell us the broad and patent fact that "no being on this earthly ball is +like another all in all"--or, in less Tennysonian words, that as regards +every specific structure there is a certain amount of individual +variability round an average mean. Indeed, in my own paper on +_Physiological Selection_--against which Mr. Wallace is here specially +arguing--I expressly said, as previously remarked, "that a specific type +may be regarded as _the average mean of all individual variations_." The +fact of such individual variability round a specific mean has always +been well known to anatomists; it constitutes one of the basal pillars +of the whole Darwinian theory; and is besides a matter of universal +recognition as regards human stature, features, and so forth. The value +of Mr. Allen's work consists in accurately measuring the _amount_ or +_range_ of individual variation; but the question of its amount or range +is without relevancy in the present connexion. For the desirability of +isolation as an aid to natural selection even where monotypic evolution +is concerned, does not arise with any reference to the amount or range +of variation: it arises with reference to the _number_ of variations +which are--or are not--_similar_ and _simultaneous_. If there be a +sufficient number which are both similar and simultaneous, the +desirability of any co-operating form of isolation is correspondingly +removed, because natural selection may then have sufficient material +wherewith to overcome the adverse influence of free intercrossing, and +so of itself to produce monotypic evolution. Now, variations may be +numerous, similar, and simultaneous, either on account of some common +cause acting on many individuals at the same time, or on account of the +structures in question being more or less variable round a specific +mean. In the latter case--which is the only case that Mr. Allen's +measurements have to do with--the law of averages will of course +determine that half the whole number of variations in any given +structure, in any given generation, will be above the mean line. But, +equally of course, no one has ever denied that where, for either of +these reasons, natural selection is provided with sufficient material, +it is correspondingly capable of improving the specific type without the +assistance of any other form of homogamy; so to speak, they protect +themselves by their very numbers, and their superiority over others +leads to their survival and accumulation. But what is the result? _The +result can only be monotypic evolution._ No matter how great the number, +or how great the range, of variations round an average specific mean, +out of such material natural selection can never produce _polytypic_ +evolution: it may _change_ the type to any extent during successive +generations, and in a single line of change; but it cannot _branch_ the +type, unless some other form of homogamy intervenes. Therefore, when Mr. +Wallace adduces the well-known fact that all structures vary more or +less round a specific mean as proof that natural selection need not be +incommoded by free intercrossing, but can of itself produce all the +known phenomena of specific evolution, he fails to perceive that his +argument refers only to one aspect of such evolution (viz. the +transformation of species in time), and does not apply to the aspect +with which alone my paper on _Physiological Selection_ was concerned +(viz. the multiplication of species in space). + +The same thing may be shown in this way. It is perfectly obvious that +where the improvement of type in a linear series is concerned (monotypic +evolution), free intercrossing, far from being a hindrance to the +process, _is the very means by which the process is accomplished_. +Improvement here ascends by successive steps, in successive generations, +simply _because_ of the general intercrossing of the generally most fit +with the result that the species, _as a whole_, gradually becomes +transformed into another species, _as a whole_. Therefore, it would be +mere fatuity in any one to adduce free intercrossing as a "difficulty" +against natural selection alone being competent to produce evolution of +this kind. But where the kind of evolution is that whereby the species +is _differentiated_--where it is required, for instance, to produce +different structures in different portions of the species, such as the +commencement of a fighting spur on the wing of a duck, or _novel_ +characters of any sort in different groups of the species--free +intercrossing is no longer a condition to, but an absolute preventive +of, the process; and, therefore, unless checked as between each portion +of the species by some form of homogamy other than natural selection, +it must effectually inhibit any _segregation_ of specific types, or +divergence of character. + +Hence it is that, while no Darwinian has ever questioned the power of +unaided selection to cause _improvement of character in successive +generations_, in common now with not a few other Darwinians I have +emphatically denied so much as the abstract possibility of selection +alone causing a _divergence of character in two or more simultaneous +lines of change_. + +And, although these opposite views cannot be reconciled, I am under the +impression that they do admit of being explained. For I take them to +indicate a continued failure to perceive the all-important distinction +between evolution as monotypic and polytypic. Unless one has fully +grasped this distinction, and constantly holds it in mind, he is not in +a position to understand the "difficulty" in question; nor can he avoid +playing fast and loose with natural selection as possibly the sole cause +of evolution, and as necessarily requiring the co-operation of some +other cause. But if he once clearly perceives that "evolution" is a +logical genus, of which the monotypic and the polytypic forms are +species, he will immediately escape from his confusion, and find that +while the monotypic form may be caused by natural selection alone the +polytypic form can never be so caused. + + * * * * * + +The second difficulty which I have to mention as at first sight +attaching to the views of Mr. Gulick and myself on the subject of +Isolation is, that in an isolated section of a species Mr. Francis +Galton's law of regression in the average character of offspring to the +typical character of the group through reversion or atavism (_Natural +Inheritance_, p. 97) must have the effect of neutralizing the +segregative influence of mere apogamy. That such, however, cannot be the +case has been well shown by Mr. Gulick in his paper on _Intensive +Segregation_. Without at all disputing the validity of Mr. Galton's law, +he proves that "it can hold in full force only where there is free +crossing, otherwise no divergent race could ever be formed by any amount +of selection and independent breeding[14]." This is so self-evident that +I need not quote his demonstration of the point. + + [14] _Zool. Journal Lin. Soc._, vol. xxiii. p. 313. + + * * * * * + +In conclusion, then, and having regard to the principle of isolation as +a whole, or in all the many and varied forms in which this principle +obtains, I trust that I have redeemed the promise with which I set +out--viz. to show that in relation to the theory of descent this +principle is of an importance second to no other, not even excepting +heredity, variability, and the struggle for existence. This has now been +fully shown, inasmuch as we have clearly seen that the importance of the +struggle for existence, and consequent survival of the fittest, arises +just because survival of the fittest is a form, and a very stringent +form, of isolation; while, as regards both heredity and variability, we +are now in a position to see that the more fully we recognize their +supreme importance as principles concerned in organic evolution, the +more must we also recognize that any rational theory of such evolution +becomes, in the last resort, a theory of the different modes in which +efficient isolation can be secured. For, in whatever degree the process +of organic evolution has been dependent upon heredity with variability, +in that degree must it also have been dependent upon the means of +securing homogamy, whereby alone the force of heredity can be made to +expend itself in the innumerable directions of progressive change, +instead of continually neutralizing the force of variability by +promiscuous intercrossing. + + + + +CHAPTER III. + +PHYSIOLOGICAL SELECTION. + + +So far we have been concerned with the principle of Isolation in +general. We have now to consider that form of isolation which arises in +consequence of mutual infertility between the members of any group of +organisms and those of all other similarly isolated groups occupying +simultaneously the same area. + + * * * * * + +Against the view that natural selection is a sufficient explanation of +the origin of species, there are two fatal difficulties: one, the +contrast between natural species and domesticated varieties in respect +of cross-sterility; the other, the fact that natural selection cannot +possibly give rise to polytypic as distinguished from monotypic +evolution. Now it is my belief that the theory of physiological +selection fully meets both these difficulties. Indeed I hold this to be +undeniable in a formal or logical sense: the only question is as to the +evidence which can be adduced for the theory in a practical or +biological sense. Therefore in this chapter, where the theory has first +of all to be stated, I shall restrict the exposition as much as possible +to the former, leaving for subsequent consideration the biological +side. + +The following is a brief outline sketch of this theory[15]. + + [15] _See Nineteenth Century_, January, 1887, pp. 61, 62. + +Of all parts of those variable objects which we call organisms, the most +variable is the reproductive system; and the variations may carry with +them functional changes, which may be either in the direction of +increased or of diminished fertility. Consequently variations in the way +of greater or less fertility frequently take place, both in plants and +animals; and probably, if we had adequate means of observing this point, +we should find that there is no one variation more common. But of course +where infertility arises--whether as a result of changed conditions of +life, or, as we say, spontaneously--it immediately becomes extinguished, +seeing that the individuals which it affects are less able (if able at +all) to propagate and to hand on the variation. If, however, the +variant, while showing some degree of infertility with the parent form, +continues to be as fertile as before when mated with similar variants, +under these circumstances there is no reason why such differential +fertility should not be perpetuated. + +Stated in another form this suggestion enables us to regard many, if not +most, species as the records of variations in the reproductive systems +of their ancestors. When variations of a non-useful kind occur in any of +the other systems or parts of organisms, they are, as a rule, +immediately extinguished by intercrossing. But whenever they arise in +the reproductive system in the way here suggested, they tend to be +preserved as new natural varieties, or incipient species. At first the +difference would only be in respect of the reproductive systems; but +eventually, on account of independent variation, other differences would +supervene, and the variety would take rank as a true species. + +Now we must remember that physiological isolation is not like those +other forms of isolation (e.g. geographical) which depend for their +occurrence on accidents of the environment, and which may therefore take +place suddenly in a full degree of completeness throughout a large +section of a species. Physiological isolation depends upon distinctive +characters belonging to organisms themselves; and it would be opposed to +the whole theory of descent with progressive modification to imagine +that absolute sterility usually arises, in a single generation between +two sections of a perfectly fertile species. Therefore evolutionists +must believe that in most, if not in all cases--could we trace the +history, say of any two species, which having sprung from a single +parent stock on a common area, are now absolutely sterile with one +another--we should find that this mutual sterility had been itself a +product of gradual evolution. Starting from complete fertility within +the limits of a single parent species, the infertility between +derivative or divergent species, _at whatever stage in their evolution +this began to occur_, must usually at first have been well-nigh +imperceptible, and thenceforth have proceeded to increase stage by +stage. + +But, if it be true that physiological isolation between genetically +allied groups must usually itself have been the product of a gradual +evolution; and if, when fully evolved, it constitutes a condition of the +first importance to any further differentiation of these groups (by +preventing fusion again into one group, more or less resembling the +original parent form), do we not perceive at least a strong probability +that in the lower stages of its evolution such mutual infertility must +have acted as a segregating influence between the diverging types, in a +degree proportional to its own development? The importance of mutual +sterility as a condition to divergent evolution is not denied, _when +this sterility is already present in an absolute degree_; and we have +just seen that, before it can have attained to this absolute degree _it +must presumably, and as a rule, itself have been the subject of a +gradual development_. Does it not therefore become, on merely antecedent +grounds, in a high degree probable, that from the moment of its +inception this isolating agency must have played the part of a +segregating cause, in a degree proportional to that of its completeness +as a physiological character? + +Whoever answers this question in the affirmative will have gone most of +the way towards accepting, on merely antecedent grounds, the theory of +physiological selection. And therefore it is that I have begun this +statement of the theory by introducing it upon these grounds, thereby +hoping to show how extremely simple--how almost self-evident--is the +theory which it will now be my endeavour to substantiate. I may here add +that the theory was foreshadowed by Mr. Belt in 1874[16], clearly +enunciated in its main features by Mr. Catchpool in 1884[17], and very +fully thought out by Mr. Gulick during a period of about fifteen years, +although he did not publish until a year after the appearance of my own +paper in 1886[18]. + + [16] _Nicaragua_, p. 207. + + [17] _Nature_, vol. xxxi. p. 4. + + [18] _Zool. Journal, Lin. Soc._, vol. xix. pp. 337-411 (1886); and + for Mr. Gulick's papers, _ibid._, vol. xx. pp. 189-274 (1887), vol. + xxiii. pp. 312-380 (1889). Mr. Gulick has recently drawn my + attention, in a private letter, to the fact that as early as 1872 a + paper of his was read at the British Association, bearing the title + _Diversity of Evolution under one set of External Conditions_, and + that here the principle of physiological segregation is stated. + Although it does not appear that Mr. Gulick then appreciated the + great importance of this principle, it entitles him to claim + priority. + +I must next proceed to state some of the leading features of +physiological selection in further detail. + +It has already been shown that Darwin clearly perceived that the very +general occurrence of some degree of infertility between allied species +cannot possibly be attributed to the _direct_ agency of natural +selection. His explanation was that the slight structural modifications +entailed by the transformation of one specific type into another, so +react upon the highly delicate reproductive system of the changing type +as to render it in some degree infertile with its parent type. Now the +theory of physiological selection begins by traversing this view. It +does not, however, deny that in _some_ cases the morphological may be +the prior change; but it strenuously denies that this must be so in +_all_ cases. Indeed, according to my statement in 1886, the theory +inclines to the view that, _as a rule_, the physiological change is +prior. At the same time, the theory, as I have always stated it, +maintains that it is immaterial whether, "in the majority of instances," +the physiological change has been prior to the morphological, or vice +versa; since in either case the physiological change will equally make +for divergence of character. + +To show this clearly the best way will be to consider the two cases +separately, taking first that in which the physiological change has +priority. In this case our theory regards any morphological changes +which afterwards supervene as due to the independent variability which +will sooner or later arise under the physiological isolation thus +secured. But to whatever causes the subsequent morphological changes may +be due, the point to notice is that they are as a general rule, +consequent upon the physiological change. For in whatever _degree_ such +infertility arises between two sections of a species occupying the same +area, in that _degree_ is their interbreeding prevented, and, therefore, +opportunity is given for a subsequent divergence of type, whether by the +influence of independent variability alone, or also by that of natural +selection, as now acting more or less independently on each of the +partially separated groups. In short, all that was said in the foregoing +chapters with respect to isolation in general, here applies to +physiological isolation in particular; and by supposing such isolation +to have been the prior change, we can as well understand the subsequent +appearance of morphological divergence on continuous areas, as in other +forms of isolation we can understand such divergence on discontinuous +areas, seeing that even a moderate degree of cross-infertility may be as +effectual for purposes of isolation as a high mountain-chain, or a +thousand miles of ocean. + +Here, then, are two sharply-defined theories to explain the very general +fact of there being some greater or less degree of cross-infertility +between allied species. The older, and hitherto current theory, +supposes the cross-infertility to be but an _accident_ of specific +divergence, which, therefore, has nothing to do with _causing_ the +divergence. The newer theory, on the other hand, supposes the +cross-infertility to have often been a necessary _condition_ to the +divergence having begun at all. Let us now consider which theory has +most evidence in its favour. + +First of all we have to notice the very general occurrence of the fact +in question. For when we include the infertility of hybrids, as well as +first crosses, the occurrence of some degree of infertility between +allied species is so usual that Mr. Wallace recommends experiments to +ascertain whether careful observation might not prove, even of species +which hybridize, "that such species, when crossed with their near +allies, do always produce offspring which are more or less sterile +_inter se_[19]." This seems going too far, but nevertheless it is the +testimony of a highly competent naturalist to the very general +occurrence of an association between the morphological differentiation +of species and the fact of a physiological isolation. Now I regard it as +little short of self-evident that this general association between +mutual infertility and innumerable secondary, or relatively variable +morphological distinctions, is due to the former having been an original +and a necessary condition to the occurrence of the latter, in cases +where intercrossing has not been otherwise prevented. + + [19] _Darwinism_, p. 169. + +The importance of physiological isolation, _when once fully developed_, +cannot be denied, for it is evident that if such isolation could be +suddenly destroyed between two allied species occupying a common area, +they would sooner or later become fused into a common type--supposing, +of course, no other form of isolation to be present. The necessity then +for this physiological form of isolation in _maintaining_ a specific +differentiation which has been already _attained_ cannot be disputed. +Yet it has been regarded as "Darwinian heresy" to suggest that it can +have been of any important service _during the process of attainment_, +or while the specific differentiation is being advanced, and this +notwithstanding that the physiological change must presumably have +developed _pari passu_ with the morphological, and notwithstanding that +in countless cases the former is associated with every conceivable +variety of the latter. + +Again, why should the physiological change be thus associated with +_every conceivable variety_ of morphological change? Throughout the +length and breadth of both vegetable and animal kingdoms we find this +association, in the great majority of cases, where new species arise. +Therefore, on the supposition that in all such cases the physiological +change has been adventitiously induced by the morphological changes, we +have to face an apparently unanswerable question--Why should the +reproductive mechanism of all organic beings have been thus arranged, as +it were, to change in immediate response to the very slightest +alteration in the complex harmony of "somatic" processes, which now more +than ever is recognized as exercising so comparatively little influence +on the _hereditary_ endowments of this mechanism? Consider the +difference between a worm and the bird that is eating it, an oak tree +and the gall-insect that is piercing it: are we to suppose that in all +cases, no matter how greatly the types differ, they must agree in this, +that when any parts of these complex structures change, ever so +slightly, the reproductive system is almost certain to be adventitiously +affected, yet always thus affected in the same peculiar way? + +If it be answered that the reproductive system is known to be very +sensitive to slight changes in the external conditions of life, the +answer proves too much. For though this is true, yet our opponents must +acknowledge that the reproductive system is not so sensitive, _in this +particular respect_, as their interpretation of the origin of specific +infertility requires. The proof of this point is overwhelming, for there +is the evidence from the entire range of our domesticated productions, +both vegetable and animal. Here the amount of structural change, which +has been slowly accumulated by artificial selection, is often much +greater in amount, and incomparably more rapid, than that which has been +induced between allied species by natural selection; and yet there is +scarcely any indication of the reproductive system having been affected +in the particular way that our opponents' theory requires. There are +many instances of its having been affected in sundry other ways +(chiefly, however, without any accompanying morphological change); but +among all the thousands of our more or less enormously modified +artificial types, there is scarcely one instance of such a peculiar +sexual relation between the modified descendants of a common type as so +usually obtains between allied species in nature. Yet in all other +respects evolutionists are bound to believe that the process of +modification has been in both cases strictly analogous. Why then this +conspicuous difference with respect to the reproductive system? + +The answer is simple. It has never been the object of breeders or of +horticulturists to select variations in the direction of +cross-infertility, for the swamping effects of intercrossing are much +more easily and rapidly prevented by artificial isolation. Consequently, +although they have been able to modify natural types in so many +directions and in such high degrees with regard to _morphology_, there +has been no accompanying physiological modification of the kind +required. But in nature there is no such thing as artificial, i.e. +intentional, isolation. Consequently, on common areas it must usually +happen that those changes of morphology which are associated with +cross-infertility are the only ones which can arise. Hence the very +remarkable contrast between our domesticated varieties and natural +species with regard to cross-infertility is just what the present theory +would expect, or, indeed, require. But on any other theory it has +hitherto remained inexplicable. + +In particular, the contrast in question has constituted one of the main +difficulties with which the theory of natural selection has hitherto had +to contend, not only in the popular mind, but also in the judgement of +naturalists, including the joint-authors of the theory themselves. Thus +Darwin says:-- + + The fertility of varieties is, with reference to my theory, of + equal importance with the sterility of species, for it seems to + make a broad and clear distinction between varieties and + species[20]. + + [20] _Origin of Species_, p. 136. + +And Mr. Wallace says:-- + + One of the greatest, or perhaps we may say the greatest, of all the + difficulties in the way of accepting the theory of natural + selection as a complete explanation of the origin of species, has + been the remarkable difference between varieties and species in + respect of fertility when crossed[21]. + + [21] _Darwinism_, p. 152. + +Now, in view of this conspicuous contrast, Darwin suggested that species +in a state of nature "will have been exposed during long periods of time +to more uniform conditions than have domesticated varieties, and [that] +this may well make a wide difference in the result." Now we have to +remember that species, living and extinct, are numbered by millions, and +represent every variety of type, constitution, and habits; is it +probable, then, that this one peculiarity of the reproductive system +should be due, in so many cases, to some merely incidental effect +produced on that system by uniform conditions of life? Again, _ex +hypothesi_, at the time when a variety is first forming, the influence +exercised by uniform conditions of life (whatever in different cases +this may happen to be) cannot be present as regards that variety: yet +this is just the time when its infertility with the parent (or allied) +form is most likely to have arisen; for it is just then that the nascent +variety would otherwise have been most liable to extinction by free +intercrossing--even supposing that in the presence of such intercrossing +the variety could ever have come into existence at all. + +Mr. Wallace meets the difficulty by arguing that sterility between +allied species may have been brought about by the direct influence of +natural selection. But, as previously remarked, this view is expressly +opposed to that of Darwin, who held that Wallace's contention is +erroneous. + +It will be seen, then, that both Darwin, and Wallace, fully recognize +the necessity of finding some explanation of the infertility of allied +species, over and above the mere reaction of morphological +differentiation on the physiology of the reproductive system, and they +both agree in suggesting additional causes, though they entirely +disagree as to what these causes are. Now, the theory of physiological +selection likewise suggests an additional cause--or, rather, a new +explanation--and one which is surely the most probable. For what is to +be explained? The very general association of a certain physiological +peculiarity with that amount of morphological change which distinguishes +species from species, of whatever kind the change may be, and in +whatever family of the animal or vegetable kingdom it may occur. Well, +the theory of physiological selection explains this very general +association by the simple supposition that, at least in a large number +of cases, it was the physiological peculiarity which first of all led to +the morphological divergence, by interposing the bar of sterility +between two sections of a previously uniform species; and by thus +isolating the two sections one from another, started each upon a +subsequently independent course of divergent evolution. + +Or, to put it in another way, if the occurrence of this physiological +peculiarity has been often the only possible means of isolating two +sections of a species occupying a common area, and thus giving rise to a +divergence of specific type (as obviously _must_ have been the case +wherever there was an absence of any other form of isolation), it is +nothing less than a necessary consequence that many allied species +should now present the physiological peculiarity in question. Thus the +association between the physiological peculiarity and the morphological +divergence is explained by the simple hypothesis, that the former has +acted as a necessary condition to the occurrence of the latter. In the +absence of other forms of isolation, the morphological divergence could +not have taken place at all, had not the physiological peculiarity +arisen; and hence it is that we now meet with so many cases where such +divergence is associated with this peculiarity. + + * * * * * + +So far we have been considering the physiological change as historically +the prior one. Here, at first sight, it may seem that the segregative +power of physiological selection must end; for it may well seem +impossible that the physiological change can ever be necessary for the +divergence of morphological varieties into true species in cases where +it has _not_ been the prior change, but has only set in after +morphological changes have proceeded far enough to have already +constituted definite varieties. A little thought, however, will show +that physiological selection is quite as potent a condition to the +differentiation of species when it occurs after varietal divergence has +begun, as it is when it occurs before the divergence--and hence that it +really makes no difference to the theory of physiological selection +whether, in particular cases, the cross-infertility arises before or +after any structural or other modifications with which it is +associated. + +For the theory does not assert that all varieties have been due to +physiological selection. There are doubtless many other causes of the +origin of varieties besides cross-infertility with parent forms; but, as +a general rule, it does not appear that they are by themselves capable +of carrying divergence beyond a merely varietal stage. In order to carry +divergence to the stage of producing _species_, it appears to be a +general condition that, sooner or later, cross-infertility should +arise--seeing that, when varieties do succeed in becoming species, we +almost invariably find that, as a matter of fact, cross-infertility has +arisen. Hence, if cross-infertility has thus usually been a necessary +condition to a varietal divergence becoming specific, it can make no +material difference when the incipient infertility arose. + +It may be asked, however, whether I suppose that, when the physiological +change is subsequent, it is directly _caused_ by change of structure, +size, colour, &c., or that it arises, so to speak, accidentally, from +other causes which may have affected the sexual system in the required +way. To this question I may briefly reply, that, looking to the absence +of any influence exercised on the reproductive systems of our +domesticated plants and animals by the great and varied changes which so +many of these forms present, it would seem that among natural varieties +such closely analogous changes are presumably not the usual causes of +the physiological change, even where the latter are subsequent to the +former. Nevertheless, I do not deny that in some of these cases changes +of structure, size, colour, &c., may be the causes of the physiological +change by reacting on the sexual system in the required way. But in +such cases free intercrossing will have prevented the perpetuation of +any morphological changes, save those which have the power of so +reacting on the reproductive system as to produce the physiological +change, and thus to protect themselves against the full and adverse +power of free intercrossing. We know that slight or initial changes of +structure, colour, &c., frequently occur as varieties, and yet that on +common areas very few of these varieties become distinct species: free +intercrossing prevents any such further divergence of character. But if +in the course of many such abortive attempts, as it were, to produce a +new species, nature happens to hit upon a structural or a colour +variation which is capable of reacting on the sexual system in the +particular way required, then this variation will be enabled to protect +itself against free intercrossing in proportion to its own development. +Or, in other words, the more it develops as a morphological change, the +more will it increase the physiological change; while the more the +physiological change is thus increased, the more will it in turn promote +the morphological. By such action and reaction the development of each +furthers the development of the other, till from an almost imperceptible +variety, apparently quite fertile with its parent form, there arises a +distinct species absolutely sterile with its parent form. In such cases, +therefore, it is still the physiological conditions which have +_selected_ the particular morphological changes capable of so reacting +on the reproductive system as to produce cross-infertility, and thus to +protect themselves against the destructive power of free intercrossing. +So to speak, free intercrossing is always on the watch to level down +any changes which natural selection, or any other cause of varietal +divergence, may attempt to produce; and therefore, in order to +produce--or to increase--such divergence in the absence of any other +form of isolation, natural selection must hit upon such changes of +structure, form, or colour, as are so correlated with the reproductive +system as to create the physiological isolation that is required. + +To show how the principle of selective fertility may be combined with +what apparently is the most improbable form of isolation for this +purpose--the geographical--I quote the following suggestion made by +Professor Lloyd Morgan in his _Animal Life and Intelligence_:-- + + Suppose two divergent local varieties were to arise in adjacent + areas, and were subsequently (by stress of competition or by + geographical changes) driven together into a single area.... If + their unions be fertile, the isolation will be annulled by + intercrossing--the two varieties will form one mean or average + variety. But if the unions be infertile, the isolation will be + preserved, and the two varieties will continue separate. Suppose + now, and the supposition is by no means an improbable one, that + this has taken place again and again in the evolution of species; + then it is clear that those varietal forms which had continued to + be fertile together would be swamped by intercrossing; while those + varietal forms which had become infertile would remain isolated. + Hence, in the long run, isolated forms occupying a common area + would be infertile, (p. 107.) + +If then cross-sterility may thus arise even in association with +geographical isolation, may it not also arise in its absence? And may it +not thus give rise to the differentiation of varieties on account of +this physiological isolation alone? + +Only two further points need be mentioned to make this statement of +physiological selection as complete as the present _résumé_ of its main +principles requires. + +The first is, that, as Mr. Wallace remarks, "every species has come into +existence coincident both in space and time with a pre-existing and +closely allied species." I regard this as important evidence that +physiological selection is one of the natural causes concerned. For the +general fact implied is that every species has come into existence on an +area occupied by its parent type, and therefore under circumstances +which render it imperative that intercrossing with that type should be +prevented. In the case of monotypic evolution by natural selection +alone, intercrossing with the parent type is prevented through the +gradual extinction of that type by successive generations of the +developing type. But in the case of polytypic evolution, intercrossing +with the parent type can only be prevented by some form of isolation +other than natural selection; and here it is evident that +cross-infertility with the parent type must be as efficient to that end +as any other form of isolation that can be imagined. Consequently we +might almost have expected beforehand that in a large proportional +number of cases cross-infertility should have been the means employed. +And the fact that this is actually the case so far corroborates the only +theory which is able to explain it. + +The second point is this. + +It appears to be comparatively rare for any cause of specific divergence +to prove effectual on common areas, unless it sooner or later becomes +associated with some degree of cross-infertility. But through this +association, the segregating influence of both the causes concerned is, +as Mr. Gulick has shown, greatly increased. For instance, if the +segregating influence of some degree of cross-infertility be associated +with that of any other form of isolation, then, not only will the two +segregating influences be added, but multiplied together. And thus, by +their mutual action and reaction, divergent evolution is promoted at a +rapidly increasing rate. + +I will now summarize the main points of the theory of physiological +isolation in a categorical form. + +1. If no other form of isolation be present, specific divergence can +only take place when some degree of cross-infertility has previously +arisen between two or more sections of a species. + +2. When such cross-infertility has arisen it may cause specific +divergence, either (_a_) by allowing independent variability in each of +the physiologically isolated groups; (_b_) by becoming associated with +any other cause of differentiation already operating; or (_c_) by both +these means combined. + +3. As some degree of cross-infertility generally obtains between allied +species, we are justified in concluding that this has been the most +frequent--or, at any rate, the most effective--kind of isolation where +the origin of species is concerned; and therefore the kind with which, +in the case of species-formation, natural selection, or any other cause +of specific divergence, has been most usually associated. + +4. Where varietal divergence has begun in the absence of +cross-infertility, such divergence seems, as a general rule, to have +been incapable of attaining to a specific value. + +5. Therefore, in the vast majority of such cases, it must have been +those varietal changes of structure, size, colour, &c., which happened +to have afterwards been assisted by the reproductive change that were on +this account _selected_ as successful candidates for specific +differentiation. + +6. It follows, that it makes no difference to the general theory of +physiological selection in what proportion of cases the physiological +change has been the initial change; for, whether prior or subsequent to +the varietal changes with which it becomes associated, its presence has +been equally important as a condition to specific divergence. + +7. When physiological isolation becomes associated with natural +selection, or any other form of homogamy, the segregative power of both +is augmented. Moreover, so great is the augmentation that even very +moderate degrees of physiological isolation--themselves capable of +effecting little or nothing--become very powerful when associated with +moderate degrees of any other kind of homogamy, and vice versa. + +8. The theory of physiological selection effectually explains the +divergent evolution of specific types and the cross-infertility of such +types when evolved. + + * * * * * + +To prevent, if possible, the continuance of certain misunderstandings +with regard to my original statement of the new theory, let me here +disclaim some views which have been assigned to me. They are: + +1. That the theory of physiological selection is opposed to the theory +of natural selection. Far from this being so, it is--at all events in my +own opinion--a very important aid to it, in preventing free +intercrossing on a common area, and thus allowing divergent evolution to +occur within that area. + +2. That, in advancing the theory of physiological selection as "an +additional suggestion on the origin of species," I wish to represent it +as being the originating cause of _all_ species. What I hold is, that +all species must have owed their origin to _isolation_, in some form or +other; but that as physiological selection is only one among many other +forms of isolation (including natural selection), and as it can only act +on common areas, a large number of species must have been formed without +its aid. + +3. That I imagine physiological varieties always to arise +"sporadically," or as merely individual "sports" of the reproductive +system. On the contrary, I expressly stated that this is _not_ the way +in which I suppose the "physiological variation" to arise, when giving +origin to a new species; but that it arises, whenever it is effectual, +as a "collective variation" affecting a number of individuals +simultaneously, and therefore characterizing "a whole race, or strain." + +4. That I suppose physiological selection always to act alone. This I +have never supposed. The essential point is, not that the physiological +isolation is unassociated with other forms of isolation, but that unless +associated with some degree of physiological isolation, no one of the +other forms is capable of originating species on common areas with any +approach to frequency. This proposition is the essence of the new +theory, and I take it to be proved, not only by general deductive +reasoning which shows that it _must_ be so, but also by the fact of an +otherwise inexplicable association between specific divergence on common +areas and some more or less considerable degree of mutual infertility. + + + + +CHAPTER IV. + +EVIDENCES OF PHYSIOLOGICAL SELECTION. + + +I will now give an outline sketch of the evidences in favour of the +theory which has been set forth in the preceding chapter, stating first +what is the nature of the verification which it requires. + +The theory is deduced from a highly general association between +distinctive specific characters of _any_ kind and a relatively constant +specific character of a _particular_ kind--namely, sexual exclusiveness. +For it is from this highly general association that the theory infers +that this relatively constant specific character has been at least one +of the needful conditions to the development of the other specific +characters with which it is found associated. Hence the necessary +verification must begin by showing the strength of the theory on these +merely deductive, or antecedent, grounds. It may then proceed to show +how far the facts of organic nature corroborate the theory in other and +independent ways. + +First, let it be carefully observed that here we have to do only with +the _fact_ of selective fertility, and with its _consequences_ as +supposed by the theory: we have nothing to do either with its _causes_ +or its _degrees_. Not with its causes, because in this respect the +theory of physiological selection is in just the same position as that +of natural selection: it is enough for both if the needful variations +are provided, without its being incumbent on either to explain the +causes which produce them. Not with its degrees, because, in the first +place, it can only be those degrees of variation which in particular +cases are supposed adequate to induce specific divergence, that fall +within the scope of the theory; and because, in the second place, +degrees which are adequate only to induce--or to assist in inducing, +_varietal_ divergence, must always tend to increase, or pass into higher +degrees. + + +_Antecedent Standing of the Theory._ + +The antecedent standing or logical basis of the theory has already been +in large measure displayed in the preceding chapter; for it was +impossible to state the theory without thereby showing in how +considerable a degree it is self-evident. A brief recapitulation is +therefore all that is here necessary. + +It has been shown that divergent or polytypic evolution on common areas +is inexplicable by natural selection alone. Hence the question arises: +What form of isolation has, under such circumstances, rendered possible +divergent evolution? In answer to this question the theory of +physiological selection suggests that variations in the reproductive +function occur in such a way as to isolate more or less perfectly from +each other different sections of a species. While cross-fertility +remains unimpaired among the members of each section, there is more or +less cross-infertility when members of either section mate with those of +the other. Thus a physiological barrier is interposed between the two +sections; and any divergences of structure, colouring, or instinct +arising in the members of either section will not in any way be affected +by such divergences as arise among the members of the other. + +In support of this suggestion, it has been shown in the preceding +chapter that the very general association of cross-infertility with +specific differentiation points most strongly to the inference that the +former has usually been an indispensable condition to the occurrence of +the latter. It cannot be denied that in many cases the specific +distinction is now maintained by means of that sexual isolation which +cross-infertility confers: it is therefore probable that such isolation +has been instrumental in securing its initial attainment. + +This probability is strengthened by the observed fact that the general +association in question is conspicuously absent in the case of +domesticated varieties, notwithstanding that their multitudinous and +diverse varietal characters usually equal, and frequently surpass, +specific characters in their degrees of divergence. + +Since, then, it would seem to be impossible for divergent evolution on +common areas to take place in the absence of some mode of isolation; +since cross-infertility appears to be the only possible mode under the +given circumstances; and since among domesticated varieties, where +isolation is otherwise secured by artificial means, cross-infertility is +usually absent, the logical foundations of the theory of physiological +selection would seem to be securely laid. + +We may therefore pass to more special lines of evidence. + + +_Evidence from Geographical Distribution._ + +Darwin has adduced very good evidence to show that large areas, +notwithstanding the disadvantages which (on his theory) must arise from +free intercrossing, are what he terms better manufactories of species +than smaller areas, such as oceanic islands. On the other hand, as a +matter of fact, oceanic islands are comparatively rich in peculiar +species. These two statements, however, are not incompatible. Smaller +areas are, as a rule, rich in peculiar species relatively to the number +of their inhabitants; but it does not follow that they are rich in +species as contrasted with larger areas containing very many more +inhabitants. Therefore, the rules are that large areas turn out an +absolutely greater number of specific types than small areas; although, +relatively to the number of individuals or amount of population, the +small areas turn out a larger number of species than the large areas. + +Now, these two complementary rules admit of being explained as Darwin +explains them. Small and isolated areas are rich in species relatively +to the amount of population, because, as we have before seen, this +population has been permitted to develop an independent history of its +own, shielded from intercrossing with parent forms, and from competition +with exotic forms; while, at the same time, the homogamy thus secured, +combined with change of environment, will give natural selection an +improved chance of finding new points of departure for its operation. On +the other hand, large and continuous areas are favourable to the +production of numerous species, first, because they contain a large +population, thus favouring the occurrence of numerous variations; and, +secondly, because the large area furnishes a diversity of conditions in +its different parts, as to food, climate, attitude, &c., and thus so +many different opportunities for the occurrence of sundry forms of +homogamy. Now, it is obvious that of all these sundry forms of homogamy, +physiological selection must have what may be termed a first-rate +opportunity of assisting in the manufacture of species on large areas. +For not only is it upon large and continuous areas that the antagonistic +effects of intercrossing are most pronounced (and, therefore, that the +influence of physiological selection must be most useful in the work of +species-making); but here also the diversity in the external conditions +of life, which the large area supplies to different parts of the +extensive population, cannot fail to furnish physiological selection +with a greater abundance of that particular variation in the +reproductive system on which its action depends. Again, and of still +more importance, on large areas there are a greater _number_ of species +already differentiated from one another as such; thus a greater number +of already sexually differentiated forms are presented for further +differentiation at the hands of physiological selection. For all these +reasons, therefore, we might have expected, upon the new theory, that +large and continuous areas would be good manufactories of species. + +Again, Darwin has shown that not only large areas, but likewise +"dominant" genera within those areas, are rich in species. By dominant +genera he meant those which are represented by numerous individuals, as +compared with other genera inhabiting the same area. This general rule +he explains by the consideration that the qualities which first led to +the form being dominant must have been useful; that these would be +transmitted to the otherwise varying offspring; and, therefore, that +when these offspring had varied sufficiently to become new species, they +would still enjoy their ancestral advantages in the struggle for +existence. And this, doubtless, is in part a true explanation; but I +also think that the reason why dominant genera are rich in species, is +chiefly because they everywhere present a great number of individuals +exposed to relatively great differences in their conditions of life: or, +in other words, that they furnish the best raw material for the +manufacture of species by physiological selection, as explained in the +last paragraph. For, if the fact of dominant genera being rich in +species is to be explained _only_ by natural selection, it appears to me +that the useful qualities which have already led to the dominance of the +ancestral type ought rather to have proved inimical to its splitting up +into a number of subordinate types. If already so far "in harmony with +its environment" as to have become for this reason dominant, one would +suppose that there is all the more reason for its not undergoing change +by the process of natural selection. Or, at least, I do not see why the +fact of its being in an unusual degree of harmony with its environment +should in itself constitute any unusual reason for its modification by +survival of the fittest. On the other hand, as just observed, I do very +plainly see why such a reason is furnished for the modifying influence +of physiological selection. + +Let us next turn to another of Darwin's general rules with reference to +distribution. He took a great deal of trouble to collect evidence of the +two following facts, namely, (1) that "species of the larger genera in +each country vary more frequently than the species of the smaller +genera"; and (2) that "many of the species included within the larger +genera resemble varieties in being very closely, but unequally, related +to each other, and in having restricted ranges[22]." By larger genera he +means genera containing many species; and he accounts for these general +facts by the principle, "that where many species of a genus have been +formed, on an average many are still forming." But _how_ forming? If we +say by natural selection alone, we should expect to find the +multitudinous species differing from one another in respect of features +presenting well-marked adaptive meanings; yet this is precisely what we +do not find. For Darwin's argument here is that "in large genera the +amount of difference between the species is often exceedingly small, so +that in this respect the species of the larger genera resemble varieties +more than do the species of the smaller genera." Therefore the argument, +while undoubtedly a very forcible one in favour of the fact of +_evolution_, appears to me scarcely consistent with the view of this +evolution being due solely to natural selection. On the other hand, the +argument tells strongly (though unconsciously) in favour of +physiological selection. For the larger a genus, or the greater the +number of its species, the greater must be the opportunity for the +occurrence of that particular kind of variation on which the principle +of physiological selection depends. The species of a genus may be +regarded as so many varieties which have already been separated from one +another physiologically; therefore each of them may now constitute a new +starting-point for a further and similar separation--particularly as, in +virtue of their previous segregation, many are now exposed to different +conditions of life. Thus, it seems to me, we can well understand why it +is that genera already rich in species tend to grow richer; while such +is not the case in so great a degree with genera that are poor in +species. Moreover, we can well understand that, multiplication of +species being as a rule, and in the first instance, determined by +changes in the reproductive system, wherever a large number of new +species are being turned out, the secondary differences between them +should be "often exceedingly small"--a general correlation which, so far +as I can see, we are not able to understand on the theory of natural +selection. + + [22] _Origin of Species_, pp. 44, 45. + +The two subsidiary facts, that very closely allied species have +restricted ranges, and that dominant species are rich in varieties, both +seem to tell more in favour of physiological than of natural selection. +For "very closely allied species" is but another name for species which +scarcely differ from one another at all except in their reproductive +systems; and, therefore, the more restricted their ranges, the more +certainly would they have become fused by intercrossing with one +another, had it not been for the barrier of sterility imposed by the +primary distinction. Or rather, I should say, had it not been for the +original occurrence of this barrier, these now closely-allied species +could never have become species. Again, that dominant species should be +rich in varieties is what might have been expected; for the greater the +number of individuals in a species, the greater is the chance of +variations taking place in all parts of the organic type, and +particularly in the reproductive system, seeing that this system is the +most sensitive to small changes in the conditions of life, and that the +greater the number of individuals composing a specific type, the more +certainty there is of some of them encountering such changes. Hence, the +richness of dominant species in varieties is, I believe, mainly due to +the greater opportunity which such species afford of some degree of +cross-infertility arising between their constituent members. + +Here is another general fact, also first noticed by Darwin, and one +which he experiences some difficulty an explaining on the theory of +natural selection. He says:-- + + In travelling from north to south over a continent, we generally + meet at successive intervals with closely-allied or representative + species, evidently filling the same place in the economy of the + land. These representative species often meet and interlock, and as + one becomes rarer and rarer, the other becomes more and more + frequent, till the one replaces the other. But if we compare these + species where they intermingle, they are generally as absolutely + distinct from each other in every detail of structure as are + specimens taken from the metropolis of each.... In the + intermediate region, having intermediate conditions of life, why do + we not now find closely-linking intermediate varieties? This + difficulty for a long time quite confounded me. But I think it can + in large part be explained[23]. + + [23] _Origin of Species_, ed. 6, pp. 134, 135. + +[Illustration:] + +His explanation is that, "as the neutral territory between two +representative species is generally narrow in comparison with the +territory proper to each, ... and as varieties do not essentially differ +from species, the same rule will probably apply to both; and, therefore, +if we take a varying species inhabiting a very large area, we shall have +to adapt two varieties to two large areas, and a third variety to a +narrow intermediate zone." It is hence argued that this third or +intermediate variety, on account of its existing in lesser numbers, will +probably be soon overrun and exterminated by the larger populations on +either side of it. But how is it possible "to adapt two varieties to two +large areas, and a third [transitional] variety to a narrow intermediate +zone," in the face of free intercrossing on a continuous area? Let _A_, +_B_, and _C_ represent the three areas in question. According to the +argument, variety _A_ passes first into variety _B_, and then into +variety _C_, while variety _B_ eventually becomes exterminated by the +inroads both from _A_ and _C_. But how can all this have taken place +with nothing to prevent intercrossing throughout the entire area _A_, +_B_, _C_? I confess that to me it seems this argument can only hold on +the supposition that the analogy between varieties and species extends +to the reproductive system; or, in a sense more absolute than the +argument has in view, that "varieties do not essentially differ from the +species" which they afterwards form, but from the first show some degree +of infertility towards one another. And, if so, we have of course to do +with the principles of physiological selection. + +That in all such cases of species-distribution these principles have +played an important part in the species-formation, appears to be +rendered further probable from the suddenness of transition on the area +occupied by contiguous species, as well as from the completeness of +it--i. e. the absence of connecting forms. For these facts combine to +testify that the transition was originally due to that particular change +in the reproductive systems of the forms concerned, which still enables +those forms to "interlock" without intercrossing. On the other hand, +neither of these facts appears to me compatible with the theory of +species-formation by natural selection alone. + +But this leads us to another general fact, also mentioned by Darwin, and +well recognized by all naturalists, namely, that closely allied species, +or species differing from one another in trivial details, usually occupy +contiguous areas; or, conversely stated, that contiguity of geographical +position is favourable to the appearance of species closely allied to +one another. Now, the large body of facts to which I here allude, but +need not at present specify, appear to me to constitute one of the +strongest of all my arguments in favour of physiological selection. +Take, for instance, a large continental area, and follow across it a +chain of species, each link of which differs from those on either side +of it by the minute and trivial distinctions of a secondary kind, but +all the links of which differ from one another in respect of the primary +distinction, so that no one member of the series is perfectly fertile +with any other member. Can it be supposed that in every case this +constant primary distinction has been superinduced by the secondary +distinctions, distributed as they are over different parts of all these +kindred organisms, and yet nowhere presenting any but a trifling amount +of morphological change? + +For my own part, I cannot believe--any more than Darwin could +believe--that all these numerous, diverse, and trivial changes have +always had the accidental effect of inducing the same peculiar change in +the reproductive system, and so producing it without any reference to +the process of specific divergence. Nor can I believe, as Darwin +incidentally and provisionally suggested, that prolonged exposure to +uniform conditions of life have so generally induced an equally +meaningless result. I can only believe that all the closely allied +species inhabiting our supposed continent, and differing from one +another in so many and such divers points of small detail, are merely so +many records of the fact that selective fertility has arisen among their +ancestry, and has thus given as many opportunities for the occurrence of +morphological differentiations as it has furnished cases of efficient +isolation. Of course, I do not deny that many, or probably most, of +these trivial morphological differentiations have been produced by +natural selection on account of their utility: I merely deny that they +could have been so produced on this common area, but for the sexual +isolation with which every distinct set of them is now found to be +associated. + + +_Evidence from Topographical Distribution of Species._ + +By topographical distribution I mean the distribution of organisms with +reference to comparatively small areas, as distinguished from larger +regions with reference to which the term geographical distribution is +appropriate. + +It will be at once apparent that a study of the topographical +distribution of organic types is of even more importance for us than a +study of their geographical distribution. For while the former study is +conducted, as it were, with a low power of our observing microscope, the +latter is conducted with a high power. The larger facts of geographical +distribution yield, indeed, all the general characters which we might +expect them to yield, on the theory that divergence of specific types on +common areas has been in chief part determined by physiological +conditions. But for the purpose of testing this theory in a still more +exacting manner, it is of the first importance to consider the more +detailed facts of topographical distribution, since we here come to +closer quarters with the problem of specific differentiation. Therefore, +as we have already considered this problem under the most general points +of view, we will now consider it under more special points of view. + + * * * * * + +It is self-evident, as we have seen in the preceding section, that the +greater the number of individuals of the same species on a given area, +the less must be the power of natural selection to split that species +into two or more allied types; because, the more crowded the population, +the greater must be the uniformitarian effect of free intercrossing. +This obvious fact has been insisted upon by several previous writers on +Darwinism; and the only reason why it has not been recognized by all +naturalists is that so few of them have observed the all-important +distinction between monotypic and polytypic evolution. The denser the +population, and therefore the greater the intercrossing and the severer +the struggle for existence within the species, the better will it be for +_transmutation_ of the species by natural selection; but the worse it +will be for _differentiation_ of the species by this form of homogamy. +On the other hand, if physiological selection be entertained as a form +of homogamy, the denser the population, the better opportunity it will +have of differentiating the species, first, because a greater number of +individuals will be present in which the physiological change may arise, +and, secondly, because, if it does arise, the severity of the struggle +for existence will _then_ give natural selection a better chance of +acting rapidly and effectually on each of the isolated sections. + +Hence, where the question is whether selective fertility has played any +large or general part in the differentiation of specific types, the best +criterion we can apply is to ascertain whether it is a general rule that +closely allied species occur in intimate association, so that their +individual members constitute, as it were, a single population, or, on +the other hand, whether they occur rather on different sides of +physical barriers. If they occur intimately associated, the form of +homogamy to which their differentiation was due must have presumably +been the physiological form; whereas, if they are proved to be +correlated with physical barriers, the form of homogamy which was +concerned in their differentiation must presumably have been the +geographical form. + +Now, at first this consideration was a trouble to me, because Moritz +Wagner had strenuously argued--and supported his argument by a +considerable wealth of illustration--that allied species are always +found correlated with physical barriers or discontinuous areas. +Weismann's answer, indeed, had shown that Wagner's statement was much +too general: nevertheless, I was disappointed to find that so much could +be said in favour of the geographical (or topographical) form of +isolation where closely allied species are concerned. Subsequently, +however, I read the writings of Nägeli on this subject, and in them I +find a very different state of matters represented. + +Seeing as clearly as Wagner that it is impossible under any +circumstances for natural selection to cause specific _differentiation_ +unless assisted by some other forms of homogamy, but committing the same +oversight as Wagner and Weismann in supposing that the only other form +of homogamy in nature is geographical isolation, Nägeli, with great +force of reasoning, and by many examples, founded his argument against +the theory of natural selection on the ground that in the vegetable +kingdom closely allied species are most frequently found in intimate +association with one another, not, that is to say, in any way isolated +by means of physical barriers. This argument is everywhere logically +intact; and, as he sustains it by a large knowledge of topographical +botany, his indictment against natural selection as a cause of specific +_differentiation_ appeared to be insurmountable. And, in point of fact, +it _was_ insurmountable; so that the whole problem of the origin of +species by _differentiation on common areas_ has hitherto been left in +utter obscurity. Nor is there now any escape from this obscurity, unless +we entertain the "supplementary factor" of selective fertility. And, +apparently, the only reason why this has not been universally +recognized, is because Darwinians have hitherto failed to perceive the +greatness of the distinction between the _differentiation_ and the +_transmutation_ of species; and hence have habitually met such +overwhelming difficulties as Nägeli presented by an illogical +confounding of these two totally distinct things. + +But if the idea of selective fertility had ever occurred to Nägeli as a +form of segregation which gives rise to specific differentiation, I can +have no doubt that so astute and logical a thinker would have perceived +that his whole indictment against natural selection was answered. For it +is incredible that he should not have perceived how this physiological +form of homogamy (supposing it to arise _before_ or _during_, and not +_after_ the specific differentiation) would perform exactly the same +function on a continuous area, as he allowed that "isolation" does on a +discontinuous one. + +However, be this as it may, there cannot be any question touching the +immense value of his facts and arguments as evidence in favour of +physiological selection--albeit this evidence was given unconsciously, +or, as it were, prophetically. Therefore I will here quote a few +examples of both, from his paper _Du Développement des Espèces +Sociales_[24]. + + [24] _Archives des Sciences physiques et naturelles_ (Genève), vol. + liii. (1875), pp. 211-236. + +After stating the theory of natural selection, he says that if the +theory is (of itself) a true explanation of the origin (or divergence) +of specific forms, it ought to follow that + + two closely allied forms, derived the one from the other, would + necessarily occupy two different geographical areas [or + topographical stations], since otherwise they would soon become + blended. Until they had already become sufficiently consolidated as + distinct species to render mutual intercrossing highly improbable, + they could not be intermingled without disadvantage [to + differentiation]. Had Darwin endeavoured to support his hypothesis + by facts, he would, at least in the vegetable kingdom, have found + little to favour his cause. I can cite many hundreds of cases, in + which species in every stage of development have been found closely + mingling with one another, and not in any way isolated. Therefore, + I do not think that one can rightly speak of natural selection in + the Darwinian sense in the vegetable kingdom; and, in my + estimation, there is a great difference between the formation of + species by nature and the production of stock by a breeder.... (p. + 212). + + Of the two kinds of distribution (i. e. growing apart and growing + together), Synoicy (or growing together) is by far the most usual + in nature. I reckon that out of a hundred allied vegetable forms, + at least ninety-five would be found to be synoical (p. 219). + +This is a most important point. That so enormous a proportion of +vegetable species should have originated in intimate association with +their parent or sister types, is clearly unintelligible on the theory of +natural selection alone; there obviously _must_ be some other form of +homogamy which, whether or not in all places _associated_ with natural +selection, is the primary condition to the differentiation. Such I hold +with Nägeli, is a logical necessity; and this whether or not I am right +in believing the other form of homogamy in question to be selective +fertility. But I go further and say, Surely there can be no rational +question that this other form of homogamy must have been, at any rate as +a highly general rule, the one which I have assigned. For how is it that +in these ninety-five per cent. of cases, where vegetable species are +growing intimately associated with their nearest allies, there is no +hybridizing, or blending and relapsing to the original undifferentiated +types? We know well the answer. These are fully differentiated species, +and, as such, are protected from mutual intercrossing by the barrier of +mutual sterility. But now, if this bar is thus necessary for preserving +the specific distinctions when they have been fully developed, much more +must it have been so to admit of their development; or, otherwise +stated, since we know that this barrier is associated with "synoical" +species, and since we clearly perceive that were it withdrawn these +species would soon cease to exist, can we reasonably doubt that their +existence (or origin) is due to the previous erection of this barrier? +If synoical species were comparatively rare, the validity of such +reasoning might be open to question; or, even if we should not doubt it +in such cases, at any rate we might well doubt the importance or extent +of selective fertility as a factor in the origination of species. But +the value of Nägeli's writings on the present subject consists in +showing that synoical species constitute so overwhelming a majority of +the vegetable kingdom, that here, at all events, it appears impossible +to rate too highly the importance of the principle I have called +physiological selection. + + + + +CHAPTER V. + +FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION. + + +_Evidence from Topographical Distribution of Varieties._ + +In the last section we have considered the topographical distribution of +closely allied _species_. I now propose to go still further into matters +of detail, by considering the case of natural _varieties_. And here we +come upon a branch of our inquiry where we may well expect to meet with +the most crucial tests of our theory. For if it should appear that these +nascent species more or less resemble fully developed species in +presenting the feature of cross-infertility, the theory would be +verified in the most direct and conclusive manner possible. These +nascent species may be called embryo species, which are actually in +course of differentiation from their parent-type; and therefore, if they +do not exhibit the feature in relation to that type which the present +theory infers to be necessary for the purposes of differentiation, the +theory must be abandoned. On the other hand, if they do exhibit this +feature, it is just the feature which the theory predicted as one that +would be found highly characteristic of such embryo types. +Contrariwise, the theory of natural selection can have no reason to form +any such anticipation; or rather its anticipation would necessarily +require to be the exact opposite. For, according to this theory, the +cross-infertility of allied species is due, either to correlation with +morphological changes which are being produced by the selection, or +else, as Darwin supposed, to "prolonged exposure to uniform conditions +of life"; and thus, in either case, the sterility variation ought to be, +as a general rule at all events, subsequent to the specific +differentiation, and, according to Darwin's view, _long_ subsequent. +Thus we ought not to find that the physiological change is ever, on any +large or general scale, the initial change; nor ought we to find that it +is, on any such scale, even so much as a contemporary change: there +ought, in fact, to be no constant or habitual association between +divergence of embryo-types and the concurrence of cross-infertility. + +Now, it will be my endeavour to prove that there is an extraordinarily +general association between _varietal_ divergence and cross-infertility, +_wherever common areas are concerned_; and in as far as this can be +proved, I take it that the evidence will make wholly in favour of +physiological selection as the prime condition to specific divergence, +while at the same time they will make no less wholly, _and quite +independently_, against natural selection as the unaided cause of such +divergence. + +I shall begin with some further quotations from Nägeli. + + Species may be synoical at all stages of relationship. We come + across varieties, scarcely distinguishable from one another, + growing in the same locality (as, for example, the _Cirsium + heterophyllum_, with smooth or jagged leaves, the _Hieracium + sylvaticum_, with or without caulinary leaves); again, we meet + other varieties more accentuated (as the _H. hoppeanum_, with under + ligules of white or red, the _Campanula_, with white or lilac + flowers, &c.), other varieties even more marked, which might almost + be elevated to the rank of species (_Hieracium alpinum_, with hairs + and glands, and the new form _H. holadenium_, which has only + glands, _Campanula rotundifolia_ with smooth and hairy leaves), or + forms still more distinct, up to well-defined species. I could + enumerate endless examples at all stages. + + It will be seen that in my definition of synoicy I do not mean to + assert that _all_ allied forms are invariably found together, but + that they are much more often seen in groups than singly. Take, for + instance, nine forms closely related (_A_ to _I_). _A_, _E_, _H_ + will be found side by side at one point, _B_, _D_ at another, _C_, + _F_ at a third, &c. These facts are plainly opposed to the theory + of isolation and amixia, and make, on the contrary, in favour of + the social development of species (_loc. cit._, p. 221). + +Not to multiply quotations to the same general effect, I will supply but +one other, referring to a particular case. + + At one spot (_Rothwand_) much exposed to the sun, and difficult of + access, I remarked two closely allied forms, so nearly related to + _H. villosum_ that this would seem to be an intermediary form + between the two. One of these (_H. villosissimum_) is distinguished + by its tongue and thick pubescence, its tolerably large capitula, + and by the lengthened and separated scales of the involucrum; the + other, on the contrary (_H. elongatum_), is less pubescent, has + smaller capitula, and more compact scales on the involucrum than + _H. villosum_. Both are finally distinguishable from the type by + their longer stalks, which are more decidedly aphyllous, and by + their later flowering. At the spot where I found them the two forms + were closely intermingled, and each was represented by a + considerable number of plants. I did not find them anywhere else on + the mountain, nor could I find at the spot where these were growing + a single specimen of the true _H. villosum_, nor a single hybrid + from these two. + + I concluded that these two new forms had, by joining their forces, + expelled the _H. villosum_ from its primitive abode, but had not + succeeded in displacing one another. As to their origin, they had + evidently developed in two different directions from a common point + of departure, namely _H. villosum_. They had succeeded, not only in + separating themselves from the original form, but also in + preventing any intermediary form from interposing. I thought myself + therefore justified in considering this as a case of varieties + which have come into existence subsequently to the Glacial epoch. + The morphological characteristics of the three forms are + sufficiently distinct for them to be designated as species by a + good many writers. They are better defined than some of MM. Frolich + and Fries' weaker species, and as well defined as some of MM. Koch + and Grisebach's (p. 222). + +Now it is clear, without comment, that all this is exactly as it ought +to be, if allied species have been differentiated on common areas by +selective fertility. For if, as Nägeli elsewhere says, "one meets forms +in nature associated with one another, and severally distinguished by +every possible degree of differentiation," not only as Nägeli adds, does +this general fact lead to the inference that species are (usually) +developed when plants grow intimately associated together; but as +certainly it leads to the further inference that such development must +be due to a prior development of cross-infertility between the diverging +varietal forms, cross-infertility which is therefore afterwards so +characteristic of the allied species, when these are found, in their +fully differentiated condition, still occupying the same area in large +and intimately mingled populations. + +To my mind there could not be any inference more strongly grounded than +this, because, with the one exception of the physiological form, no +other form of homogamy can be conceived which shall account for the +origin and permanence of these synoical varieties, in all degrees of +differentiation up to well-defined synoical species. Least of all, as we +have seen, can natural selection alone have had anything to do with such +a state of matters; while, as we have likewise seen, in all its details +it is exactly the state of matters which the theory of physiological +selection requires. + +Nevertheless, although this inference is so strongly grounded, we ought +to remember that it is only an inference. In order fully to verify the +theory of physiological selection, we ought to prove by experiment the +fact of cross-infertility between these synoical varieties, as we learn +that it afterwards obtains between synoical species. It is to be +regretted that the theory of physiological selection did not occur to +the mind of Nägeli, because he would then, no doubt, have ascertained +this by actual experiment. As it is, the great value of his observations +goes no further than establishing a strong presumption, that it _must_ +be selective fertility which causes the progressive differentiation of +synoical varieties; and also that, if so, this _must_ be the principal +factor in the differentiation of vegetable species, seeing that some +ninety-five per cent. are of synoical origin. + + +_Evidence from Experimental Research._ + +My paper on _Physiological Selection_ pointed out that the whole theory +would have to stand or fall with the experimental proof of the presence +or the absence of cross-infertility between varieties of the same +species growing on common areas. From the facts and considerations which +we have hitherto been dealing with, it did indeed appear to me that +there was the strongest conceivable ground for inferring that +cross-infertility between such varieties would be found by experiment to +be a phenomenon of highly general occurrence--amply sufficient ground to +prove that allied species on common areas for the most part owed their +origin to this character of mutual sterility, and not vice versa as +previously supposed. At that time I was not aware that any experiments +had been made in this direction. Soon after the paper was published, +however, my attention was directed to a laborious research which had +been directed to this very point, and carried on for more than thirty +years, by M. Jordan[25]. This had not attracted the general notice which +it undoubtedly deserved; and I have since ascertained that even Darwin +began to look into it only a few months before his death. + + [25] _Remarques sur le fait de l'existence en société à l'état + sauvage des espèces végétales affines et sur d'autres faits relatifs + à la question de l'espèce_, par Alexis Jordan; lues au congrès de + l'Association Française pour l'Avancemeat des Sciences, 2^me + session, Lyon, séance de 28 Août, 1873. + +Having devoted his life to closely observing in divers stations +multitudes of different species of plants--annuals and perennials, +bulbous and aquatic, trees and shrubs--M. Jordan has been able to +satisfy himself, and the French school of botanists to which this line +of observation has given rise, that in most cases (or "nearly +everywhere"), when a Linnean species is indigenous to a country and is +there of common occurrence, this species within that district is +represented by more or less numerous and perfectly constant varieties. +These varieties are constituted by such minute differences of +morphological character that their very existence eluded the +observation of botanists, until M. Jordan began to search specially for +them as the special objects of his scrutiny. Moreover, these varieties +of a Linnean species occupy common areas, and there grow in intimate +association with one another, or as M. Jordan says, "_pêle-mêle_." So +far, be it noticed, Jordan was proceeding on exactly the same lines as +Nägeli; only he carried his observations over a still wider range of +species on the one hand, and into a still minuter search for varieties +on the other. But the all-important point for us is, that he further +proceeded to test by experiment the physiological relations between +these morphological varieties; and found, in many hundreds of cases, +that they not only came true to seed (i. e. are hereditary and not +merely climatic), but likewise cross-sterile _inter se_. For these +reasons, M. Jordan, who is opposed to the theory of evolution, regards +all such varieties as separately created species; and the inspiring +motive of his prolonged investigations has been a desire to multiply +these proofs of creative energy. But it clearly makes no difference, so +far as evolutionists are concerned with them, whether all this multitude +of sexually isolated forms be denominated species or varieties. + +The points which are of importance to evolutionists--and of the first +order of importance in the present connexion--may be briefly summarized +as follows:-- + +(1) The research embraces large numbers of species, belonging to very +numerous and very varied orders of plants; (2) in the majority of +cases--although not all--indigenous species which are of common +occurrence present constant varieties; (3) these varieties, +nevertheless, may be morphologically so slight as to be almost +imperceptible; (4) they occupy common areas and grow in intimate +association; (5) although many of them have undergone so small an amount +of morphological change, they have undergone a surprising amount of +physiological change; for (6) not only do very many of these varieties +come true to seed; but, (7) when they do, they are always more or less +cross-infertile _inter se_. + +Now, it is self-evident that every one of these seven points is exactly +what the theory of physiological selection requires, while there is not +one of them which it does not require. For if the theory be sound, we +should expect to find large numbers of species belonging to numerous and +varied orders of plants presenting constant varieties on common areas; +we should expect this to be a highly general, though not a universal, +rule; and we should expect it to apply only to species which are +indigenous. Moreover, we should expect these varieties, although but +slightly differentiated morphologically, to present a great +differentiation physiologically--and this in the special direction of +selective fertility, combined, of course, with heredity. + +On the other hand, as I have said, this catalogue of evidences leaves +nothing to be supplied. It gives us all the facts--and no more than all +the facts--which my paper on _Physiological Selection_ anticipated as +the eventual result of a prolonged experimental research. And if I have +to regret my ignorance of these facts when that paper was published, at +any rate it now furnishes the best proof that my anticipations were not +guided by the results of a verification which had already been supplied. +These anticipations were deduced exclusively from the theory itself, as +representing what _ought_ to be the case if the theory were true; and, I +must confess, if I had then been told that they had already been +realized--that it had actually been found to be a general rule that +endemic species present constant and hereditary varieties, intimately +commingled on common areas, morphologically almost indistinguishable, +but physiologically isolated by selective fertility--I should have felt +that the theory had been verified in advance. For there are only two +alternatives: either these things are due to physiological selection, or +else they are due--as M. Jordan himself believes--to special creation. +Which is equivalent to saying that, for evolutionists, the facts must be +held to verify the former theory in as complete a manner as it is +logically possible for the theory to be verified. + + +_Evidence from Prepotency._ + +We have now to consider the bearing of what is called "prepotency" on +the theory of physiological selection. + +Speaking of the vast number of species of Compositae, Darwin says:-- + + There can be no doubt that if the pollen of all these species could + be simultaneously or successively placed on the stigma of any one + species, this one would elect with unerring certainty its own + pollen. This elective capacity is all the more wonderful, as it + must have been acquired since the many species of this great group + of plants branched off from a common progenitor. + +Darwin is here speaking of elective affinity in its fully developed +form, as absolute cross-sterility between fully differentiated species. +But we meet with all lower degrees of cross-infertility--sometimes +between "incipient species," or permanent varieties, and at other times +between closely allied species. It is then known as "prepotency" of the +pollen belonging to the same variety or species over the pollen of the +other variety or species, when both sets of pollen are applied to the +same stigma. Although in the absence of the prepotent pollen the less +potent will fertilize the seed, yet, such is the appetency for the more +appropriate pollen, that even if this be applied to the stigma some +considerable time after the other, it will outstrip or overcome the +other in fertilizing the ovules, and therefore produce the same result +on the next generation as if it had been applied to the mother plant +without any admixture of the less potent pollen, although in some cases +such incipient degrees of cross-infertility are further shown by the +number or quality of the seeds being fewer or inferior. + +Now, in different varieties and in different allied species, all degrees +of such prepotency have been noticed by many observers, from the +faintest perceptible amount up to complete impotency of the alien +pollen--when, of course, there is absolute sterility between the two +varieties or allied species. The inference is obvious. In this graduated +scale of prepotency--beginning with an experimentally almost +imperceptible amount of sexual differentiation between two varieties, +and ending in an absolute partitioning of two allied species--we have +the only remaining fact that is required to complete the case in favour +of the present theory. We are here brought back to the very earliest +stages of physiological differentiation or to the stages which lie +behind Jordan's "Physiological Species"; and therefore, when taken in +conjunction with his results, the phenomena of prepotency may be said to +give us the complete and final demonstration of one continuous +development, which, beginning in an almost imperceptible amount of +cross-infertility, ends in absolute cross-sterility. The "elective +capacity" to which Darwin alludes as having been "acquired" by all the +species of Compositae since they "branched off from a common +progenitor," is thus seen among innumerable other species actually in +process of acquisition; and so we can perfectly well understand, what is +otherwise unintelligible, that closely allied species of plants occur, +in ninety-five per cent. of cases, intimately associated on common +areas, while exhibiting towards one another the character of mutual +sterility. + +But more than this. The importance of the widespread phenomena of +prepotency to the theory of physiological selection does not consist +merely in thus supplying the last link in the chain of evidence touching +the origin of species by selective fertility, or "elective capacity." +These phenomena are of further importance as showing how in plants, at +all events, physiological selection appears to be frequently capable of +differentiating specific types without the necessary assistance of any +other form of homogamy. In my original statement of the theory, I was +careful to insist upon the great value, as differentiating agents, of +even small degrees of other forms of homogamy when co-operating with +physiological selection. But I also stated my belief that in many cases +selective fertility is presumably of itself capable of splitting a +specific type; and the reason why I still believe this is, that I do not +otherwise understand these phenomena of prepotency. I cannot believe +that in all the innumerable cases where they arise, they have been +super-induced by some prior morphological changes going on in some other +part of the organism, or by "prolonged exposure to uniform conditions of +life," on the part of two well-nigh identical forms which have arisen +intimately commingled in exactly the same environment, and under the +operation of a previously universal intercrossing. Even if such a thing +could be imagined as happening occasionally, I feel it difficult to +imagine that it can happen habitually, and yet this view must be held by +those who would attribute prepotency to natural selection. + +It must never be forgotten that the relatively enormous changes as to +size, structure, habit, &c., which are presented by our domesticated +plants as results of artificial selection, do not entail the +physiological character of cross-sterility in any degree, save possibly +in some small number of cases. Although in wild species any +correspondingly small percentage of cases (where natural selection +happens to hit upon parts of the organism modifications of which produce +the physiological change by way of correlation) would doubtless be the +ones to survive on common areas, still it is surely incredible that such +an accidental association between natural selection and +cross-infertility is so habitually the means of specific differentiation +as the facts of prepotency (together with the observations of Jordan +and Nägeli) would necessarily demand. + +Moreover, this view of the matter is still further corroborated by +certain other facts and considerations. For example, the phenomena of +prepotency (whether as between varieties or between closely allied +species) are found to occur when the two forms occupy a common area, +i.e. are growing intermingled with one another. Therefore, but for this +physiological differentiation, there could be absolutely nothing to +prevent free intercrossing. Yet the fact that hybrids are so +comparatively rare in a state of nature--a fact which Sir Joseph Hooker +has pointed out to me as otherwise inexplicable--proves the efficacy of +even a low degree of such differentiation in preventing the +physiologically-differentiated forms from intercrossing. Even in cases +where there is no difficulty in producing artificial hybrids or mongrels +between species or varieties growing on common areas, it is perfectly +astonishing what an extremely small percentage of the hybrid or mongrel +forms are found to occur in nature. And there can be no question that +this is due to the very efficient manner in which prepotency does its +work--efficient, I mean, from the point of view of the new theory; for +upon any other theory prepotency is a meaningless phenomenon, which, +notwithstanding its frequent occurrence, plays no part whatever in the +process of organic evolution. + +I attach considerable importance to the phenomena of prepotency in view +of the contrast which is presented between plants and animals in the +relation of their species to physical barriers. For animals--and +especially the higher animals--appear to depend for their specific +differentiations upon such barriers much more than in the case with +plants. This is no more than we should expect; for, in accordance with +our theory, selective fertility is not so likely to work alone in the +case of the higher animals which mate together, as in plants which are +fertilized through the agency of wind or insects. In the former case +there is no opportunity given for the first rise of cross-infertility, +in the form of prepotency; and even where selective fertility has gained +a footing in other ways, the chances against the suitable mating of +"physiological complements" must be much greater than it is in the +latter case. Hence, among the higher animals, selective fertility ought +much more frequently to be found in association with other forms of +homogamy than it is among plants. And this is exactly what we find. Thus +it seems to me that this contrast between the comparative absence and +presence of physical barriers, where allied species of plants and of +higher animals are respectively concerned, is entitled to be taken as a +further corroboration of our theory. For while it displays exactly such +a general correlation as this theory would expect, the correlation is +one which cannot possibly be explained on any other theory. It is just +where physiological selection can be seen to have the best opportunity +of acting (viz. in the vegetable kingdom) that we find the most +unequivocal evidence of its action; while, on the other hand, it is just +where it can be seen to have the least opportunity of asserting itself +(viz. among the higher animals) that we find it most associated with, +and therefore assisted by, other forms of homogamy, i. e. not only +geographical isolation, but also by sexual preference in pairing, and +the several other forms of homogamy, which Mr. Gulick has shown to arise +in different places as the result of intelligence. + + +_Evidence from Special Cases._ + +Hitherto I have been considering, from the most general point of view, +the most widespread facts and broadest principles which serve to +substantiate the theory of physiological selection. I now pass to the +consideration of one of those special cases in which the theory appears +to have been successfully applied. + +Professor Le Conte has adduced the fossil snails of Steinheim as serving +to corroborate the theory of physiological selection[26]. + + [26] _Evolution and its Relations to Religious Thought_, &c. pp. + 236-7. + +The facts are these. The snail population of this lake remain for a long +time uniform and unchanged. Then a small percentage of individuals +suddenly began to vary as regards the form of their shells, and this in +two or three directions at the same time, each affected individual, +however, only presenting one of the variations. But after all these +variations had begun to affect a proportionally large number of +individuals, some individuals occur in which two or more of the +variations are blended together, evidently, as Weismann says, by +intercrossing of the varieties so blended. Later still, both the +separate varieties and their blended progeny became more and more +numerous, and eventually a single blended type, comprising in itself all +the initial varieties, supplanted the parent form. Then another long +period of stability ensued until another eruption of new variations took +place; and these variations, after having affected a greater and greater +number of individuals, eventually blended together by intercrossing and +supplanted their parent form. So the process went on, comparatively +short periods of variation alternating with comparatively long periods +of stability, the variations, moreover, always occurring suddenly in +crops, then multiplying, blending together, and in their finally blended +type eventually supplanting their parent form. + +Now, the remarkable fact here is that whenever the variations arose, +they only intercrossed between themselves, they did not intercross with +their parent form; for, if they had, not only could they never have +survived (having been at first so few in number and there having been no +geographical barriers in the small lake), but we should have found +evidence of the fact in the half-bred progeny. Moreover, natural +selection can have had nothing to do with the process, because not only +are the variations in the form of the shells of no imaginable use in +themselves; but it would be preposterous to suppose that at each of +these "variation periods" several different variations should always +have occurred simultaneously, all of which were of some hidden use, +although no one of them ever occurred during any of the prolonged +periods of stability. How, then, are we to explain the fact that the +individuals composing each crop of varieties, while able to breed among +themselves, never crossed with their parent form? These varieties, each +time that they arose, were intimately commingled with their parent +form, and would certainly have been reabsorbed into it had intercrossing +in that direction been possible. With Professor Le Conte, therefore, I +conclude that there is only one conceivable answer to this question. +Each crop of varieties must have been _protected from intercrossing with +their parent form_. + +They must have been the result of a variation, which rendered the +affected individuals sterile with their parent form, whilst leaving them +fertile amongst themselves. The progeny of these individuals would then +have dispersed through the lake, physiologically isolated from the +parent population, and especially prone to develop secondary variations +as a direct result of the primary variation. Thus, as we might expect, +two or three variations arose simultaneously, as expressions of so many +different lines of family descent from the original or physiological +variety; these were everywhere prevented from intercrossing with their +parent form, yet capable of blending whenever they or their +ever-increasing progeny happened to meet. Thus, without going into +further details, we are able by the theory of physiological selection to +give an explanation of all these facts, which otherwise remain +inexplicable. + + * * * * * + +In view of the evidence which has now been presented, I will now ask +five questions which must be suitably answered by critics of the theory +of physiological selection. + +1. Can you doubt that the hitherto insoluble problem of inter-specific +sterility would be solved, supposing cross-infertility were proved to +arise before or during the process of specific differentiation, instead +of after that process had been fully completed? + +2. Can you doubt, after duly considering the circumstances under which +allied species of plants have been differentiated--viz. in ninety-five +per cent. of cases intimately commingled on common areas, and therefore +under identical environments--that cross-infertility _must_ have arisen +before or during the specific differentiation? + +3. Can you doubt, after duly considering the facts of prepotency on the +one hand and those of Jordan's physiological varieties on the other, +that cross-infertility _does_ arise before or during the specific +differentiation? + +4. If you cannot express a doubt upon any of these points, can you +explain why you refuse to accept the theory of the origin of species by +means of physiological selection, together with the explanation which +this theory affords of the continued cross-fertility of domesticated +varieties? + +5. Supposing this theory to be true, can you conceive of any other +classes of facts which, either quantitatively or qualitatively, could +more directly or more effectually prove its truth than those which have +now been adduced? + +On these five heads I entertain no doubt. I am convinced that the theory +of physiological selection is the only one that can explain the facts of +inter-specific sterility on the one hand, and, on the other hand, the +contrast which these facts display to the unimpaired fertility of our +domesticated varieties. + +In conclusion, it seems desirable once more to insist that there is no +antagonism or rivalry between the theories of natural and of +physiological selection. For which purpose I will quote the final +paragraph of my original paper. + + So much, then, for the resemblances and the differences between the + two theories. It only remains to add that the two are + complementary. I have already shown some of the respects in which + the newer theory comes to the assistance of the older, and this in + the places where the older has stood most in need of assistance. In + particular, I have shown that segregation of the fit entirely + relieves survival of the fittest from the difficulty under which it + has hitherto laboured of explaining why it is that sterility is so + constantly found between species, while so rarely found between + varieties which differ from one another even more than many + species; why so many features of specific distinction are useless + to the species presenting them; and why it is that incipient + varieties are not obliterated by intercrossing with parent forms. + Again, we have seen that physiological selection, by preventing + such intercrossing, enables natural selection to promote diversity + of character, and thus to evolve species in ramifying branches + instead of in linear series--a work which I cannot see how natural + selection could possibly perform unless thus aided by physiological + selection. Moreover, we have seen that although natural selection + alone could not induce sterility between allied types, yet when + this sterility is given by physiological selection, the forms which + present it would be favoured in the struggle for existence; and + thus again the two principles are found playing, as it were, into + each other's hands. And here, as elsewhere, I believe that the + co-operation enables the two principles to effect very much more in + the way of species-making than either of them could effect if + working separately. On the one hand, without the assistance of + physiological selection, natural selection would, I believe, be all + but overcome by the adverse influences of free + intercrossing--influences all the more potent under the very + conditions which are required for the multiplication of species by + divergence of character. On the other hand, without natural + selection, physiological selection would be powerless to create any + differences of specific type, other than those of mutual sterility + and trivial details of structure, form, and colour--differences + wholly without meaning from a utilitarian point of view. But in + their combination these two principles appear to me able to + accomplish what neither can accomplish alone--namely, a full and + satisfactory explanation of the origin of species. + + + + +CHAPTER VI. + +A BRIEF HISTORY OF OPINIONS ON ISOLATION AS A FACTOR OF ORGANIC +EVOLUTION. + + +This historical sketch must begin with a consideration of Darwin's +opinions on the subject; but as these were considerably modified from +time to time during a period of thirty years by the publications of +other naturalists, it will be impossible to avoid cross-references as +between his writings and theirs. It may also be observed that the _Life +and Letters of Charles Darwin_ was not published until the year 1887, so +that the various opinions which I shall quote from the letters, and +which show some considerable approximation in his later years to the +views which have been put forward by Mr. Gulick and myself, were not +before us at the time when our papers were read. + +The earliest allusion that I can find to geographical isolation in the +writings of Darwin occurs in a correspondence with Sir Joseph Hooker, as +far back as 1844. He there says:-- + + I cannot give my reasons in detail; but the most general conclusion + which the geographical distribution of all organic beings appears + to me to indicate is, that isolation is the chief concomitant or + cause of the appearance of _new_ forms (I well know there are some + staring exceptions)[27]. + + [27] _Life and Letters_, vol. ii. p. 28. + +And again:-- + + With respect to original creation or production of new forms, I + have said that isolation appears the chief element[28]. + + [28] _Ibid._ + +Next, in the earlier editions of the _Origin of Species_ this view is +abandoned, and in its stead we meet with the opinion that geographical +isolation lends a certain amount of assistance to natural selection, by +preventing free intercrossing. But here we must note two things. First, +the distinction between monotypic and polytypic evolution is not +defined. Secondly, the levelling effect of free intercrossing in nature, +and hence its antagonism to divergence of character by natural +selection, is not sufficiently recognized; while, on the other hand, and +in consequence of this, the importance of isolation as a factor of +evolution is underrated--not only in its geographical, but likewise in +all its other forms. + +Taking these two points separately, the only passages in Darwin's +writings, so far at least as I can find, in which any distinction is +drawn between evolution as monotypic and polytypic, are those in which +he deals with a somewhat analogous distinction between artificial +selection as intentional and unconscious. He says, for example:-- + + In the case of methodical selection, a breeder selects for some + definite object, and if the individuals be allowed freely to + intercross, his work will completely fail. But when many men, + without intending to alter the breed, have a nearly common + standard of perfection, and all try to procure and breed from the + best animals, improvement surely but slowly follows from this + unconscious process of selection, notwithstanding that there is no + separation of selected individuals. Thus it will be under + nature[29]. + + [29] _Origin of Species_, p. 80, 6th ed. (1872). + +Here we have what may perhaps be regarded as a glimmering of the +distinction between monotypic and polytypic evolution. But that it is +only a glimmering is proved by the immediately ensuing sentences, which +apply this analogy of unconscious selection _not_ to the case of +monotypic, _but_ to that of polytypic evolution. So likewise, in the +succeeding discussion on "divergence of character," the analogy is again +resorted to for the purpose of showing how polytypic evolution may occur +in nature. + +Thus far, then, it may be said that we have scarcely so much as a +glimmering of the distinction between monotypic and polytypic evolution; +and as the same discussion (with but a few verbal alterations) runs +through all the editions of the _Origin_, it may well be asked why I +should have alluded to such passages in the present connexion. Well, I +have done so because it is apparent that, during the last years of his +life, the distinction between selection as "methodical" and +"unconscious" enabled Darwin much more clearly to perceive that between +evolution as monotypic and polytypic. Thus in 1868 he wrote to Moritz +Wagner (who, as we shall presently see, entirely failed to distinguish +between monotypic and polytypic evolution), expressing his belief-- + + That in many large areas all the individuals of the same species + have been slowly modified, in the same manner, for instance, as the + English racehorse has been improved, that is, by the continued + selection of the fleetest individuals, without any separation. But + I admit that by this process two or more new species could hardly + be formed within the same limited area[30]. + + [30] _Life and Letters_, vol. iii. p. 158. + +Again, in 1876 he wrote another letter to Wagner, in which the following +passage occurs:-- + + I believe that all the individuals of a species can be slowly + modified within the same district, in nearly the same manner as man + effects by what I have called the process of unconscious selection. + I do not believe that one species will give birth to two or more + new species as long as they are mingled together within the same + district[31]. + + [31] _Ibid._ p. 159. + +Two years later he wrote to Professor Semper:-- + + There are two different classes of cases, it appears to me, viz. + those in which species becomes slowly modified in the same country, + and those cases in which a species splits into two, or three, or + more new species; and, in the latter case, I should think nearly + perfect separation would greatly aid in their "specification," to + coin a new word[32]. + + [32] _Ibid._ p. 160. + +Now, these passages show a very much clearer perception of the +all-important distinction between monotypic and polytypic evolution than +any which occur in the _Origin of Species_; and they likewise show that +he was led to this perception through what he supposed to be a somewhat +analogous distinction between "unconscious" and "methodical" selection +by man. The analogy, I need hardly say, is radically unsound; and it is +a curious result of its unsoundness that, whereas in the _Origin of +Species_ it is adduced to illustrate the process of polytypic evolution, +as previously remarked, in the letters above quoted we find it adduced +to illustrate the process of monotypic evolution. But the fact of this +analogy being unsound does not affect the validity of the distinction +between monotypic and polytypic evolution to which it led Darwin, in his +later years, so clearly to express[33]. + + [33] The analogy is radically unsound because unconscious selection + differs from methodical selection only in the _degree_ of + "separation" which it effects. These two forms of selection do not + necessarily differ from one another in regard to the _number_ of + characters which are being simultaneously diversified; for while it + may be the object of methodical selection to breed for modification + of a single character alone, it may, on the other hand, be the + result of unconscious selection to diversify an originally uniform + stock, as Darwin himself observes with regard to horse-breeding. The + real distinction between monotypic and polytypic evolution is, not + at all with reference to the _degree_ of isolation (i. e. _amount_ + of "separation"), but to the _number of cases_ in which any + efficient degree of it occurs (i. e. whether in but a single case, + or in two or more cases). + +Turning next to the second point which we have to notice, it is easy to +show that in the earlier editions of his works Darwin did not +sufficiently recognize the levelling effects of free intercrossing, and +consequently failed to perceive the importance of isolation (in any of +its forms) as a factor of organic evolution. This may be most briefly +shown by quoting his own more matured opinion upon the subject. Thus, +with reference to the swamping effects of intercrossing, he wrote to Mr. +Wallace in 1867 as follows:-- + + I must have expressed myself atrociously: I meant to say exactly + the reverse of what you have understood. F. Jenkin argued in the + _North British Review_ against single variations being perpetuated, + and has convinced me, though not in quite so broad a manner as here + put. I always thought individual differences more important; but I + was blind, and thought that single variations might be preserved + much oftener than I now see is possible or probable. I mentioned + this in my former note merely because I believed that you had come + to a similar conclusion, and I like much to be in accord with you. + I believe I was mainly deceived by single variations offering such + simple illustrations, as when man selects [i.e. isolates][34]. + + [34] _Life and Letters_, vol. iii. pp. 157-8. + +Again, somewhere about the same time, he wrote to Moritz Wagner:-- + + Although I saw the effects of isolation in the case of islands and + mountain-ranges, and knew of a few instances of rivers, yet the + greater number of your facts were quite unknown to me. I now see + that, from the want of knowledge, I did not make nearly sufficient + use of the views which you advocate[35]. + + [35] _Ibid._ pp. 157-8. + +Now it would be easy to show the justice of these self-criticisms by +quoting longer passages from earlier editions of the _Origin of +Species_; but as this, in view of the above passages, is unnecessary, we +may next pass on to another point. + +The greatest oversight that Wagner made in his otherwise valuable essays +on geographical isolation, was in not perceiving that geographical +isolation is only one among a number of other forms of isolation: and, +therefore, that although it is perfectly true, as he insisted, that +polytypic evolution cannot be effected by natural selection alone, it is +very far from true, as he further insisted, that _geographical_ +isolation is the only means whereby natural selection can be assisted in +this matter. Hence it is that, when Darwin said he had not himself "made +nearly sufficient use" of geographical isolation as a factor of specific +divergence, he quite reasonably added that he could not go so far as +Wagner did in regarding such isolation as a condition, _sine qua non_, +to divergent evolution in all cases. Nevertheless, he adds the +important words, "I almost wish I could believe in its importance to the +same extent with you; for you well show, in a manner which never +occurred to me, that it removes many difficulties and objections." These +words are important, because they show that Darwin had come to feel the +force of the "difficulties and objections" with regard to divergent +evolution being possible by means of natural selection alone, and how +readily they could be removed by assuming the assistance of isolation. +Hence, it is much to be deplored that Wagner presented a single kind of +isolation (geographical) as equivalent to the principle of isolation in +general. For he thus failed to present the complete--and, therefore, the +true--philosophy of the subject to Darwin's mind; and in this, as in +certain other respects which I shall notice later on, served rather to +confuse than to elucidate the matter as a whole. + +To sum up. Although in his later years, as shown by his correspondence, +Darwin came to recognize more fully the swamping effects of free +intercrossing, and the consequent importance of "separation" for the +prevention of these effects, and although in this connexion he likewise +came more clearly to distinguish between the "two cases" of monotypic +and polytypic evolution, it is evident that he never worked out any of +these matters--"thinking it prudent," as he wrote with reference to them +in 1878, "now I am growing old, to work at easier subjects[36]." +Therefore he never clearly saw, on the one hand, that free +intercrossing, far from constituting a "difficulty" to _monotypic_ +evolution by natural selection, is the very means whereby natural +selection is in this case enabled to operate; or, on the other hand, +that, in the case of _polytypic_ evolution, the "difficulty" in question +is so absolute as to render such evolution, by natural selection alone, +absolutely impossible. Hence, although in one sentence of the _Origin of +Species_ he mentions three forms of isolation (besides the geographical +form) as serving in some cases to assist natural selection in causing +"divergence of character" (i. e. polytypic evolution[37]), on account of +not perceiving how great and how sharp is the distinction between the +two kinds or "cases" of evolution, he never realized that, where "two or +more new species" are in course of differentiation, _some_ form of +isolation other than natural selection must _necessarily_ be present, +whether or not natural selection be likewise so. The nearest approach +which he ever made to perceiving this necessity was in one of his +letters to Wagner above quoted, where, after again appealing to the +erroneous analogy between monotypic evolution and "unconscious +selection," he says:--"But I admit that by this process (i. e. +unconscious selection) two or more new species could hardly be formed +within the same limited area: some degree of separation, if not +indispensable, would be highly advantageous; and here your facts and +views will be of great value." But even in this passage the context +shows that by "separation" he is thinking exclusively of _geographical_ +separation, which he rightly enough concludes (as against Wagner) need +certainly not be "indispensable." Had he gone a step further, he must +have seen that separation, _in some form or another, is_ "indispensable" +to polytypic evolution. Instead of taking this further step, however, +two years later he wrote to Semper as follows:-- + + [36] _Life and Letters_, vol. iii. p. 161. + + [37] Page 81. The three forms of isolation mentioned are, "from + haunting different stations, from breeding at slightly different + seasons, or from the individuals of each variety preferring to pair + together." + + I went as far as I could, perhaps too far, in agreement with Wagner + [i. e. in the last edition of the _Origin of Species_]; since that + time I have seen no reason to change my mind; but then I must add + that my attention has been absorbed on other subjects[38]. + + [38] _Life and Letters_, vol. iii. p. 159. + +And he seems to have ended by still failing to perceive that the +explanation which he gives of "divergence of character" in the _Origin +of Species_, can only hold on the unexpressed assumption that free +intercrossing is in some way prevented at the commencement, and +throughout the development, of each diverging type. + +Lastly, we have to consider Darwin's opinion touching the important +principle of "Independent Variability." This, it will be remembered, is +the principle which ensures that when a portion (not too large) of a +species is prevented from interbreeding with the rest of the species, +sooner or later a divergence of type will result, owing to the fact that +the average qualities of the separated portion at the time of its +separation cannot have been exactly the same as the average qualities of +the specific type as a whole. Thus the state of Amixia, being a state of +what Mr. Gulick calls Independent Generation, will of itself--i.e. even +if unassisted by natural selection--induce divergence of type, in a +ratio that has been mathematically calculated by Delboeuf. + +Darwin wrote thus to Professor Weismann in 1872:-- + + I have now read your essay with very great interest. Your view of + the origin of local races through "Amixia" is altogether new to me, + and seems to throw an important light on an obscure question[39]. + + [39] _Life and Letters_, vol. iii. p. 155. + +And in the last edition of the _Variation of Animals and Plants_ he adds +the following paragraph:-- + + This view may throw some light on the fact that the domestic + animals which formerly inhabited the several districts in Great + Britain, and the half-wild cattle lately kept in several British + parks, differed slightly from one another; for these animals were + prevented from wandering over the whole country and intercrossing, + but would have crossed freely within each district or park[40]. + + [40] _Variation_, &c., vol. ii. p. 262. + +Now, although I allow that Darwin never attributed to this principle of +Amixia, or Independent Variability, anything like the degree of +importance to which, in the opinion of Delboeuf, Gulick, Giard, and +myself, it is entitled, the above passage appears to show that, as soon +as the "view" was clearly "suggested" to his mind, he was so far from +being unfavourably disposed towards it, that he added a paragraph to the +last edition of his _Variation_ for the express purpose of countenancing +it. Nevertheless, later on the matter appears to have entirely escaped +his memory; for in 1878 he wrote to Semper, that he did "not see at all +more clearly than I did before, from the numerous cases which he +[Wagner] has brought forward, how and why it is that a long isolated +form should almost always become slightly modified[41]." I think this +shows entire forgetfulness of the principle in question, because, if +the latter is good for explaining the _initial_ divergence of type as +between separated stocks of "domesticated animals," much more must it be +competent to explain the _further_ divergence of type which is "almost +always" observable in the case of "a long isolated form" under nature. +The very essence of the principle being that, when divergence of type +has once begun, this divergence must _ipso facto_ proceed at an +ever-accelerating pace, it is manifestly inconsistent to entertain the +principle as explaining the first commencement of divergence, and then +to ignore it as explaining the further progress of divergence. Hence, I +can only conclude that Darwin had forgotten this principle altogether +when he wrote his letter to Semper in 1878--owing, no doubt, as he says +in the sentence which immediately follows, to his having "not attended +much of late years to such questions." + + [41] _Life and Letters_, vol. iii. p. 161. + + * * * * * + +So much, then, for Darwin's opinions. Next in order of time we must +consider Moritz Wagner's essays on what he called the "Law of +Migration[42]." The merit of these essays was, first, the firm expression +of opinion upon the swamping effects of free intercrossing; and, second, +the production of a large body of facts showing the importance of +geographical isolation in the prevention of these effects, and in the +consequent differentiation of specific types. On the other hand, the +defect of these essays was, first, not distinguishing between evolution +as monotypic and polytypic; and, second, not perceiving that +geographical isolation is only one among a number of other forms of +isolation. From these two radical oversights--which, however, were +shared by all other writers of the time, with the partial exception of +Darwin himself, as previously shown--there arose the following and most +lamentable errors. + + [42] _Die Darwin'sche Theorie und das Migrationsgesetz_ (1868): + _Ueber den Einfluss der geographischen Isolirung_, &c. (1870). + +Over and over again Moritz Wagner insists, as constituting the +fundamental doctrine of his attempted reform of Darwinism, that +evolution by natural selection is impossible, unless natural selection +be assisted by geographical isolation, in order to prevent the swamping +effects of intercrossing[43]. Now, if instead of "evolution" he had said +"divergence of type," and if instead of "geographical isolation" he had +said "prevention of intercrossing," he would have enunciated the general +doctrine which it has been the joint endeavour of Mr. Gulick and myself +to set forth. But by not perceiving that "evolution" is of two radically +different kinds--polytypic and monotypic--he entirely failed to perceive +that, while for one of its kinds the _prevention_ of intercrossing is an +absolute necessity, for the other of its kinds the _permission_ of +intercrossing is a necessity no less absolute. And, again, in missing +the fact that geographical isolation is but one of the many ways +whereby intercrossing may be prevented, he failed to perceive that, even +as regards the case of polytypic evolution, he greatly erred in +representing this one form of isolation as being universally a necessary +condition to the process. The necessary condition to this process is, +indeed, the prevention of intercrossing _by some means or another_; but +his unfortunate insistence on geographical separation as the only +possible means to this end--especially when coupled with his no less +unfortunate disregard of monotypic evolution--caused him to hinder +rather than to advance a generalization which he had only grasped in +part. And this generalization is, as now so repeatedly stated, that +while the form of isolation which we know as natural selection depends +for its action upon the intercrossing of all the individuals which it +isolates (i. e. selects), when acting alone it can produce only +monotypic evolution; but that when it is supplemented by any of the +other numerous forms of isolation, it is furnished with the necessary +condition to producing polytypic evolution--and this in as many lines of +divergent change as there may be cases of this efficient separation. + + [43] For instance, speaking of common, or continuous areas, he + says:--"In this case a constant variety, or new species, cannot be + produced, because the free crossing of a new variety with the old + unaltered stock will always cause it to revert to the original type; + in other words, will destroy the new form. The formation of a real + variety, which Darwin, as we know, regards as the commencement of a + new species, will only succeed when a few individuals, having + crossed the barrier of their habitat, are able to separate + themselves for a long time from the old stock." And the last + sentence, given as a summary of his whole doctrine, is--"The + geographical isolation of the form, a necessary consequence of + migration, is the cause of its typical character." + +Nevertheless, while we must lament these shortcomings on the part of +Wagner, we ought to remember that he rendered important services in the +way of calling attention to the swamping effects of free intercrossing, +and, still more, in that of showing the high importance of geographical +isolation as a factor of organic evolution. Therefore, although in an +elaborate criticism of his views Weismann was easily able to dispose of +his generalizations in the imperfect form that they presented, I do not +think it was just in Weismann to remark, "if Wagner had confined himself +to the statement that geographical isolation materially assists the +process of natural selection, and thus also promotes the origination of +new species, he would have met with little or no opposition; but then, +of course, in saying this much, he would not have been saying anything +new." No doubt, as I have just shown, he _ought_ thus (as well as in +other and still more important respects not perceived by Prof. Weismann) +to have limited his statement; but, had he done so, it does not follow +that he would not have been saying anything new. For, in point of fact, +in as far as he said what was true, he did say a great deal that was +also new. Thus, most of what he said of the _principle of separation_ +(apogamy) was as new as it was true, although, as we have seen, he said +it to very little purpose on account of his identifying this principle +as a whole with that of but one of its forms. Again, notwithstanding +this great error, or oversight, he certainly showed of the particular +form in question--viz. geographical isolation--that it was of +considerably _more_ importance than had previously been acknowledged. +And this was so far a valuable contribution to the general theory of +descent. + + * * * * * + +Prof. Weismann's essay, to which allusion has just been made[44], was, +however, in all respects a great advance upon those of Wagner. It was +not only more comprehensive in its view of the whole subject of +geographical isolation, but likewise much more adequate in its general +treatment thereof. Its principal defects, in my judgement, were, first, +the inordinately speculative character of some of its parts, and, +second, the restriction of its analysis to but one form of isolation--a +defect which it shares with the essays of Wagner, and in quite as high a +degree. Furthermore, although this essay had the great merit of +enunciating the principle of Amixia, it did so in a very inefficient +manner. For not only was this principle adduced with exclusive reference +to _geographical_ isolation, but even in regard to this one kind of +isolation it was presented in a highly inconsistent manner, as I will +now endeavour to show. + + [44] _Ueber den Einfluss der Isolirung auf die Artbildung_ (1872). + +Weismann was led to perceive the principle in question by the +consideration that new specific characters, when they first appear, do +not all appear together in the same individuals: they appear one in one +individual, another in another, a third in a third, &c.; and it is only +in the course of successive generations that they all become blended in +the same individuals by free intercrossing. Hence, the eventually +emerging constant or specific type is the resultant of all the +transitory or varietal types, when these have been fused together by +intercrossing. From which Weismann deduces what he considers a general +law--namely, that "the constancy of a specific type does not arise +suddenly, but gradually; and it is established by the promiscuous +crossing of all individuals[45]." From which again it follows, that this +constancy must cease so soon as the condition which maintains it +ceases--i. e. so soon as free intercrossing is prevented by the +geographical isolation of a portion of the species from its parent +stock. + + [45] _Loc. cit._, p. 43. + +Now, to begin with, this statement of the principle in question is not a +good statement of it. There was no need while stating the doctrine that +separation induces differentiation, to found the doctrine on any such +highly speculative basis. In point of fact, there is no real evidence +that specific types do attain their constancy in the way supposed; nor, +for the purposes of the doctrine in question, is it necessary that there +should be. For this doctrine does not need to show how the constancy has +been _attained_; it only has to show that the constancy is _maintained_ +by free intercrossing, with the result that when free intercrossing is +_by any means_ prevented, divergence of character ensues. In short, the +correct way of stating the principle is that which has been adopted by +Delboeuf and Gulick--namely, the average characters of a separated +portion of a species are not likely to be the same as those of the whole +species; with the result that divergence of type will be set up in the +separated portion by intercrossing within that portion. Or the principle +may be presented as I presented it under the designation of "Independent +Variability"--namely, "a specific type may be regarded as the average +mean of all individual variations, any considerable departure from this +average mean being, however, checked by intercrossing," with the result +that when intercrossing is prevented between a portion of a species and +the rest of the species, "this population is permitted to develop an +independent history of its own, shielded from intercrossing with its +parent form[46]." + + [46] _Physiological Selection_, pp. 348, 389. + +Not only, however, is Weismann's principle of "Amixia" thus very +differently stated from that of my "Independent Variability" (apogamy), +or Gulick's "Independent Generation"; but, apparently owing to this +difference of statement, the principle itself is not the same. In +particular, while Weismann holds with us that when new characters arise +in virtue of the mere prevention of intercrossing with parent forms +these new characters will be of non-utilitarian kind[47], he appears to +think that divergence of character under such circumstances is not +likely to go on to a _specific_ value. Now, it is of importance to +observe why he arrives at this conclusion, which is not only so +different from that of Delboeuf, Gulick, and myself, but apparently so +inconsistent with his own recognition of the diversifying effect of +"Amixia" as regards the formation of _permanent varieties_. For, as we +have already seen while considering Darwin's views on this same +principle of "Amixia," it is highly inconsistent to recognize its +diversifying effect up to the stage of constituting fixed varieties, and +then not to recognize that, so much divergence of character having been +already secured by the isolation alone, much more must further +divergence continue, and continue at an ever accelerating pace--as +Delboeuf and Gulick have so well shown. What, then, is the explanation +of this apparent inconsistency on Weismann's part? The explanation +evidently is that, owing to his erroneous statement of the principle, he +misses the real essence of it. For, in the first place, he does not +perceive that this essence consists in an initial difference of average +characters on the part of the isolated colony as compared with the rest +of their species. On the contrary, he loses himself in a maze +of speculation about all species having had what he calls +"variation-periods," or eruptions of general variability alternating +with periods of repose--both being as unaccountable in respect of their +causation as they are hypothetical in respect of their occurrence. From +these speculations he concludes, that isolation of a portion of a +species will then only lead to divergence of character when the +isolation happens to coincide with a "variation-period" on the part of +the species as a whole, and that the divergence will cease so soon as +the "variation-period" ceases. Again, in the second place as previously +remarked, equally with Wagner whom he is criticizing, he fails to +perceive that _geographical_ isolation is not the only kind of +isolation, or the only possible means to the prevention of free +intercrossing. And the result of this oversight is, that he thinks +amixia can act but comparatively seldom upon sufficiently small +populations to become a factor of much importance in the differentiation +of species. Lastly, in the third place, owing to his favourite +hypothesis that all species pass through a "variation-period," he +eventually concludes that the total amount of divergence of type +producible by isolation alone (even in a small population) can never be +greater than that between the extremes of variation which occur within +the whole species at the date of its partition (p. 75). In other words, +the possibility of change due to amixia alone is taken to be limited by +the range of deviation from the general specific average, as manifested +by different individual variations, before the species was divided. Thus +the doctrine of amixia fails to recognize the law of Delboeuf, or the +_cumulative_ nature of divergence of type when once such divergence +begins in a separated section. Therefore, in this all-important--and, +indeed, essential--respect, amixia differs entirely from the principle +which has been severally stated by Delboeuf, Gulick, and myself. + + [47] _Loc. cit._, p. 54. + +Upon the whole, then, we must say that although Professor Weismann was +the first to recognize the diversifying influence of merely +indiscriminate isolation _per se_ (apogamy), he did so only in part. He +failed to distinguish the true essence of the principle, and by +overlaying it with a mass of hypothetical speculation, concealed even +more of it than he revealed. + + * * * * * + +The general theory of Isolation, as independently worked out by Mr. +Gulick and myself, has already been so fully explained, that it will +here be sufficient merely to enumerate its more distinguishing features. +These are, first, drawing the sharpest possible line between evolution +as monotypic and polytypic; second, showing that while for the former +the peculiar kind of isolation which is presented by natural selection +suffices of itself to _transform_ a specific type, in order to work for +the latter, or to _branch_ a specific type, natural selection must +necessarily be assisted by some other kind of isolation; third, that +even in the absence of natural selection, other kinds of isolation may +be sufficient to effect specific divergence through independent +generation alone; fourth, that, nevertheless, natural selection, where +present, will always accelerate the process of divergence; fifth, that +monotypic evolution by natural selection depends upon the _presence_ of +intercrossing, quite as much as polytypic evolution (whether with or +without natural selection) depends upon the _absence_ of it; sixth, +that, having regard to the process of evolution throughout all taxonomic +divisions of organic nature, we must deem the physiological form of +isolation as the most important, with the exception only of natural +selection. + +The only difference between Mr. Gulick's essays and my own is, that, on +the one hand, he has analyzed much more fully than I have the various +forms of isolation; while, on the other hand, I have considered much +more fully than he has the particular form of physiological isolation +which so frequently obtains between allied _species_. This particular +form of physiological isolation I have called "physiological selection," +and claim for it so large a share in the differentiation of specific +types as to find in it a satisfactory explanation of the contrast +between natural species and artificial varieties in respect of +cross-infertility. + + * * * * * + +Mr. Wallace, in his _Darwinism_, has done good service by enabling all +other naturalists clearly to perceive how natural selection alone +produces monotypic evolution--namely, through the free intercrossing of +all individuals which have not been eliminated by the isolating process +of natural selection itself. For he very lucidly shows how the law of +averages must always ensure that in respect of any given specific +character, half the individuals living at the same time and place will +present the character above, and half below its mean in the population +as a whole. Consequently, if it should ever be of advantage to a +species that this character should undergo either increase or decrease +of its average size, form, colour, &c., there will always be, in each +succeeding generation, a sufficient number of individuals--i. e. half of +the whole--which present variations in the required direction, and which +will therefore furnish natural selection with abundant material for its +action, without the need of any other form of isolation. It is to be +regretted, however, that while thus so clearly presenting the fact that +free intercrossing is the very means whereby natural selection is +enabled to effect monotypic evolution, he fails to perceive that such +intercrossing must always and necessarily render it impossible for +natural selection to effect polytypic evolution. A little thought might +have shown him that the very proof which he gives of the necessity of +intercrossing where the _transmutation_ of species is concerned, +furnishes, measure for measure, as good a proof of the necessity of its +absence where the _multiplication_ of species is concerned. In justice +to him, however, it may be added, that this distinction between +evolution as monotypic and polytypic (with the important consequence +just mentioned) still continues to be ignored also by other well-known +evolutionists of the "ultra-Darwinian" school. Professor Meldola, for +example, has more recently said that in his opinion the "difficulty from +intercrossing" has been in large part--if not altogether--removed by Mr. +Wallace's proof that natural selection alone is capable of effecting +[monotypic] evolution; while he regards the distinction between +monotypic and polytypic evolution as mere "verbiage[48]." + + [48] _Nature_, vol. xliii. p. 410, and vol. xliv. p. 29. + +It is in relation to my presentment of the impossibility of natural +selection alone causing polytypic evolution, that Mr. Wallace has been +at the pains to show how the permission of intercrossing (panmixia) is +necessary for natural selection in its work of causing monotypic +evolution. And not only has he thus failed to perceive that the +"difficulty" which intercrossing raises against the view of natural +selection being of itself capable of causing polytypic evolution in no +way applies to the case of monotypic; but as regards this "difficulty," +where it does apply, he says:-- + + Professor G. J. Romanes has adduced it as one of the difficulties + which can alone be overcome by his theory of physiological + selection[49]. + + [49] _Darwinism_, p. 143. + +This, however, is a misapprehension. I have by no means represented that +the difficulty in question can alone be overcome by this theory. What I +have represented is, that it can be overcome by any of the numerous +forms of isolation which I named, and of which physiological selection +is but one. And although, _where common areas are concerned_, I believe +that the physiological form of isolation is the most important form, +this is a very different thing from entertaining the supposition which +Mr. Wallace here assigns to me. + + * * * * * + +I may take this opportunity of correcting a somewhat similar +misunderstanding which has been more recently published by Professor W. +A. Herdman, of Liverpool; and as the case which he gives is one of +considerable interest in itself, I will quote his remarks in extenso. In +his _Opening Address to the Liverpool Biological Society_, Professor +Herdman said:-- + + Some of you will doubtless remember that in last year's address, + while discussing Dr. Romanes' theory of physiological selection, I + quoted Professor Flemming Jenkin's imaginary case of a white man + wrecked upon an island inhabited by negroes, given as an + illustration of the supposed swamping effect by free intercrossing + of a marked variety with the parent species. I then went on to say + in criticism of the result at which Jenkin arrived, viz. that the + characteristics of the white man would be stamped out by + intercrossing with the black:-- + + "Two influences have, I think, been ignored, viz. atavism, or + reversion to ancestral characters, and the tendency of the members + of a variety to breed with one another. Keeping to the case + described above, I should imagine that the numbers of intelligent + young mulattoes produced in the second, third, fourth, and few + succeeding generations would to a large extent intermarry, the + result of which would be that a more or less white aristocracy + would be formed on the island, including the king and all the chief + people, the most intelligent men and the bravest warriors. Then + atavism might produce every now and then a much whiter + individual--a reversal to the characteristics of the ancestral + European--who, by being highly thought of in the whitish + aristocracy, would have considerable influence on the colour and + other characteristics of the next generation. Now such a white + aristocracy would be in precisely the same circumstances as a + favourable variety competing with its parent species," &c. + + You may imagine then my pleasure when, a few months after writing + the above, I accidentally found, in a letter[50] written by the + celebrated African traveller Dr. David Livingstone to Lord + Granville, and dated "Unyanyembe, July 1st, 1872," the following + passage:-- + + [50] In Appendix to H. M. Stanley's _How I found Livingstone_, 2nd + ed. London, 1872, p. 715. + + "About five generations ago, a white man came to the highlands of + Basañgo, which are in a line east of the watershed. He had six + attendants, who all died, and eventually their headman, called + Charura, was elected chief by the Basañgo. In the third generation + he had sixty able-bodied spearmen as lineal descendants. This + implies an equal number of the other sex. They are very light in + colour, and easily known, as no one is allowed to wear coral beads + such as Charura brought except the royal family. A book he brought + was lost only lately. The interest of the case lies in its + connexion with Mr. Darwin's celebrated theory on the 'origin of + species,' for it shows that an improved variety, as we whites + modestly call ourselves, is not so liable to be swamped by numbers + as some have thought." + + Here we have a perfect fulfilment of what I last year, in ignorance + of this observation of Livingstone's, predicted as being likely to + occur in such a case. We have the whitish aristocracy in a dominant + condition, and evidently in a fair way to spread their + characteristics over a larger area and give rise to a marked + variety, and it had clearly struck Livingstone fourteen years + before the theory of physiological selection had been heard of, + just as it must strike us now, as an instance telling strongly + against the "swamping" argument as used by Flemming Jenkin and + Romanes. + +Here we have a curious example of one writer supporting the statements +of another, while appearing to be under the impression that he is +controverting those statements. Both Professor Herdman's imaginary case, +and its realization in Livingstone's account, go to show "the tendency +of the members of a variety to breed with one another." This is what I +have called "psychological selection," and, far from "ignoring" it, I +have always laid stress upon it as an obviously important form of +isolation or _prevention_ of free intercrossing. But it is a form of +isolation which can only occur in the higher animals, and, therefore, +the whole of Professor Herdman's criticism is merely a restatement of my +own views as already published in the paper which he is criticizing. +For all that his argument goes to prove is, first, the necessity for +_some_ form of isolation if the overwhelming effects of intercrossing +are to be obviated; and, secondly, the manifest consequence that where +the psychological form is unavailable (as in many of the lower animals +and in all plants), some other form must be present if divergent +evolution is taking place on a common area. + + * * * * * + +Seeing that so much misunderstanding has been shown with reference to my +views on "the swamping effects of intercrossing," and seeing also that +this misunderstanding extends quite as much to Mr. Gulick's views as to +my own, I will here supply brief extracts from both our original papers, +for the double purpose of showing our complete agreement, and of leaving +it to be judged whether we can fairly be held responsible for the +misunderstanding in question. After having supplied these quotations, I +will conclude this historical sketch by considering what Mr. Wallace has +said in reply to the views therein presented. I will transcribe but a +single passage from our papers, beginning with my own. + + Any theory of the origin of species in the way of descent must be + prepared with an answer to the question, Why have species + _multiplied_? How is it that, in the course of evolution, species + have not simply become transmuted in linear series instead of + ramifying into branches? This question Mr. Darwin seeks to answer + "from the simple circumstance that the more diversified the + descendants from any one species becomes in structure, + constitution, and habits, by so much will they be better enabled to + seize on many and widely diversified places in the economy of + nature, and so be enabled to increase in numbers." And he proceeds + to illustrate this principle by means of a diagram, showing the + hypothetical divergence of character undergone by the descendants + of seven species. Thus, he attributes divergence of character + exclusively to the influence of natural selection. + + Now, this argument appears to me unassailable in all save one + particular; but this is a most important particular: the argument + wholly ignores the fact of intercrossing with parent forms. + Granting to the argument that intercrossing with parent forms is + prohibited, and nothing can be more satisfactory. The argument, + however, sets out with showing that it is in limited areas, or in + areas already overstocked with the specific form in question, that + the advantages to be derived from diversification will be most + pronounced. It is where they "jostle each other most closely" that + natural selection will set a premium upon any members of the + species which may depart from the common type. Now, inasmuch as + this jostling or overcrowding of individuals is a needful condition + to the agency of natural selection in the way of diversifying + character, must we not feel that the general difficulty from + intercrossing previously considered is here presented in a special + and aggravated form? At all events, I know that, after having duly + and impartially considered the matter, to me it does appear that + unless the swamping effects of intercrossing with the parent form + on an overcrowded area is in some way prevented to begin with, + natural selection could never have any material supplied by which + to go on with. Let it be observed that I regard Mr. Darwin's + argument as perfectly sound where it treats of the divergence of + _species_, and of their further divergence into _genera_; for in + these cases the physiological barrier is known to be already + present. But in applying the argument to explain the divergence of + individuals into varieties, it seems to me that here, more than + anywhere else, Mr. Darwin has strangely lost sight of the + formidable difficulty in question; for in this particular case so + formidable does the difficulty seem to me, that I cannot believe + that natural selection alone could produce any divergence of + specific character, so long as all the individuals on an + overcrowded area occupy that area together. Yet, if any of them + quit that area, and so escape from the unifying influence of free + intercrossing, these individuals also escape from the conditions + which Mr. Darwin names as those that are needed by natural + selection in order to produce divergence. Therefore, it appears to + me that, under the circumstances supposed, natural selection alone + could not produce divergence; the most it could do would be to + change the whole specific type in some one direction, and thus + induce transmutation of species in a linear series, each succeeding + member of which might supplant its parent form. But in order to + secure _diversity_, _multiplication_, or _ramification_ of species, + it appears to me obvious that the primary condition required is + that of preventing intercrossing with parent forms at the origin of + each branch, whether the prevention be from the first absolute, or + only partial. + +Now for Mr. Gulick, a portion of whose more lengthy discussion of the +subject, however, is all that I need quote:-- + + Having found that the evolution of the fitted is secured through + the prevention of crossing between the better fitted and the less + fitted, can we believe that the evolution of a special race, + regularly transmitting a special kind of fitness, can be realized + without any prevention of crossing with other races that have no + power to transmit that special kind of fitness? Can we suppose that + any advantage, derived from new powers that prevent severe + competition with kindred, can be permanently transmitted through + succeeding generations to one small section of the species while + there is free crossing equally distributed between all the families + of the species? Is it not apparent that the terms of this + supposition are inconsistent with the fundamental laws of heredity? + Does not inheritance follow the lines of consanguinity; and when + consanguinity is widely diffused, can inheritance be closely + limited? When there is free crossing between the families of one + species, will not any peculiarity that appears in one family either + be neutralized by crosses with families possessing the opposite + quality, or, being preserved by natural selection, while the + opposite quality is gradually excluded, will not the new quality + gradually extend to all the branches of the species; so that, in + this way or in that, increasing divergence of form will be + prevented? + + If the advantage of freedom from competition in any given variation + depends on the possession, in some degree, of new adaptations to + unappropriated resources, there must be some cause that favours the + breeding together of those thus specially endowed, and interferes + in some degree with their crossing with other variations, or, + failing this, the special advantage will in succeeding generations + be lost. As some degree of Independent Generation is necessary for + the continuance of the advantage, it is evident that the same + condition is necessary for the accumulation through Natural + Selection of the powers on which the advantage depends. The + advantage of divergence of character cannot be retained by those + that fail to retain the divergent character; and divergent + character cannot be retained by those that are constantly crossing + with other kinds; and the prevention of free crossing between those + that are equally successful is in no way secured by Natural + Selection. + +So much, then, as expressive of Mr. Gulick's opinion upon this subject. +To exactly the same effect Professor Lloyd Morgan has recently published +his judgement upon it thus:-- + + That perfectly free intercrossing, between any or all of the + individuals of a given group of animals, is, so long as the + characters of the parents are blended in the offspring, fatal to + divergence of character, is undeniable. Through the elimination of + less favourable variations, the swiftness, strength, and cunning of + a race may be gradually improved. But no form of elimination can + possibly differentiate the group into swift, strong, and cunning + varieties, distinct from each other, so long as all three varieties + freely interbreed, and the characters of the parents blend in the + offspring. Elimination may and does give rise to progress in any + given group, _as a group_; it does not and cannot give rise to + differentiation and divergence, so long as interbreeding with + consequent interblending of characters be freely permitted. Whence + it inevitably follows, as a matter of simple logic, that where + divergence has occurred, intercrossing and interbreeding must in + some way have been lessened or prevented. Thus a new factor is + introduced, that of _isolation_ or _segregation_. And there is no + questioning the fact that it is of great importance. Its + importance, indeed, can only be denied by denying the swamping + effects of intercrossing, and such denial implies the tacit + assumption that interbreeding and interblending are held in check + by some form of segregation. The isolation explicitly denied is + implicitly assumed[51]. + + [51] _Animal Life and Intelligence_, pp. 98, 99 (1890-1891). + +Similarly, and still more recently, Professor Le Conte writes:-- + + It is evident, then, as Romanes claims, that natural selection + alone tends to _monotypic_ evolution. Isolation of some sort seems + necessary to _polytypic_ evolution. The tree of evolution under the + influence of natural selection alone grows palm-like from its + terminal bud. Isolation was necessary to the starting of lateral + buds, and thus for the profuse ramification which is its most + conspicuous character[52]. + + [52] _The Factors of Evolution_ (1891). + +In order to complete this historical review, it only remains to consider +Mr. Wallace's utterances upon the subject. + +It is needless to say that he stoutly resists the view of Weismann, +Delboeuf, Gulick, and myself, that specific divergence can ever be +due--or, as I understand him, even so much as assisted--by this +principle of indiscriminate isolation (apogamy). It will be remembered, +however, that Mr. Gulick has adduced certain general principles and +certain special facts of geographical distribution, in order to prove +that apogamy eventually leads to divergence of character, provided that +the isolated section of the species does not contain any very large +number of individuals. Now, Mr. Wallace, without making any reference to +this argument of Mr. Gulick, simply states the reverse--namely, that, as +a matter of fact, indiscriminate isolation is not found to be +associated with divergence of character. For, he says, "there is an +entire absence of change, where, if this were a _vera causa_, we should +expect to find it[53]." But the only case which he gives is that of +Ireland. + + [53] _Darwinism_, p. 151. + +This, he says, furnishes "an excellent test case, for we know that it +[Ireland] has been separated from Britain since the end of the glacial +epoch: ... yet hardly one of its mammals, reptiles, or land molluscs has +undergone the slightest change[54]." Here, however, Mr. Wallace shows +that he has failed to understand "the views of those who, like Mr. +Gulick, believe isolation itself to be a cause of modification of +species"; for it belongs to the very essence of these views that the +efficiency of indiscriminate isolation as a "_vera causa_" of organic +evolution varies inversely with the number of individuals (i. e. the +size of the species-section) exposed to its influence. Therefore, far +from being "an excellent test case," the case of Ireland is +unsatisfactory. If we are in search of excellent test cases, in the +sense intended by Mr. Wallace, we ought not to choose a large island, +which from the time of its isolation must have contained large bulks of +each of the geographically separated species concerned: we ought to +choose cases where as small a number as possible of the representatives +of each species were in the first instance concerned. And, when we do +this, the answer yielded by any really "excellent test case" is +unequivocal. + + [54] _Ibid._ + +No better test case of this kind has ever been furnished than that of +Mr. Gulick's land-shells, which Mr. Wallace is specially considering in +the part of his book where the sentence above quoted occurs. How, then, +does he meet this case? He meets it by assuming that in all the numerous +adjacent valleys of a small island there must be as many differences of +environment, each of which is competent to induce slight varietal +changes on the part of its occupants by way of natural selection, +although in no one case can the utility of these slight changes be +surmised. Now, against this explanation there are three overwhelming +considerations. In the first place, it is purely gratuitous, or offered +merely in order to save the hypothesis that there _can_ be no other +cause of even the most trivial change in species than that which is +furnished by natural selection. In the second place, as Mr. Gulick +writes to me in a private letter, "if the divergence of Sandwich Island +land molluscs is wholly due to exposure to different environments, as +Mr. Wallace argues on pages 147-150, then there must be completely +occult influences in the environment that vary progressively with each +successive mile. This is so violent an assumption that it throws doubt +on any theory that requires such support." In the third place, the +assumption that the changes in question must have been due to natural +selection, is wholly incompatible with the facts of isolation +elsewhere--namely, in those cases where (as in that of Ireland) a large +section of species, instead of a small section, has been +indiscriminately isolated. Mr. Wallace, as we have seen, inadvertently +alludes to these "many other cases of isolation" as evidence against +apogamy being _per se_ a cause of specific change. But although, for +the reason above stated, they are without relevancy in this respect, +they appear to me fatal to the explanation which he gives of specific +changes under apogamy where only small sections of species are +concerned. For example, can it be rationally maintained that there are +more differences of environment between every two of the many contiguous +valleys of a small island, such as Mr. Gulick describes, than there are +in the incomparably larger area of the whole of Ireland? But, if not, +and if natural selection is able to work such "occult" wonders in each +successive mile on the Sandwich Islands, why has it so entirely lost +this magic power in the case of Ireland--or in the "many other cases of +isolation" to which Mr. Wallace refers? On his theory there is no +coherent answer to be given to this question, while on our theory the +answer is given in the very terms of the theory itself. The facts are +plainly just what the theory requires that they should be; and +therefore, if they were not as they are, the theory would be deprived of +that confirmation which it now derives from them. + +Thus, in truth, though in an opposite way, the case of Ireland is, as +Mr. Wallace says, "an excellent test case," when once the theory of +apogamy as a "_vera causa_" of specific change is understood; and the +effect of applying the test is fully to corroborate this theory, while +at the same time it as fully negatives the other. For the consideration +whereby Mr. Wallace seeks to explain the inactivity of natural selection +in the case of Ireland is not "coherent." What he says is, "That changes +have not occurred through natural selection, is perhaps due to the less +severe struggle for existence, owing to the smaller number of competing +species[55]." But even with regard to molluscs alone, there is a greatly +larger number of species in Ireland than occurs in any one valley of the +Sandwich Islands; while if we have regard to all the other classes of +animal life, comparison entirely fails. + + [55] _Loc. cit._, p. 151. + +Much more to the point are certain cases which were adduced long ago by +Weismann in his essay previously considered. Nevertheless, although this +essay was published as far back as 1872, and, although it expressly +deals with the question of divergence of character through the mere +prevention of intercrossing (Amixia), Mr. Wallace nowhere alludes to +these cases _per contra_, which are so much more weighty than his own +"test case" of Ireland. Of such are four species of butterflies, +belonging to three genera[56], which are identical in the polar regions +and in the Alps, notwithstanding that the sparse Alpine populations have +been presumably separated from their parent stocks since the glacial +period; or of certain species of fresh water crustaceans (_Apus_), the +representatives of which are compelled habitually to form small isolated +colonies in widely separated ponds, and nevertheless exhibit no +divergence of character, although apogamy has probably lasted for +centuries. These cases are unquestionably of a very cogent nature, and +appear of themselves to prove that apogamy alone is not invariably +capable of inducing divergence--at any rate, so rapidly as we might +expect. There appears, however, to be another factor, the presence or +absence of which makes a great difference. This as stated in the text, +is the degree in which a specific type is stable or unstable--liable or +not liable to vary. Thus, for example, the Goose is what Darwin calls an +"inflexible" type as compared with most other domesticated birds. +Therefore, if a lot of geese were to be indiscriminately isolated from +the rest of their species, the probability is that in a given time their +descendants would not have diverged from the parent type to such an +extent as would a similar lot of ducks under similar circumstances: the +more stable specific type would require a longer time to change under +the influence of apogamy alone. Now, the butterflies and crustaceans +quoted by Weismann may be of a highly stable type, presenting but a +small range of individual variability; and, if so, they would naturally +require a long time to exhibit any change of type under the influence of +apogamy alone. But, be this as it may, Weismann himself adduces these +cases merely for the sake of showing that there are cases which seem to +tell against the general principle of modification as due to apogamy +alone--i.e. the general principle which, under the name amixia, he is +engaged in defending. And the conclusion at which he himself arrives is, +that while it would be wrong to affirm that apogamy _must_ in all cases +produce divergence, we are amply justified in affirming that in many +cases it _may_ have done so; while there is good evidence to prove that +in not a few cases it _has_ done so, and therefore should be accepted +as one of the factors of organic evolution[57]. + + [56] Namely, _Lycaena denzelii_, _L. pheretes_, _Argynnis pales_, + _Erebia mante_. + + [57] Since the above was written, I have heard of some cases which + seem to present greater difficulties to our theory than those above + quoted. These refer to some of the numerous species of land mollusca + which inhabit the isolated rocks near Madeira (Dezertas). My + informant is Dr. Grabham, who has himself investigated the matter, + and reports as follows:-- + + "It is no uncommon thing to meet with examples of the same species, + sub-fossil, recent, and living upon one spot, and presenting no + variation in the long record of descent." Then, after naming these + examples, he adds, "All seem to vary immediately on attaining new + ground, assuming many aspects in different districts." + + Unquestionably these statements support, in a very absolute manner, + Mr. Wallace's opinion, while making directly against my own. It is + but fair, however, to add that the cases are not numerous (some + half-dozen at the most, and all within the limits of a single + genus), and that, even in the opinion of my informant himself, the + facts have not hitherto been sufficiently investigated for any + decisive judgement to be formed upon them. + +My view from the very first has been that variations in the way of +cross-infertility are of frequent occurrence (how, indeed, can they be +otherwise, looking to the complex conditions that have to be satisfied +in every case of full fertility?); and, therefore, however many of such +variations are destined to die out, whenever one arises, "under suitable +conditions," "it must inevitably tend to be preserved as a new natural +variety, or incipient species." Among the higher animals--which are +"comparatively few in number"--I think it probable that some slight +change of form, colour, habit, &c., must be usually needed either to +"superinduce," or, which is quite a different thing, to _coincide_ with +the physiological change But in the case of plants and the lower +invertebrata. I see no reason for any frequent concomitance of this +kind; and therefore believe the physiological change to be, "as a +general rule," the primordial change. At the same time, I have always +been careful to insist that this opinion had nothing to do with "the +essence of physiological selection"; seeing that "it was of no +consequence" to the theory in what proportional number of cases the +cross-sterility had begun _per se_, had been superinduced by +morphological changes, or only enabled to survive by happening to +coincide with any other form of homogamy. In short, "the essence of +physiological selection" consists in _all_ cases of the diversifying +_effect_ of cross-infertility, whensoever and howsoever it may happen in +particular cases to have been _caused_. + +Thus I emphatically reaffirm that "from the first I have always +maintained that it makes no essential difference to the theory _in what +proportional number of cases_ they [the physiological variations] have +arisen 'alone in an otherwise undifferentiated species'"; therefore, +"even if I am wrong in supposing that physiological selection can _ever_ +act alone, the _principle_ of physiological selection, as I have stated +it, is not thereby affected. And this principle is, as Mr. Wallace has +re-stated it, 'that some amount of infertility characterizes the +distinct varieties which are in process of differentiation into +species'--infertility whose absence, 'to obviate the effects of +intercrossing, may be one of the _usual_ causes of their failure to +become developed into distinct species.'" + +These last sentences are quoted from the correspondence in _Nature_[58], +and to them Mr. Wallace replied by saying, "if this is not an absolute +change of front, words have no meaning"; that "if this is 'the whole +essence of physiological selection,' then physiological selection is but +a re-statement and amplification of Darwin's views"; that such a "change +of front" is incompatible, not only with my term "physiological +selection," but also with my having "acknowledged that Mr. Catchpool had +'very clearly put forward the theory of physiological selection'"; and +much more to the same effect. + + [58] Vol. xliii. p. 127. + +Now, to begin with, it is due to Mr. Catchpool to state that his only +publication upon this subject is much too brief to justify Mr. +Wallace's, inference, that he supposes variations in the way of +cross-infertility always to arise "alone in an otherwise +undifferentiated species." What Mr. Catchpool's opinion on this point +may be, I have no knowledge; but, whatever it is, he was unquestionably +the first writer who "clearly stated the leading principles" of +physiological selection, and this fact I am very glad to have +"acknowledged." In my correspondence with Mr. Wallace, however, I not +only named Mr. Catchpool: I also named--and much more prominently--Mr. +Gulick. For even if I were to grant (which I am far indeed from doing) +that there was any want of clearness in my own paper touching the point +in question, I have now repeatedly shown that it is simply impossible +for any reader of Mr. Gulick's papers to misunderstand _his_ views with +regard to it. Accordingly, I replied to Mr. Wallace in _Nature_ by +saying:-- + + Not only have I thus from the first fully recognized the sundry + other causes of specific change with which the physiological + variations may be associated; but Mr. Gulick has gone into this + side of our common theory much more fully, and elaborately + calculated out the high ratio in which the differentiating agency + of any of these other causes must be increased when assisted by--i. + e. associated with--even a moderate degree of the selective + fertility, and vice versa. Therefore, it is simply impossible for + Mr. Wallace to show that "our theory" differs from his in this + respect. Yet it is the only respect in which his reply alleges any + difference. (Vol. xliii. p. 127.) + +I think it is to be regretted that, in his answer to this, Mr. Wallace +alludes only to Mr. Catchpool, and entirely ignores Mr. Gulick--whose +elaborate calculations above alluded to were communicated to the +Linnaean Society by Mr. Wallace himself in 1887. + +The time has now come to prove, by means of quotations, that I have from +the first represented the "principle," or "essence," of physiological +selection to consist in selective fertility furnishing a needful +condition to specific differentiation, in at least a large proportional +number of allied species which afterwards present the reciprocal +character of cross-sterility; that I have never represented variations +in the way of this selective fertility as necessarily constituting the +initial variations, or as always arising "alone, in an otherwise +undifferentiated species"; and that, although I have uniformly given it +as my opinion that these variations do _in some cases_ thus arise +(especially among plants and lower invertebrata), I have as uniformly +stated "that it makes no difference to the theory in what proportional +number of cases they have done so"--or even if, as Mr. Wallace supposes, +they have never done so in any case at all[59]. These statements (all of +which are contradictory of the only points of difference alleged) have +already been published in my article in the _Monist_ of October, 1890. +And although Mr. Wallace, in his reply to that article, ignores my +references to the "original paper," it is scarcely necessary to quote +the actual words of the paper itself, since the reader who is further +interested in this controversy can readily refer to it in the _Journal +of the Linnaean Society_ (vol. xix. pp. 337-411). + + [59] This refers to what I understand Mr. Wallace to say in the + _Nature_ correspondence is the supposition on which his own theory + of the origin of species by cross-infertility is founded. But in the + original statement of that theory itself, it is everywhere + "supposed" that when species are originated by cross-infertility, + the _initial_ change _is_ the physiological change. In his original + statement of that theory, therefore, he literally went further than + I had gone in my "original paper," with reference to supposing the + physiological change to be the initial change. I do not doubt that + this is due to some oversight of expression; but it is curious that, + having made it, he should still continue his endeavour to fix + exactly the same oversight upon me. + +Having arrived at these results with regard to the theory of Isolation +in general and of Physiological Isolation in particular, I arrive also +at the end of this work. And if, while dealing with the post-Darwinian +period, I have imparted to any general reader the impression that there +is still a great diversity of expert opinion; I must ask him to note +that points with reference to which disagreement still exists are but +very subordinate to those with regard to which complete agreement now +prevails. The noise of wrangling disputations which has so filled the +camp of evolutionists since the death of their captain, is apt to hide +from the outside world the solid unanimity that prevails with regard to +all the larger and more fundamental questions, which were similarly the +subjects of warfare in the past generation. Indeed, if we take a fair +and general view of the whole history of Darwinism, what must strike us +as the really significant fact is the astonishing unanimity which has +been so rapidly attained with regard to matters of such immeasurable +importance. It is now but little more than thirty years since the +publication of the _Origin of Species_; and in that period not only have +all naturalists unequivocally embraced the doctrine of descent +considered as a fact; but, in one degree or another, they have all as +unequivocally embraced the theory of natural selection considered as a +method. The only points with regard to which any difference of opinion +still exist, have reference to the precise causation of that mighty +stream of events which, under the name of organic evolution, we have now +all learnt to accept as scientifically demonstrated. But it belongs to +the very nature of scientific demonstration that, where matters of great +intricacy as well as of high generality are concerned, the process of +demonstration must be gradual, even if it be not always slow. It is only +by the labours of many minds working in many directions that, in such +cases, truth admits of being eventually displayed. Line upon line, +precept upon precept, here a little and there a little--such is the +course of a scientific revelation; and the larger the subject-matter, +the more subtle and the more complex the causes, the greater must be the +room for individual differences in our reading of the book of Nature. +Now, if all this be true, must we not feel that in the matter of organic +evolution the measure of agreement which has been attained is out of all +proportion to the differences which still remain--differences which, +although of importance in themselves, are insignificant when compared +with those which once divided the opinions of not a few still living +men? And if we are bound to feel this, are we not bound further to feel +that the very intensity of our disputations over these residual matters +of comparative detail, is really the best earnest that can be given of +the determination of our quest--determination which, like that of our +fathers, cannot fail to be speedily rewarded by the discovery of truth? + +Nevertheless, so long as this noise of conflict is in the Senate, we +cannot wonder if the people are perplexed. Therefore, in conclusion, I +may ask it to be remembered exactly what are the questions--and the only +questions--which still divide the parties. + +Having unanimously agreed that organic evolution is a fact and that +natural selection is a cause, or a factor in the process, the primary +question in debate is whether natural selection is the only cause, or +whether it has been assisted by the co-operation of other causes. The +school of Weismann maintain that it is the only cause; and therefore +deem it worse than useless to search for further causes. With this +doctrine Wallace in effect agrees, excepting as regards the particular +case of the human mind. The school of Darwin, on the other hand--to +which I myself claim to belong--believe that natural selection has been +to a considerable extent supplemented by other factors; and, therefore, +although we further believe that it has been the "main" factor, we agree +with Darwin himself in strongly reprobating all attempts to bar _a +priori_ the progress of scientific investigation touching what, if any, +these other factors may be. Lastly, there are several more or less +struggling schools, chiefly composed of individual members who agree +with each other only to the extent of holding that the causal agency of +natural selection is not so great as Darwin supposed. The Duke of +Argyll, Mr. Mivart and Mr. Geddes may be named in this connexion; +together with the self-styled neo-Lamarckians, who seek to magnify the +Lamarckian principles at the expense of the distinctively Darwinian. + +This primary difference of opinion leads deductively to certain +secondary differences. For if a man starts with the premiss that natural +selection must necessarily be the "exclusive" cause of organic +evolution, he is likely to draw conclusions which another man would not +draw who starts with the premiss that natural selection is but the +"main" cause. Of these subordinate differences the most important are +those which relate to the possible transmission of acquired characters, +to the necessary (or only general) utility of specific characters, and +to the problem touching the inter-sterility of allied species. But we +may well hope that before another ten years shall have passed, even +these still outstanding questions will have been finally settled; and +thus that within the limits of an ordinary lifetime the theory of +organic evolution will have been founded and completed in all its parts, +to stand for ever in the world of men as at once the greatest +achievement in the history of science, and the most splendid monument of +the nineteenth century. + +In the later chapters of the foregoing treatise I have sought to +indicate certain matters of general principle, which many years of study +specially devoted to this great movement of contemporary thought have +led me to regard as almost certainly sound in themselves, and no less +certainly requisite as complements of the Darwinian theory. I will now +conclude by briefly summarizing these matters of general principle in +the form of twelve sequent propositions. And, in doing so, I may ask it +to be noticed that the system which these propositions serve to express +may now claim, at the least, to be a strictly logical system. For the +fact that, not merely in its main outlines, but likewise in its details, +it has been independently constructed by Mr. Gulick, proves at any rate +this much; seeing that, where matters of such intricacy are concerned, +nothing but accurate reasoning from a common foundation of _data_ could +possibly have yielded so exact an agreement. The only difference between +us is, that Mr. Gulick has gone into much further detail than I have +ever attempted in the way of classifying the many and varied forms of +isolation; while I have laid more special stress upon the physiological +form, and found in it what appears to me a satisfactory solution of "the +greatest of all the difficulties in the way of accepting the theory of +natural selection as a complete explanation of the origin of +species"--namely, "the remarkable difference between varieties and +species when crossed." + + + + +GENERAL CONCLUSIONS. + +1. NATURAL SELECTION IS PRIMARILY A THEORY OF THE CUMULATIVE DEVELOPMENT +OF ADAPTATIONS WHEREVER THESE OCCUR; AND THEREFORE IS ONLY INCIDENTALLY, +OR LIKEWISE, A THEORY OF THE ORIGIN OF SPECIES IN CASES WHERE ALLIED +SPECIES DIFFER FROM ONE ANOTHER IN RESPECT OF PECULIAR CHARACTERS, WHICH +ARE ALSO ADAPTIVE CHARACTERS. + +2. HENCE, IT DOES NOT FOLLOW FROM THE THEORY OF NATURAL SELECTION THAT +ALL SPECIES--MUCH LESS ALL SPECIFIC CHARACTERS--MUST NECESSARILY HAVE +OWED THEIR ORIGIN TO NATURAL SELECTION; SINCE IT CANNOT BE PROVED +DEDUCTIVELY FROM THE THEORY THAT NO "MEANS OF MODIFICATION" OTHER THAN +NATURAL SELECTION IS COMPETENT TO PRODUCE SUCH SLIGHT DEGREES OF +MODIFICATION AS GO TO CONSTITUTE DIAGNOSTIC DISTINCTIONS BETWEEN CLOSELY +ALLIED SPECIES; WHILE, ON THE OTHER HAND, THERE IS AN OVERWHELMING MASS +OF EVIDENCE TO PROVE THE ORIGIN OF "A LARGE PROPORTIONAL NUMBER OF +SPECIFIC CHARACTERS" BY CAUSES OF MODIFICATION OTHER THAN NATURAL +SELECTION. + +3. THEREFORE, AND UPON THE WHOLE, AS DARWIN SO EMPHATICALLY HELD, +"NATURAL SELECTION HAS BEEN THE MAIN, BUT NOT THE EXCLUSIVE MEANS OF +MODIFICATION." + +4. EVEN IF IT WERE TRUE THAT ALL SPECIES AND ALL SPECIFIC CHARACTERS +MUST NECESSARILY OWE THEIR ORIGIN TO NATURAL SELECTION, IT WOULD STILL +REMAIN ILLOGICAL TO DEFINE THE THEORY OF NATURAL SELECTION AS +INDIFFERENTLY A THEORY OF SPECIES OR A THEORY OF ADAPTATIONS; FOR, EVEN +UPON THIS ERRONEOUS SUPPOSITION, SPECIFIC CHARACTERS AND ADAPTIVE +CHARACTERS WOULD REMAIN VERY FAR INDEED FROM BEING CONTERMINOUS--MOST OF +THE MORE IMPORTANT ADAPTATIONS WHICH OCCUR IN ORGANIC NATURE BEING THE +COMMON PROPERTY OF MANY SPECIES. + +5. IN NO CASE CAN NATURAL SELECTION HAVE BEEN THE CAUSE OF MUTUAL +INFERTILITY BETWEEN ALLIED, OR ANY OTHER, SPECIES--_I.E._ OF THE MOST +GENERAL OF ALL "SPECIFIC CHARACTERS." + +6. WITHOUT ISOLATION, OR THE PREVENTION OF FREE INTERCROSSING, ORGANIC +EVOLUTION IS IN NO CASE POSSIBLE. THEREFORE, IT IS ISOLATION THAT _HAS_ +BEEN "THE EXCLUSIVE MEANS OF MODIFICATION," OR, MORE CORRECTLY, THE +UNIVERSAL CONDITION TO IT. THEREFORE, ALSO, HEREDITY AND VARIABILITY +BEING GIVEN, THE WHOLE THEORY OF ORGANIC EVOLUTION BECOMES A THEORY OF +THE CAUSES AND CONDITIONS WHICH LEAD TO ISOLATION. + +7. ISOLATION MAY BE EITHER DISCRIMINATE OR INDISCRIMINATE. WHEN +DISCRIMINATE, IT HAS REFERENCE TO RESEMBLANCES BETWEEN INDIVIDUALS +CONSTITUTING THE ISOLATED COLONY OR GROUP; WHEN INDISCRIMINATE, IT HAS +NO SUCH REFERENCE. IN THE FORMER CASE THERE ARISES HOMOGAMY, AND IN THE +LATTER CASE THERE ARISES APOGAMY. + +8. EXCEPT WHERE VERY LARGE POPULATIONS ARE CONCERNED, INDISCRIMINATE +ISOLATION ALWAYS TENDS TO BECOME INCREASINGLY DISCRIMINATE; AND, IN THE +MEASURE THAT IT DOES SO, APOGAMY PASSES INTO HOMOGAMY, BY VIRTUE OF +INDEPENDENT VARIABILITY. + +9. NATURAL SELECTION IS ONE AMONG MANY OTHER FORMS OF DISCRIMINATE +ISOLATION, AND PRESENTS IN THIS RELATION THE FOLLOWING PECULIARITIES:-- +(_A_) THE ISOLATION IS WITH REFERENCE TO SUPERIORITY OF FITNESS; (_B_) +IS EFFECTED BY DEATH OF THE EXCLUDED INDIVIDUALS; AND (_C_) UNLESS +ASSISTED BY SOME OTHER FORM OF ISOLATION, CAN ONLY EFFECT MONOTYPIC AS +DISTINGUISHED FROM POLYTYPIC EVOLUTION. + +10. IT IS A GENERAL LAW OF ORGANIC EVOLUTION THAT THE NUMBER OF +POSSIBLE DIRECTIONS IN WHICH DIVERGENCE MAY OCCUR CAN NEVER BE MORE THAN +EQUAL TO THE NUMBER OF CASES OF EFFICIENT ISOLATION; BUT, EXCEPTING +NATURAL SELECTION, ANY ONE FORM OF ISOLATION NEED NOT NECESSARILY +REQUIRE THE CO-OPERATION OF ANOTHER FORM IN ORDER TO CREATE AN +ADDITIONAL CASE OF ISOLATION, OR TO CAUSE POLYTYPIC AS DISTINGUISHED +FROM MONOTYPIC EVOLUTION. + +11. WHERE COMMON AREAS AND POLYTYPIC EVOLUTION ARE CONCERNED, THE MOST +GENERAL AND MOST EFFICIENT FORM OF ISOLATION HAS BEEN THE PHYSIOLOGICAL, +AND THIS WHETHER THE MUTUAL INFERTILITY HAS BEEN THE ANTECEDENT OR THE +CONSEQUENT OF MORPHOLOGICAL CHANGES ON THE PART OF THE ORGANISMS +CONCERNED, AND WHETHER OR NOT THESE CHANGES ARE OF AN ADAPTIVE +CHARACTER. + +12. THIS FORM OF ISOLATION--WHICH, IN REGARD TO INCIPIENT SPECIES, I +HAVE CALLED PHYSIOLOGICAL SELECTION--MAY ACT EITHER ALONE OR IN +CONJUNCTION WITH OTHER FORMS OF ISOLATION ON COMMON AREAS: IN THE FORMER +CASE ITS AGENCY IS OF MOST IMPORTANCE AMONG PLANTS AND THE LOWER CLASSES +OF ANIMALS; IN THE LATTER CASE ITS IMPORTANCE CONSISTS IN ITS GREATLY +INTENSIFYING THE SEGREGATIVE POWER OF WHATEVER OTHER FORM OF ISOLATION +IT MAY BE WITH WHICH IT IS ASSOCIATED. + + + + +_APPENDICES_ + + + + + +APPENDIX A. + +MR. GULICK'S CRITICISM OF MR. WALLACE'S VIEWS ON PHYSIOLOGICAL +SELECTION. + + +I have received from Mr. Gulick the results of his consideration of Mr. +Wallace's criticism. As these results closely resemble those which I +have myself reached, and as they were independently worked out on the +other side of the globe, I deem it desirable to publish them here for +the sake of comparison. + +In his covering letter Mr. Gulick writes:-- + + Mr. Wallace has most certainly adopted the fundamental principles + of our theory, and in an arbitrary way attempted to claim the + results produced by these principles as the effects of natural + selection. He takes our principles, which in the previous chapter + he has combated; but he makes such disjointed use of them that I am + not willing to recognize his statement as an intelligible + exposition of our theory.... I have endeavoured to indicate at what + points Mr. Wallace has deserted his own principles, and at what + points he has failed to make the best use of ours. To bring out + these points distinctly has been no easy task; but if you regard + this paper on _The Preservation and Accumulation of + Cross-infertility_ as giving any help in elucidating the true + principles, and in showing Mr. Wallace's position in regard to + them, I shall be satisfied. Please make any use of it that may seem + desirable, and then forward it to Professor Dana. + +The following is a general summary of Mr. Gulick's results:-- + + Mr. Wallace's criticism of the theory of Physiological Selection is + unsatisfactory; (l) because he has accepted the fundamental + principle of that theory on pages 173-9, in that he maintains that + without the cross-infertility the incipient species there + considered would be swamped; (2) because he assumes that + physiological selection pertains simply to the infertility of first + crosses, and has nothing to do with the infertility of mongrels and + hybrids; (3) because he assumes that infertility between first + crosses is of rare occurrence between species of the same genus, + ignoring the fact that in many species of plants the pollen of the + species is pre-potent on the stigma of the same species when it has + to compete with the pollen of other species of the same genus; (4) + because he not only ignores Mr. Romanes' statement that + cross-infertility often affects "a whole race or strain," but he + gratuitously assumes that the theory of Physiological Selection + excludes this "racial incompatibility" (which Mr. Romanes maintains + is the more probable form), and bases his computation on the + assumption that the cross-infertility is not associated with any + other form of segregation; (5) because he claims to show that "all + infertility not correlated with some _useful_ variation has a + constant tendency to effect its own elimination," while his + computation only shows that, if the cross-infertility is not + associated with some form of _positive_ segregation, it will + disappear[60]; and (6) because he does not observe that the positive + segregation may be secured by the very form of the physiological + incompatibility.... Without here entering into any computation, it + is evident that, e.g. the prepotency of pollen of each kind with + its own kind, if only very slight, will prevent cross-fertilization + as effectually as a moderate degree of instinctive preference in + the case of an animal. + + [60] "Positive segregation" is Mr. Gulick's term for forms of + homogamy other than that which is due to selective fertility. Of + these other, or "positive" forms, natural selection is one; but as + it is far from being the _only_ one, the criticism points out that + utility is not the _only_ conserving principle with which selective + fertility may be associated. + +The paper likewise indicates a point which, in studying Mr. Wallace's +theory, I have missed. It will be remembered that the only apparent +difference between his theory and mine has been shown to consist in +this--that while I was satisfied to state, in a general way, that +natural selection is probably able to increase a selective fertility +which has already been begun by other causes, Mr. Wallace has sought to +exhibit more in detail the precise conditions under which it can do so. +Now, Mr. Gulick shows that the particular conditions which Mr. Wallace +describes, even if they do serve to promote an increase of +cross-infertility, are conditions which preclude the possibility of +natural selection coming into play at all. So that if, under these +particular conditions, a further increase of cross-infertility does take +place, it does not take place in virtue of natural selection. To me it +appears that this criticism is sound; and, if so, it disposes of even +the one very subordinate addition to our theory which Mr. Wallace +"claims" as the most "distinctive" part of his. + +The following is the criticism in question:-- + + On pages 173-186 Mr. Wallace maintains that "Natural selection is, + in some probable cases at all events, able to accumulate variations + in infertility between incipient species" (p. 174); but his + reasoning does not seem to me conclusive. Even if we grant that the + increase of this character [cross-infertility] occurs by the steps + which he describes, _it is not a process of accumulation by natural + selection_. In order to be a means of cumulative modification of + varieties, races, or species, selection, whether artificial or + adaptational [i.e. natural], must preserve certain forms of an + intergenerating stock, to the exclusion of other forms of the same + stock. Progressive change in the size of the occupants of a + poultry-yard may be secured by raising only bantams the first, only + common fowls the second, and only Shanghai fowls the third year; + but this is not the form of selection that has produced the + different races of fowls. So in nature, rats may drive out and + supplant mice; but this kind of selection modifies neither rats + nor mice. On the other hand, if certain variations of mice prevail + over others, through their superior success in escaping their + pursuers, then modification begins. Now, turning to page 175, we + find that, in the illustrative case introduced by Mr. Wallace, the + commencement of infertility between the incipient species is in the + relations to each other of two portions of a species that are + locally segregated from the rest of the species, and partially + segregated from each other by different modes of life. These two + local varieties, being by the terms of his supposition better + adapted to the environment than the freely interbreeding forms in + other parts of the general area, increase till they supplant these + original forms. Then, in some limited portion of the general area, + there arise two still more divergent forms, with greater mutual + infertility, and with increased adaptation to the environment, + enabling them to prevail throughout the whole area. The process + here described, if it takes place, is not modification by natural + selection. + +On the other hand, it _is_ modification by physiological selection. For, +among the several other forms of isolation which are called +into requisition, the physiological (i.e. ever accumulating +cross-infertility) is supposed to play an important part. That the +modification is not modification by natural selection may perhaps be +rendered more apparent by observing, that in as far as _any_ other mode +of isolation is involved or supposed, so far is the _possible_ agency of +natural selection eliminated _as between the two or more otherwise +isolated sections of a species_; and yet it is modes of isolation other +than that furnished by natural selection (i.e. perishing of the less +fit), that Mr. Wallace here supposes to have been concerned--including, +as I have before shown, the physiological form, to which, indeed, he +really assigns most importance of all. Or, as Mr. Gulick states the +matter in his independent criticism:-- + + In the supposed case pictured by Mr. Wallace, the principle by + which the two segregating forms are kept from crossing, and so are + eventually preserved as permanently distinct forms, is no other + than that which Mr. Romanes and myself have discussed under the + terms Physiological Selection and Segregate Fecundity. Not only is + Mr. Wallace's exposition of the divergence and the continuance of + the same in accord with these principles which he has elsewhere + rejected, but his whole exposition is at variance with his own + principle, which, in the previous chapter, he vigorously maintains + in opposition to my statement that many varieties and species of + Sandwich Island land molluscs have arisen, while exposed to the + same environment, in the isolated groves of the successive valleys + of the same mountain range. If he adhered to his own theory, "the + greater infertility between the two forms in one portion of the + area" would be attributed to a difference between the _environment_ + presented in that portion and that presented in the other portions; + and the difficulty would be to consistently show how this greater + infertility could continue unabated when the varieties thus + characterized spread beyond the environment on which the character + depends. But, without power to continue, the process which he + describes would not take place. Therefore, in order to solve the + problem of the _origin_ and _increase_ of infertility between + species, he tacitly gives up his own theory, and adopts not only + the theory of Physiological Selection but that of Intensive + Segregation[61] through Isolation, though he still insists on + calling the process natural selection; for on page 183 he says, "No + form of infertility or sterility between the individuals of a + species can be increased by natural selection unless correlated + with some useful variation, while all infertility not so correlated + has a constant tendency to effect its own elimination." Even this + claim he seems to unwittingly abandon when on page 184 he says: + "The moment it [a species] becomes separated either by geographical + or selective isolation, or by diversity of station or of habits, + then, while each portion must be kept fertile _inter se_, there is + nothing to prevent infertility arising between the two separated + portions." + + [61] By Intensive Segregation Mr. Gulick means what I have called + Independent Variability. + +The criticism proceeds to show yet further inconsistencies and +self-contradictions in Mr. Wallace's treatment of this subject; but it +now seems needless to continue. Nor, indeed, should I have quoted this +much but for the sake of so fully justifying my own criticism by showing +the endorsement which it has received from a completely independent +examination. + + + + +APPENDIX B. + +AN EXAMINATION BY MR. FLETCHER MOULTON OF MR. WALLACE'S CALCULATION +TOUCHING THE POSSIBILITY OF PHYSIOLOGICAL SELECTION EVER ACTING ALONE. + + +We have seen that the only important point of difference between Mr. +Wallace's more recent views and my own on the problem of inter-specific +sterility, has reference to the question whether variations in the way +of cross-infertility can _ever_ arise and act "alone, in an otherwise +undifferentiated species," or whether they can _never_ so arise and act. +It is Mr. Wallace's opinion that, even if they ever do arise alone, at +all events they can never act in differentiating a specific type, seeing +that the chances against their suitable mating must be so great: only if +they be from the first associated with some other form of homogamy, +which will have the effect of determining their suitable mating, does he +think that they can act in the way supposed by our theory of "selective +fertility"[62]. On the other hand, as previously and frequently stated, +I have so strong a belief in the segregating power of physiological +selection, or selective fertility, that I do not think it is necessary +for this principle to be _always___ associated with some other form of +homogamy. From the first, indeed, I have laid great stress (as, also, +has Mr. Gulick) on the re-enforcing influence which association with any +other form of homogamy must exercise upon the physiological form, and +vice versa; but I have also said that, in my opinion, the physiological +form may in many cases be able to act entirely alone, or without +assistance derived from any other source. The question here is, as we +have already so fully seen, a question of but secondary importance; +since, whether or not the physiological form of homogamy ever acts +alone, even Mr. Wallace now allows, or rather argues, that it acts in +combination--and this so habitually, as well as with so much effect, +that it constitutes a usual condition to the origination of species. +Nevertheless, although the only relevancy of his numerical computation +of chances--whereby he thinks that he overturns my theory _in toto_--is +such relevancy as it bears to this question of secondary importance, I +have thought it desirable to refer the question, together with Mr. +Wallace's views upon it, to the consideration of a trained +mathematician. + + [62] His sentence, "all fertility not correlated with some _useful_ + variation has a constant tendency to effect its own elimination," + still further restricts the possible action of physiological + selection to cases where at least one of the other forms of homogamy + with which it is associated is natural selection. Or, in other + words, it is represented that physiological selection must always be + associated with natural selection, even if it be likewise associated + with any other form of exclusive breeding. But as this further + limitation appears to me self-evidently unjustifiable (seeing that + utility is not the only possible means of securing effective + isolation) I here neglect it, and take the wider ground marked out + above. It is needless to say that this is giving Mr. Wallace every + possible advantage, by not holding him to his still narrower ground. + +As this "subordinate question" depends entirely on numerical +computations involving the doctrine of chances, I should first of all +like to remark, that in reference to biological problems of the kind now +before us, I do not myself attach much importance to a merely +mathematical analysis. The conditions which such problems involve are so +varied and complex, that it is impossible to be sure about the validity +of the _data_ upon which a mathematical analysis is founded. +Nevertheless, for the sake of meeting these criticisms upon their own +ground, I will endeavour to show that, even as mathematical +calculations, they are quite untrustworthy. And, in order to do this +effectually, I will quote the results of a much more competent, as well +as a much more thorough, inquiry. I applied to Mr. Moulton for this +purpose, not only because he is one of the ablest mathematicians of my +acquaintance; but also because his interest in biology, and his +knowledge of Darwinian literature, render him well fitted to appreciate +exactly, and in all their bearings, the questions which were submitted +to his consideration. I need only add that his examination was +completely independent, and in no way influenced by me. Having +previously read my paper on _Physiological Selection_, Mr. Gulick's +paper on _Divergent Evolution_, and Mr. Wallace's book on _Darwinism_, +he was in possession of all the materials; and I merely requested the +favour of his opinion upon the whole case from a mathematical point of +view. The following is his reply; and I give it _in extenso_, because it +serves to place in another light some of the general considerations +which it has already been my endeavour to present[63]. + + [63] In our _Nature_ correspondence of 1890-1891, Mr. Wallace + remarked: "If Dr. Romanes will carefully work out numerically (as I + have attempted to do) a few cases showing the preservative and + accumulative agency of pure physiological selection within an + otherwise undifferentiated species, he will do more for his theory + than volumes of general disquisition or any number of assertions + that it _does_ possess this power." Several months before this was + written I had already in my hands Mr. Moulton's letter, with its + accompanying calculations. + +After some introductory remarks on Mr. Wallace's "adoption of the theory +of physiological selection pure and simple," and "the pure caricature of +it which he puts forward as" mine, the letter proceeds thus:-- + + The reason why it is so easy to attack your theory is that it is so + easy to confuse the survival of an _individual_ with the survival + of a _peculiarity_ of _type_. No one has ever said that an + _individual_ is _assisted_ by the possession of selective + fertility: that is a matter which cannot affect his chance of + _life_. Nor has any one said that the possession of selective + fertility in an _individual_ will _of itself_ increase the chance + of his having _progeny_ that will survive, and in turn become the + progenitors of others that will survive. Taken by itself, the fact + that an _individual_ is capable of fertility with some only of the + opposite sex lessens the chance of his having progeny. Whether or + not he is more or less favourably situated than his _confreres_ for + the battle of life must be decided by the _total sum_ of his + peculiarities; and the question whether or not this selective + fertility will be a hindrance must be decided by considerations + depending on the other peculiarities associated with it. + + But when we come to consider the survival or permanence of a _type_ + or _peculiarity_, the case is quite different. It then becomes not + only a favourable circumstance, but, in my opinion, almost a + necessary condition, that the peculiarity should be associated with + selective fertility[64]. + + [64] As, for example, in the case of sexuality in general. It is not + to the advantage of such individual male Arthropoda as perish after + the performance of the sexual act that they should perform it; but + its performance is necessary for the perpetuation of their + species.--G. J. R. + + Take the case of the Jews. I don't think that intermarriage with + other nations would lessen their fertility, or diminish the number + of their progeny; nor is there any reason to think that this + progeny would be unequal to the struggle for existence. But no one + doubts that the abandonment of their voluntary isolation (which + operates so far as this is concerned as a selective fertility), + would lead to the disappearance of the familiar Jewish type. All + the world would get some of it; but as a whole it would be + "swamped." + + Now although no doubt Wallace would admit all this, he fails to + give it the weight it ought to have. In discussing the question of + its operation he considers too exclusively the case of the + individual. + + Of course, a type can only be perpetuated through the medium of + individuals, and all that his argument amounts to is, that + selective fertility would be so fatal to individuals that _no_ type + which presents it could be formed or perpetuated--a conclusion + which is not only absurd in itself, but contradicted by his own + subsequent adoption of your theory. Besides, apart from + calculations (with which I will deal when I write next), such + reasoning brings its own refutation. Selective fertility is not in + the same category as some of the other influences to which an + important share has been ascribed in the formation of the existing + types. _It exists as a recognized phenomenon._ Hence all these + numerical proofs that it would lead to extinction, because it is so + disadvantageous to the possessor, prove too much. They would show + that the degree of selective fertility which so frequently + characterizes species is a most onerous gift; and that, were it not + present, there would be a vastly increased chance of fertility, + which would render the races fitter and lead to their increased + survival. Why then has it not been got rid of? + + The two answers which no doubt would be given seem to me to support + rather than to make against your theory. In the first place, + Wallace might say that this infertility is an advantage because it + keeps pure a type which is specially fitted to its surroundings, as + shown by its continued existence. But if this be so, and it is + necessary to protect the _developed_ type, how much more necessary + to protect the _incipient_ type! In the second place, he might say + that this selective fertility is not so disadvantageous when the + species has been formed, because the individual can choose his mate + from his like; whereas, when it is beginning to be formed, he must + mate blindly, or without what you call "psychological selection." + But this seems to me to be wholly inapplicable to at least half the + animal, and to all the vegetable kingdom. Moreover, with regard to + the other half of the animal kingdom, it merely raises the + question,--How soon will such an incipient type recognize itself? + Seeing it is probable that many families [broods] will belong to + the same [incipient] type, I should not be surprised if it were + found that this sexual recognition and preference sets in very + early. + + But this leads me to the question of your letter. I understand you + to want me to examine and criticize the attempted numerical + arguments against or for your theory. Now it seems to me that it + will be best to take, in the first instance, the vegetable kingdom, + and with regard to it I cannot see how there can be any numerical + argument against the theory. For we often have species side by side + with others nearly allied, but much more numerous. The condition of + these is precisely analogous to that of your incipient species. + They are exposed to fertilization from, say, ten times as numerous + individuals of the allied species. They reject this in favour of + that from the relatively few individuals of their own. Yet the two + species are in competition. I could go through the numerical + arguments of your assailant word for word, applying them to such a + case as this, and they would triumphantly show that the specific + fertility of the rarer kind would lead to its certain extinction. + Yet we know that this is not so. + + Indeed, the too triumphant character of the logic used against you + seems to me to be capable of being turned to your use. If + cross-infertility is so intensely disadvantageous to the + individuals presenting it, it cannot have been _that_ which made + these individuals and their progeny survive. It is therefore a + burden which they have carried. But we find that it is more or less + present in all the closely allied types that occur on common areas: + therefore it must be a necessary feature in the formation of such + types; for it cannot be an accident that it is present in so many. + In other words, it must be the price which the individual and his + progeny pay for their formation into a type. And this is your + theory pure and simple. + + The more I consider the matter, the more I feel that it is + impossible to decide as to the sufficiency of selective fertility + to explain the formation of species, if we consider merely the + effect it would have on the number of individuals, as contrasted + with what it would be if no such peculiarity had developed itself. + Indeed, I may say that on pondering over the matter I have come to + the conclusion, that mere fertility is probably a comparatively + unimportant factor in the preservation of the species, after a + certain sufficient degree of fertility is attained. I do not wish + to be misunderstood. To a certain point fertility is not only + advantageous but necessary, in order to secure survival of the + type; but I feel that little reliance can be placed on calculations + based on the numerical co-efficient of fertility (i. e. the ratio + of the number of offspring to the number of parents) in determining + the relative chance of type-survival. + + Take, for instance, the oak tree. It produces thousands of acorns, + almost the whole of which die without producing any progeny. Have + we any reason to believe that if the number of acorns borne by oak + trees were diminished, even so much as to one-tenth, the race of + oaks would perish? It may of course be said that, if all other + things are equal, the probabilities of survival must be increased + by increased fertility of this kind; but I feel convinced that when + numerical fertility has attained to a high point in circumstances + in which actual increase of the race cannot take place to any + substantial extent, the numerical value of this fertility sinks + down into a factor of the second or third order of importance--that + is to say, into the position of a factor whose effects are only to + be considered when we have duly allowed for the full effects of all + the main factors. Until we have done that, we gain little or + nothing in the way of accuracy of conclusion by taking into + consideration the minor factors. It may be very well to neglect the + effect of the attraction of Jupiter in our early researches on the + motion of the Moon; and our doing so will not prevent the results + being approximate and having considerable value, because we are + retaining the two main factors that establish the motion, viz. the + effects of the Earth and the Sun. But if we exclude the effect of + one of these main factors, our results would be worthless; and it + would not be rendered substantially less so by the fact that we had + taken Jupiter into account in arriving at them. + + You must not imagine, however, that I think it wholly profitless to + see whether there would be any substantial effect on numerical + fertility were _selective_ fertility to manifest itself. But if we + want to derive any assistance from calculation, it must be by + applying it with a good deal more precision and definiteness than + anything that Wallace shows. And, in the first place, it is useless + to confuse the vegetable and animal kingdoms. In the former you + have union unaffected by choice; in the latter, so far at all + events as the higher animals are concerned, you have "psychological + selection." In order to give you a specimen of what can safely be + done by calculation if you take a problem of sufficient + definiteness, I have chosen the case of a flowering plant in which + a certain proportion of the race have developed the peculiarity of + being sterile with the remainder, while retaining the normal + fertility of the race in unions among themselves. In order to give + the greatest advantage to your critics, I have assumed that such + flowers as possess the peculiarity are not self-fertilizable; for + it is clear that if we suppose that they are self-fertilizable, the + fertility need be very slightly affected. + + As I have excluded self-fertilization, it is necessary, if we are + to get any trustworthy results, that one should consider the mode + in which fertilization will be produced. I have taken the case of + fertilization by insects, and have assumed that each flower is + visited a certain number of times by insects during the period when + fertilization is possible; and, further, that the insects which + visit it have on the average visited a certain number of flowers of + the same species before they came there. Of course nothing but + observation can fix these latter numbers; but I should not be + surprised at finding that they are of considerable magnitude[65]. In + order to make the results a little more intelligible, I have + grouped them under the numbers which represent the average number + of flowers that an insect visits in a journey. This is a little + more than twice as great as the number which represents the number + of flowers he has on the average visited before coming to the + individual whose fertility we are considering. + + [65] In this anticipation Mr. Moulton is right. The well-known + botanist, Mr. Bennett, read a most interesting paper on the subject + before the British Association in 1881. His results have since been + corroborated by other observers. In particular, Mr. R. M. Christy + has recorded the movements of 76 insects while visiting at least + 2,400 flowers. (_Entomologist_, July 1883, and _Zool. Journal Lin. + Soc._, August 1883.) The following is an analysis of his results. In + the case of butterflies, in twelve observations on nearly as many + species, there are recorded altogether 99 visits to fifteen species + of flowers; and of these 99 visits 94 were constant to the same + species, leaving only 5 visits to any other, or second species. In + the case of the hive-bee, there were 8 individuals observed: these + visited altogether 258 flowers, and all the visits paid by the same + individual were paid to the same species in each of the eight cases. + Lastly, as regards bumble-bees, there were altogether observed 55 + individuals belonging to four species. These paid altogether 1751 + visits to 94 species of flowers. Of these 1751 visits, 1605 were + paid to one species, 131 to two species, 16 to three, 6 to four, and + 1 to five. Adding all these results together, we find that 75 + insects (butterflies and bees) visited 117 species of flowers: of + these visits, 1957 were constant to one species of flower; 136 were + paid also to a second species, 16 also to a third, 6 also to a + fourth, and 1 also to a fifth. Or, otherwise stated, while 1957 were + absolutely constant, from such absolute constancy there were only + 159 deviations. Moreover, if we eliminate three individual humble + bees, which paid nearly an equal number of visits to two species + (and, therefore, would have ministered to the work of physiological + selection almost as well as the others), the 159 deviations become + reduced to 72, or about four per cent. of the whole.--G. J. R. + + I send you the formula and the calculation on which it is based in + an Appendix; but as I know you have a holy horror of algebraical + formulae, I give you here a few numerical results. + + The cases I have worked out are those in which the number of + insects visiting each flower is 5, or 10, or 15; and I have also + taken 5, 10, and 15, to represent the number of flowers which an + insect visits each journey. This makes nine cases in all; and I + have applied these to two instances--viz. one in which one-fifth of + the whole race have developed cross-infertility, and the other in + which one-tenth only have done so. Taking first the instance where + one-fifth have developed the peculiarity, I find that if on the + average five insects visit a flower, and each insect on the average + visits five flowers on a journey, the fertility is diminished by + about one-tenth. If, however, the average number of flowers the + insect visits is ten, the reduction of fertility is less than one + per cent. And it becomes inappreciable if the average number is + fifteen. If on the average ten insects visit each flower, then, if + each insect visits on the average five flowers on a journey, the + reduction of fertility is a little over one per cent.; but if it + visits ten or fifteen the reduction is inappreciable. If fifteen + insects visit the flower on an average, then, if these insects on + the average visit five or more flowers on a journey, the reduction + of fertility is inappreciable. + + By the term inappreciable I mean that it is not substantially + greater than one-tenth of one per cent.--i.e. not more than + one-thousandth. + + Of course, if the proportion of individuals acquiring the + peculiarity is less, the effect on the fertility under the above + hypothesis will be greater; and it will not be counteracted so + fully unless the number of insect visits is larger, or unless the + insects visit more flowers on a journey. Thus if only one-tenth of + the race have developed the peculiarity, then, if each flower is + visited on the average by five insects who visit five flowers on + each trip, the fertility will be reduced about one-third. If, + however, the insects visit on the average ten flowers per trip, it + will be only diminished about one-tenth; and if they visit fifteen + on each trip, it will be only diminished about one-fortieth. If in + the same case we suppose that each flower receives ten insect + visits, then, if the insects visit on an average five flowers per + trip, the fertility will be diminished about one-eighth. If they + visit ten on a trip, it will be diminished about one-hundredth, and + the diminution is inappreciable if they visit fifteen on a trip. + Similarly, if a flower receives fifteen insect visits, the + diminution is about one-twenty-fifth, if insects visit on the + average five flowers on a trip; and is inappreciable if they visit + ten or fifteen. + + These figures will show you that it is exceedingly possible that a + peculiarity like this, the effect of which at first sight would + seem to be so prejudicial to fertility, may in fact have little or + no influence upon it; and if you set against this the overwhelming + importance of such a peculiarity in segregating the type so as to + give it a chance of becoming a fixed species, you will, I think, + feel that your hypothesis has nothing to fear from a numerical + examination. + + I have not examined the case of fertilization by other means; nor + have I examined the case of fertilization in animals, where + psychological selection can come in. To obtain any useful results, + one would have to consider very carefully the circumstances of each + case; and at present, at all events, I do not think it would be + useful to do so. Nor have I attempted to show the converse of the + problem--viz. the effect of swamping where cross-fertilization is + possible. I shall be very glad to examine any one of these cases if + you want me to do so; but I should prefer to leave it until I hear + from you again. + + If you contrast the results that I have given above with those + given on pages 181 to 183 of Wallace's book, you will see the + enormous difference. His calculations can only apply to the animal + kingdom in those cases in which there is only a union between one + individual of each sex; and before you can deal with the question + of such animals, you will have to take into consideration many + elements besides that of mere fertility, if you wish to get any + tolerably accurate result[66]. + + [66] Here follows the Appendix presenting the calculations on which + the above results are founded; but it seems unnecessary to reproduce + it on the present occasion.--G. J. R. + +The above analysis leaves nothing to be added by me. But, in conclusion, +I may once more repeat that the particular point with which it is +concerned is a point of very subordinate importance. For even if Mr. +Wallace's computation of chances had been found by Mr. Moulton to have +been an adequate computation--and, therefore, even if it had been thus +proved that physiological homogamy must always be associated with some +other form of homogamy in order to produce specific divergence--still +the importance of selective fertility as a factor of organic evolution +would not have been at all diminished. For such a result would merely +have shown that, not only "in many cases" (as I originally said), but +actually in all cases, the selective fertility which I hold to have been +so generally concerned in the differentiation of species has required +for this purpose the co-operation of some among the numerous other forms +of homogamy. But inasmuch as, by hypothesis, no one of these other or +co-operating factors would of itself have been capable of effecting +specific divergence in any of the cases where its association with +selective fertility is concerned, the mathematical proof that such an +association is _always_--and not merely _often_--necessary, would not +have materially affected the theory of the origin of species by means of +physiological selection. We have now seen, however, that a competent +mathematical treatment proves the exact opposite; and, therefore, that +Mr. Wallace's criticism fails even as regards the very subordinate point +in question. + + + + +APPENDIX C. + +SOME EXTRACTS FROM THE AUTHOR'S NOTE-BOOKS. + + +_Bearing of Weismannism on Physiological Selection._--If in view of +other considerations I could fully accept Professor Weismann's theory of +heredity, it would appear to me in no small measure to strengthen my own +theory of physiological selection. For Weismann's theory supposes that +all changes of specific type must have their origin in variations of a +continuous germ-plasm. But _the more the origin of species is referred +directly to variations arising in the sexual elements, the greater is +the play given to the principles of physiological selection_[67]; while, +on the other hand, the less standing-ground is furnished to the theory +that cross-infertility between allied species is due to "external +conditions of life," "prolonged exposure to uniform change of +conditions," "structural modifications re-acting on the sexual +functions"; or, in short, that "somatogenetic" changes of any kind can +of themselves induce the "blastogenetic" change of cross-infertility +between progeny of the same parental stock. + + [67] _Doctrine of Descent and Darwinism_, Eng. trans. p. 139. + + +_Cross-infertility and Diversity of Life._--Observe that one great +consequence of duly recognizing the importance of intercrossing is +indefinitely to raise our estimate of the part played by the principle +of cross-infertility in diversifying organic nature. For whenever in any +line of descent the bar of sterility arises, there the condition is +given for a new crop of departures (species of a genus); and when genera +are formed by the occurrence of this bar, there natural selection and +all other equilibrating causes are supplied with new material for +carrying on adaptational changes in new directions. Thus, owing to +cross-infertility, all these causes are enabled to work out numberless +adaptations in many directions (i. e. lines of descent) simultaneously. + + +_Cross-infertility and Stability._--The importance of sterility as a +diagnostic feature is obvious if we consider that more than any other +feature it serves to give _stability_ to the type; and unless a type is +stable or constant, it cannot be ranked as a species. That Darwin +himself attributes the highest importance to this feature as diagnostic, +see _Forms of Flowers_, pp. 58, 64. + + +_Cross-infertility and Specific Differentiation._--In their elaborate +work on the many species of the genus Hieracium, Nägeli and Peter are +led to the general conclusion that the best defined species are always +those which display absolute sterility _inter se_; while the species +which present most difficulty to the systematist are always those which +most easily hybridize. Moreover, they find, as another general rule +applicable to the whole genus, that there is a constant correlation +between inability to hybridize and absence of intermediate varieties, +and, conversely, between ability to hybridize and the presence of such +varieties. + + +_Cross-infertility in Domesticated Cattle._--Mr. J. W. Crompton, who has +had a large experience as a professional cattle-breeder, writes to me +(March 2, 1887)-- + + "That form of barrenness, very common in some districts, which + makes heifers become what are called 'bullers'--that is, + irregularly in 'season,' wild, and failing to conceive--is + certainly produced by excess of iron in their drinking-water, and I + suspect also by a deficiency of potash in the soil." + +He also informs me that pure white beasts of either sex are so well +known by experienced breeders to be comparatively infertile together, +that they are never used for breeding purposes, so that "in some parts +of the country, where a tendency to sterility had become so confirmed in +the white race that they utterly died out," only the coloured breeds are +now to be found. He goes on to say that if "a lot of white heifers were +put to a lot of white bulls, I think you would probably get a fertile +breed of pure white cattle.... I think, in short, that domestication has +produced just what your theory suggests, a new variety inclined to prove +sterile with its parent stock." + +Commenting on the origin of domesticated cattle, Professor Oscar Schmidt +remarks (_Doctrine of Descent_, p. 139)-- + + "Rütimeyer's minute researches on domestic cattle have shown that, + in Europe at least, three well-defined species of the diluvial + period have contributed to their formation--_Bos primigenius_, + _longifrons_, and _frontosus_. These species once lived + geographically separate, but contemporaneously; and they and their + specific peculiarities have perished, to rise again in our domestic + races. These races breed together with unqualified fertility. In + the form of skull and horns they recall one or other of the extinct + species; but collectively they constitute a new main species. That + from their various breeds, the three or any one of the aboriginal + species would ever emerge in a state of pristine purity, would be + an utterly ludicrous assertion." + +Now, seeing that these "aboriginal species," although living +"contemporaneously," were "geographically separate," we can well +understand that their divergence of type from a common ancestor did not +require, as a condition to their divergence, that any cross-sterility +should have arisen between them. The geographical isolation was enough +to secure immunity from mutual intercrossing, and therefore, as our +present theory would have expected as probable, morphological divergence +occurred without any corresponding physiological divergence, as must +almost certainly have been the case if such polytypic evolution had +occurred on a common area. Indeed, one of the two lines of experimental +verification of our theory consists in selecting cases where nearly +allied species are separated by geographical barriers, and proving that, +in such cases, there is no cross-sterility. + + +_Fertility of Domesticated Varieties._--Some writers have sought to +explain the contrast between domesticated varieties and natural species +in respect of fertility when crossed, by the consideration that it is +only those natural species which have proved themselves so far flexible +as to continue fertile under changed conditions of life that can have +ever allowed themselves to become domesticated. But although this +condition may well serve to explain the unimpaired fertility under +domestication of such species as for this very reason have ever become +domesticated, I fail to see how it explains the further and altogether +different fact, that this fertility continues unimpaired between all the +newly differentiated morphological types which have been derived from +the original specific type. It is one thing that this type should +continue fertile after domestication: it is quite another thing that +fertility should continue as between all its modified descendants, even +although the amount of modification may extend much further than that +which usually obtains between different natural species. + + +_Testing for Cross-infertility_ among varieties growing on the same area +is a much more crucial line of verification than testing for unimpaired +fertility between allied species which occupy different areas, because +while in the former case we are dealing with "incipient species" with a +view to ascertaining whether the divergence which they have already +undergone is accompanied by physiological isolation, in the latter case +we can never be sure that two allied species, which are now widely +disconnected geographically, have always been so disconnected. They may +both have originated on the same area; or one may have diverged from the +other before it migrated from that area; or even if, when it migrated, +it was unchanged, and if in its new home it afterwards split into two +species by physiological selection, the newer species would probably +prove infertile, not only with its parent type, but also with its +grand-parent in any other part of the world. + + +_Seebohm on Isolation._--Seebohm is so strongly influenced by the +difficulty from "the swamping effects of free intercrossing," that he is +driven by it to adopt Asa Gray's hypothesis of variations as +teleological. Indeed, he goes as far as Wagner, for he maintains that in +no case can there be divergence or multiplication of species without +isolation. He makes the important statement that "the more the +geographical distribution of birds is studied, the more doubtful it +seems to be that any species of bird has ever been differentiated +without the aid of geographical isolation" (_Charadriidae_, p. 17). If +this is true, it makes in favour of physiological selection by showing +the paramount importance of the swamping effects of intercrossing, and +consequent importance of isolation. But it makes against physiological +selection by showing that the geographical form of isolation is +sufficient to explain all the cases of specific differentiation in +birds. But I must remember that the latter point rests largely on +negative inference, and that birds, owing to their highly locomotive +habits, are the class of animals where physiological selection is likely +to be most handicapped. + + +_Herbert on Hybridization._--Herbert tells us that when he first +astonished the Horticultural Society by laying before them the results +of his experiments on hybridization, his brother botanists took serious +alarm. For it appeared to them that this "intermixture of species would +confuse the labours of botanists, and force them to work their way +through a wilderness of uncertainty." Therefore he was bluntly told by +several of these gentlemen, "I do not thank you for your mules." Now, +although naturalists have travelled far and learnt much since those +days, it appears to me that a modern evolutionist might still turn to +the horticulturist with the same words. For assuredly he has no reason +to thank the horticulturist for his mules, until he has found a +satisfactory answer to the question why it is that natural species +differ so profoundly as regards their capacity for hybridizing. + + +_Advance on Herbert's Position._--- If it be said that all my work +amounts to showing what Herbert said long ago--viz. that the only true +or natural distinction between organic types is the sexual +distinction--I answer that my work does much more than this. For it +shows that the principle of sterility is the main condition to the +differentiation, not merely of species and genera, but also to the +evolution of adaptations everywhere, in higher as well as in lower +taxonomic divisions. Moreover, even though naturalists were everywhere +to consent to abandon specific designations, and, as Herbert advises, to +"entrench themselves behind genera," there would still remain the facts +of what are now called specific differences (of the secondary or +morphological kind), and by whatever name these are called, they alike +demand explanation at the hands of the evolutionist. + + +_Fritz Müller on Cross-infertility._--Fritz Müller writes, "Every plant +requires, for the production of the strongest possible and most prolific +progeny, a certain amount of difference between male and female elements +which unite. Fertility is diminished as well when this degree is too low +(in relatives too closely allied) as when it is too high (in those too +little related)." Then he adds, as a general rule, "Species which are +wholly sterile with pollen of the same stock, and even with pollen of +nearly allied stocks, will generally be fertilized very readily by the +pollen of another species. The self-sterile species of the genus +Abutilon, which are, on the other hand, so much inclined to +hybridization, afford a good example of this theory, which appears to be +confirmed also by Lobelia, Passiflora, and Oncidium" (_American +Naturalist_, vol. viii, pp. 223-4, 1874). + + +_Different groups of plants exhibit remarkable differences in the +capability of their constituent species to hybridize._--In so far as +these differences have reference only to first crosses, they have no +bearing either for or against my theory. Only in so far as the +differences extend to the production of fertile hybrids does any +question arise for me. First of all, therefore, I must ascertain whether +(or how far) there is any correlation between groups whose species +manifest aptitude to form first crosses, and groups where first crosses +manifest aptitude to produce fertile hybrids. Next, whatever the result +of this inquiry should be, if I find that certain natural groups of +plants exhibit comparatively well-marked tendencies to form fertile +hybrids, the question will arise, Are these tendencies correlated with +_paucity_ of species? If they are, the fact would make strongly in +favour of physiological selection. For the fact would mean that in these +natural groups, owing to "the nature of the organisms" included under +them, less opportunity is given to physiological selection in its work +of differentiating specific types than is given by other natural groups +where the nature of the organism renders them more prone to mutual +sterility. But in prosecuting this branch of verification, I must +remember to allow for possibilities of differential degrees of +geographical isolation in the different groups compared. + +On this subject Focke writes me as follows:--"In a natural group +(family, order, genus) showing considerable variability in the structure +of the flower, we may expect to find [or do find] a greater number of +mules than in a group whose species are only distinguished by +differences in the shape of the leaves, or in growth, &c. I do not +know, however, which in this connexion of things is the cause and which +the effect. A useful ancestral structure of the flower may be conserved +by an otherwise varying progeny, on condition that the progress of +diversity be not disturbed by frequent intercrossings. [Therefore, if +this condition be satisfied, the structure of the flower in different +members of the group will continue constant: here the cause of +_constancy_ in the flower (however much variability there may be in the +leaves, &c.) is its original _inability_ to hybridize.] On the other +hand, in species or groups ready to hybridize [or capable of +hybridizing], the fixation of a new specific type will require some +change in the structure of the flower, and a change considerable enough +to alter the conditions of fertilization. [Here the reason of the +_in_constancy of the flower in different members of the group is the +original _aptitude_ of their ancestral forms to hybridize.] Perhaps +there is something in this suggestion, but certainly there are other +efficient physiological relations, which are at present unknown. Your +theory of physiological selection may serve to explain many difficult +facts." + + +_The Importance of Prepotency._--A. Kerner shows by means of his own +observations on sundry species of plants which hybridize in the wild +state, that they do so very much more frequently if both, or even if +only one of the parent forms be rare in the neighbourhood. This fact can +only be explained by supposing that, even in species most prone to +hybridizing under Nature, there is some degree of prepotency of pollen +of the same species over that of the other species; so that where both +species are common, it is correspondingly rare that the foreign pollen +gets a chance. But if there were no prepotency, the two species would +blend; and this Kerner supposes must actually take place wherever two +previously separated species, thus physiologically circumstanced, happen +to be brought together. (Kerner's paper is published in _Oester. Bot. +Zeitschrift_, XXI, 1871, where he alludes to sundry other papers of his +own advocating similar views.) + +The relation of these observations to Jordan's _espèces affines_ is +obvious. We have only to suppose that some such slight and constant +difference characterizes the sexual elements of these allied varieties +as demonstrably characterizes their morphology, and we can understand +how pollen-prepotency would keep the forms distinct--such forms, +therefore, being so many records of such prepotency. + +Both from Kerner's work, and still more from that of Jordan and Nägeli, +I conclude that (at all events in plants) prepotency is the way in which +physiological selection chiefly acts. That is to say, _sudden_ and +_extreme_ variations in the way of sexual incompatibility are probably +rare, as compared with some degree of prepotency. According as this +degree is small or great so will be the amount of the corresponding +separation. This view would show that in plants the principle of +physiological selection is one of immensely widespread influence, +causing (on the same areas) more or less permanent varieties much below +specific rank. And when we remember on how delicate a balance of +physiological conditions complete correspondency of pollen to ovules +depends, we may be prepared to expect that the phenomenon of prepotency +is not of uncommon occurrence. + + +_Self-fertilization and Variability._--It occurred to Count Berg Sagnitz +that, if physiological selection is a true principle in nature, +vegetable species in which self-fertilization obtains ought to be more +rich in constant varieties than are species in which cross-fertilization +rules. For, although even in the latter case physiological isolation may +occasionally arise, it cannot be of such habitual or constant occurrence +as it must be in the former case. Acting on this idea, Count Berg +Sagnitz applied himself to ascertain whether there is any general +correlation between the habit of self-fertilization and the fact of +high variability; and he says that in all the cases which he has +hitherto investigated, the correlation in question is unmistakable. + + +_Additional Hypothesis concerning Physiological Selection._--In +reciprocal crosses _A_ × _B_ is often more fertile than _B_ × _A_. If +hybrid _AB_ is more fertile with _A_, and hybrid _BA_ with _B_, than +vice versa, there would be given a good analogy on which to found the +following hypothesis. + +Let _A_ and _B_ be two intergenerating groups in which segregate +fecundity is first beginning. Of the hybrids, _AB_ will be more fertile +with _A_, and _BA_ with _B_, than vice versa. The interbreeding of _AB_ +with _A_ will eventually modify sexual characters of _A_ by assimilating +it to those of _AB_, while the interbreeding of _BA_ with _B_ will +similarly modify sexual characters of _B_ by assimilating it to those of +_BA_. Consequently, _A_ will become more and more infertile with _B_, +while _B_ becomes more and more infertile with _A_. Fewer and fewer +hybrids will thus be produced till mutual sterility is complete. + +To sustain this hypothesis it would be needful to prove experimentally, +(1) that hybrid forms _AB_ are more fertile with _A_ than with _B_, +while hybrid forms _BA_ are more fertile with _B_ than with _A_ [or, it +may be possible that the opposite relations would be found to obtain, +viz. that _AB_ would be more fertile with _B_, and _BA_ with _A_]; (2) +that, if so, effect of intercrossing _AB_ with _A_ is to make progeny +more fertile with _A_ than with _B_, while effect of intercrossing _BA_ +with _B_ is to make progeny more fertile with _B_ than with _A_. + +Such experiments had best be tried with species where there is already +known to be a difference of fertility between reciprocal crosses (e.g. +Matthiola annua and M. glabra, see _Origin of Species_, p. 244). + + + + +INDEX + + +A. + +ALLEN, Mr. J. A., on variation under nature, 34. + +Amixia, 12-28, 110-115, 117-133. + +Apogamy, 5, 6, 10, 18, 28. + + +B. + +BELT, on physiological selection, 44. + +BERG SAGNITZ, Count, on self-fertilization and variability, 177. + +Breeding, separate and segregate, 5. + +Butterflies of polar regions and Alps, 133. + + +C. + +CATCHPOOL, Mr., on physiological selection, 44, 137. + +Cross-infertility, 46; + and varietal divergence, 82; + and diversity of life, 169; + and stability, 170; + and specific differentiation, 170; + in domesticated cattle, 170; + testing for, 172; + Fritz Müller on, 174. + + +D. + +_Darwin_, Charles, on isolation, 2, 106; + on diversity under nature, 31; + on the fertility of varieties, 50; + on the origin of cross-infertility, 51; + on distribution, 68; + on prepotency, 89; + on geographical isolation, 101, 108; + on methodical selection, 102; + on modification in large areas, 103; + on the swamping effects of intercrossing, 105; + on independent variability, 109; + on domestic animals, 110. + +DELBOEUF, law of independent variability, 13. + +Differentiation under natural selection, 37. + +Diversity of life and cross-infertility, 169. + +Domesticated cattle and cross-infertility, 170, 172. + + +E. + +Evidences of physiological selection, 62. + +Evolution, monotypic and polytypic, 21, 75, 102, 107, 112, 129. + +Experimental research in physiological selection, 85. + + +F. + +Fertility of domesticated varieties, 172. + +FOCKE, Herr, on hybridization, 175. + + +G. + +GALTON, Mr. Francis, law of regression, 39. + +General conclusions, 144. + +Geographical distribution and physiological selection, 65. + +GIARD, M., on apogamy, 14. + +GRABHAM, Dr., on mollusca of Madeira, 135. + +GULICK, Rev. J., on natural Selection as a mode of isolation, 9; + on divergence, 11; + on segregate breeding, 19; + on geographical distribution, 27; + on the prevention of intercrossing, 127; + on Mr. Wallace's criticisms, 151. + + +H. + +HERBERT, on hybridization, 173; + advance on his position, 174. + +HERDMAN, Prof., on physiological isolation, 123. + +Historical sketch of opinions on isolation, 101. + +Homogamy, 5, 6; + forms of, 7, 19, 29. + +Hybridization, HERBERT on, 173; + in plants, 175. + +Hypothesis, additional, concerning physiological selection, 178. + + +I. + +Independent variability, 12-29. + +Isolation, defined, 2; + forms of, 3, 6; + geographical, 3; + discriminate and indiscriminate, 5; + physiological, 9, 41, 58; + its importance, 39; + sketch of opinions on, 101; + general conclusions, 144; + SEEBOHM on, 173. + + +J. + +JORDAN, M., on cross sterile varieties of plants, 86; + his researches summarized, 87. + + +K. + +KERNER, Prof. A., on prepotency, 176. + + +L. + +LANKESTER, Prof. Ray, on divergent evolution, 15. + +LE CONTE, Prof., on fossil snails of steinheim, 95; + on isolation, 129. + +LIVINGSTONE, Dr. David, Quoted, 123. + + +M. + +MELDOLA, Prof., on difficulty from intercrossing, 121. + +Misunderstandings of Physiological selection, 59. + +Monotypic evolution, see Evolution. + +MORGAN, Prof. Lloyd, on sterility, 56; + on isolation, 128. + +MOULTON, Mr. Fletcher, an examination of Mr. Wallace's calculations on +physiological selection, 157. + +MÜLLER, Fritz, on cross-infertility, 174. + + +N. + +NÄGELI, on isolation, 76; + on synoicy, 78, 82. + +Natural selection, a form of discriminate isolation, 9, 10, 23; + leads to monotypic evolution, 24-29; + difficulties of, 41, 51. + + +P. + +Panmixia, 12. + +Physiological selection, 9, 41; + summarized, 58; + misunderstandings of, 59; + evidences of, 81-119; + and Weismannism, 169; + additional hypothesis, 178. + +Polytypic evolution, see Evolution. + +Prepotency, 89; importance of, 176. + + +S. + +SCHMIDT, Prof. Oscar, on domesticated cattle, 171. + +SEEBOHM on isolation, 173. + +Segregation, 28. + +Selection, physiological, see Physiological selection. + +Self-fertilization and variability, 177. + +Snails of Sandwich Islands, 16, 130; + fossil of Steinheim, 95. + +Specific differentiation and cross-infertility, 170. + +Stability and cross-infertility, 170. + +Synoicy, 78. + + +T. + +Topographical distribution and physiological selection, 74; + of varieties, 81. + +Transformation, serial and divergent, 21, 121. + + +V. + +Variability and self-fertilization, 177. + +Variation in birds, 34. + +Varieties, topographical distribution of, 81. + + +W. + +WAGNER, Maritz, 3; + on geographical isolation, 76; + quoted, 103; + law of migration, 111. + +WALLACE, Mr. A. R., 3, 17; + quoted, 34, 47, 51, 57,130-136; + criticized by Gulick, 152. + +WEISMANN, Prof., on geographical isolation, 76, 114-118. + +Weismannism and physiological selection, 169. + + + + +TITLE LIST OF OPEN COURT PUBLICATIONS ARRANGED ALPHABETICALLY BY AUTHORS + + +ANESAKI, M. + +345. BUDDHIST AND CHRISTIAN GOSPELS, Being Gospel Parallels from Pali +Texts. Now first compared from the originals by Albert J. Edmunds. +Edited with parallels and notes from the Chinese Buddhist Triptaka by +_M. Anesaki._ $1.50 net. + + +BAYNE, JULIA TAFT. + +323. HADLEY BALLADS. _Julia Taft Bayne._ 75c net. + + +BERKELEY, GEORGE. + +307. A TREATISE CONCERNING THE PRINCIPLES OF HUMAN KNOWLEDGE. _George +Berkeley._ Cloth, 60c net. (3s. net.) + +308. THREE DIALOGUES BETWEEN HYLAS AND PHILONOUS. _George Berkeley._ +Cloth, 60c net. (3s. net.) + + +BINET, ALFRED. + +201. THE PSYCHIC LIFE OF MICRO-ORGANISMS. _Alfred Binet._ 75c. (3s. 6d.) + +270. 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(22 instances) and "i.e." (14 instances). + +Different hyphenation patterns were left as in the original text: + + prepotent (1 instance) pre-potent (1 instance) + presupposes (1) pre-supposes (1) + reacting(5) re-acting (1) + restatement (1) re-statement (2) + superinduced (2) super-induced (1) + + + + + + + + +End of the Project Gutenberg EBook of Darwin, and After Darwin (Vol 3 of 3), by +George John Romanes + +*** END OF THIS PROJECT GUTENBERG EBOOK DARWIN, AFTER DARWIN (VOL 3 OF 3) *** + +***** This file should be named 37777-8.txt or 37777-8.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/3/7/7/7/37777/ + +Produced by Marilynda Fraser-Cunliffe, LN Yaddanapudi and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Darwin, and After Darwin (Vol 3 of 3) + Post-Darwinian Questions: Isolation and Physiological Selection + +Author: George John Romanes + +Release Date: October 17, 2011 [EBook #37777] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK DARWIN, AFTER DARWIN (VOL 3 OF 3) *** + + + + +Produced by Marilynda Fraser-Cunliffe, LN Yaddanapudi and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + + + + + +</pre> + + + + + +<h1>DARWIN, AND AFTER DARWIN</h1> + +<h2>III</h2> + +<h2>POST-DARWINIAN QUESTIONS<br /> +ISOLATION AND PHYSIOLOGICAL SELECTION</h2> + +<hr class="r65" /> + +<h3>BY THE SAME AUTHOR.</h3> +<hr class="r5" /> + +<p>DARWIN, AND AFTER DARWIN. An Exposition of the +Darwinian Theory and a Discussion of Post-Darwinian +Questions.</p> + +<div class="blockquot"><p>1. <span class="smcap">The Darwinian Theory.</span> With portrait of Darwin. +460 pages. 125 illustrations. Cloth, $2.00.</p> + +<p>2. <span class="smcap">Post-Darwinian Questions.</span> Edited by Prof. C. +Lloyd Morgan. With portrait of G. J. Romanes. +338 pages. Cloth, $1.50.</p> + +<p>3. <span class="smcap">Post-Darwinian Questions. Isolation and Physiological +Selection.</span> Edited by Prof. C. Lloyd +Morgan. With portrait of Mr. J. T. Gulick. 181 +pages. Cloth, $1.00.</p> + +<p>All three volumes together, $4.00 net.</p></div> + +<p>AN EXAMINATION OF WEISMANNISM. With portrait of +Weismann. 236 pages. Cloth, $1.00.</p> + +<p>THOUGHTS ON RELIGION. Edited by Charles Gore, M.A., +Canon of Westminster. Third Edition. 184 pages. Cloth, +gilt top, $1.25.</p> + +<hr class="r5" /> + +<p class="center">THE OPEN COURT PUBLISHING COMPANY,<br /> +<span class="smcap">324 Dearborn Street, Chicago.</span></p> + +<hr class="r65" /> + +<div class="figcenter" style="width: 332px;"> +<img src="images/illus-004.jpg" width="332" height="600" alt="John J. Gulick" title="John J. Gulick" /></div> + +<hr class="chap" /> + + + + +<h1>DARWIN, AND AFTER DARWIN</h1> + +<h3><i>AN EXPOSITION OF THE DARWINIAN THEORY<br /> +AND A DISCUSSION OF<br /> +POST-DARWINIAN QUESTIONS</i></h3> + +<h3><span style="font-size:small;">BY THE LATE</span><br /> +GEORGE JOHN ROMANES, M.A., LL.D., F.R.S.<br /> +<span style="font-size:small;"><i>Honorary Fellow of Gonville and Caius College, Cambridge</i></span></h3> + + +<h2>III<br /> +POST-DARWINIAN QUESTIONS<br /> +ISOLATION<br /> +AND PHYSIOLOGICAL SELECTION</h2> + + +<h4>Chicago<br /> +THE OPEN COURT PUBLISHING COMPANY<br /> +1906</h4> + +<hr class="r65" /> + +<p class="center">CHAPTER 1. COPYRIGHTED BY<br /> +THE OPEN COURT PUBLISHING CO.<br /> +1897.</p> + + + +<p class="center">The Lakeside Press<br /> +R. R. DONNELLEY & SONS CO., CHICAGO</p> + +<hr class="chap" /> + +<p class="pagenum"><a name="Page_v" id="Page_v">[Pg v]</a></p> + + + + +<h2>PREFACE</h2> + + +<p>Of the six chapters which constitute this concluding +volume of G. J. Romanes' <i>Darwin, and after +Darwin</i>, three, the first two and the last, were in +type at the time of his death. I have not considered +myself at liberty to make any alterations of moment +in these chapters. For the selection and arrangement +of all that is contained in the other three +chapters I am wholly responsible.</p> + +<p>Two long controversial Appendices have been +omitted. Those marked A and B remain in accordance +with the author's expressed injunctions. In +a third, marked C, a few passages from the author's +note-books or MSS. have been printed.</p> + +<p>The portrait of the Rev. J. Gulick, which forms the +frontispiece, was prepared for this volume before the +author's death. Mr. Gulick's chief contributions to +the theory of physiological selection are to be found +in the Linnean Society's <i>Journal</i> (<i>Zoology</i>, vols. xx<span class="pagenum"><a name="Page_vi" id="Page_vi">[Pg vi]</a></span> +and xxiii), and in four letters to <i>Nature</i> (vol. xli. +p. 536; vol. xlii. pp. 28 and 369; and vol. xliv. +p. 29).</p> + +<p>I have to thank Mr. Francis Galton, D.C.L., F.R.S. +and Mr. F. Howard Collins for valuable assistance +generously rendered for the sake of one whom all +who knew him held dear. For he was, if I may +echo the words of Huxley, "a friend endeared to +me, as to so many others, by his kindly nature, and +justly valued by all his colleagues for his powers +of investigation and his zeal for the advancement of +science."</p> + +<p><span class="smcap" style="margin-left:80%;">C. Lloyd Morgan.</span></p> + +<p style="margin-left:10%;"><span class="smcap">Bristol</span>, <i>May 1897</i>.</p> + +<hr class="chap" /> + +<p class="pagenum"><a name="Page_vii" id="Page_vii">[Pg vii]</a></p> + + + + +<h2>CONTENTS</h2> + + +<p class="center">CHAPTER I.</p> + +<p class="blockquot"><span class="smcap">Isolation</span> <span class="ralign"><a href="#Page_1">1</a></span></p> + +<p class="center">CHAPTER II.</p> + +<p class="blockquot"><span class="smcap">Isolation</span> (<i>continued</i>) <span class="ralign"><a href="#Page_28">28</a></span></p> + +<p class="center">CHAPTER III.</p> + +<p class="blockquot"><span class="smcap">Physiological Selection</span> <span class="ralign"><a href="#Page_41">41</a></span></p> + +<p class="center">CHAPTER IV.</p> + +<p class="blockquot"><span class="smcap">Evidences of Physiological Selection</span> <span class="ralign"><a href="#Page_62">62</a></span></p> + +<p class="center">CHAPTER V.</p> + +<p class="blockquot"><span class="smcap">Further Evidences of Physiological Selection</span> <span class="ralign"><a href="#Page_81">81</a></span></p> + +<p class="center">CHAPTER VI.</p> + +<p><span class="smcap blockquot">A Brief History of Isolation as a Factor in +Organic Evolution</span> <span class="ralign"><a href="#Page_101">101</a></span></p> + +<p class="blockquot"><span class="smcap">General Conclusions</span> <span class="ralign"><a href="#Page_144">144</a></span></p> + +<hr class="r5" /> + +<p class="blockquot"><span class="smcap">Appendix A. Mr. Gulick's Criticism of Mr. Wallace's +<span class="pagenum"><a name="Page_viii" id="Page_viii">[Pg viii]</a></span>Views on Physiological Selection</span> <span class="ralign"><a href="#Page_151">151</a></span></p> + +<p class="blockquot"><span class="smcap">Appendix B. An Examination by Mr. Fletcher +Moulton of Mr. Wallace's Calculation touching +the Possibility of Physiological Selection +ever acting alone</span> <span class="ralign"><a href="#Page_157">157</a></span></p> + +<p class="blockquot"><span class="smcap">Appendix C. Some Extracts from the Author's +Note-books</span> <span class="ralign"><a href="#Page_169">169</a></span></p> +<hr class="full" /> + + + + +<p class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></p> +<h2><i>ISOLATION</i></h2> +<hr class="chap" /> + + + + +<h2>CHAPTER I.<br /> +<span class="smcap">Isolation.</span></h2> + + +<p>This treatise will now draw to a close by considering +what, in my opinion, is one of the most important +principles that are concerned in the process of organic +evolution—namely, Isolation. I say in <i>my</i> opinion +such is the case, because, although the importance of +isolation is more or less recognized by every naturalist, +I know of only one other who has perceived all that +the principle involves. This naturalist is the Rev. J. +Gulick, and to his essays on the subject I attribute +a higher value than to any other work in the field of +Darwinian thought since the date of Darwin's death<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a>. +For it is now my matured conviction that a new point +of departure has here been taken in the philosophy of +Darwinism, and one which opens up new territories +for scientific exploration of an endlessly wide and +varied character. Indeed I believe, with Mr. Gulick,<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span> +that in the principle of Isolation we have a principle +so fundamental and so universal, that even the great +principle of Natural Selection lies less deep, and +pervades a region of smaller extent. Equalled only +in its importance by the two basal principles of +Heredity and Variation, this principle of Isolation +constitutes the third pillar of a tripod on which is +reared the whole superstructure of organic evolution.</p> + +<p>By isolation I mean simply the prevention of intercrossing +between a separated section of a species or +kind and the rest of that species or kind. Whether +such a separation be due to geographical barriers, to +migration, or to any other state of matters leading +to exclusive breeding within the separated group, +I shall indifferently employ the term isolation for the +purpose of designating what in all cases is the same +result—namely, a prevention of intercrossing between +A and B, where A is the separated portion and B the +rest of the species or kind.</p> + +<p>The importance of isolation as against dissimilar +forms has always been fully appreciated by breeders, +fanciers, horticulturists, &c., who are therefore most +careful to prevent their pedigree productions from +intercrossing with any other stock. Isolation is indeed, +as Darwin has observed, "the corner-stone of the +breeder's art." And similarly with plants and animals +in a state of nature: unless intercrossing with allied +(i.e. dissimilar) forms is prevented, the principle of +heredity is bound to work for uniformity, by blending +the dissimilar types in one: only when there is +exclusive breeding of similarly modified forms can the +principle of heredity work in the direction of change—i.e. +of evolution.</p> + +<p><span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span> +Now, the forms of isolation—or the conditions +which may lead to exclusive breeding—are manifold. +One of the most important, as well as the most obvious, +is geographical isolation; and no one questions that +this has been an important factor in the process of +evolution, although opinions still vary greatly as to +the degree of its importance in this respect. At one +end of the series we may place the opinion of Mr. +Wallace, who denies that any of what may be termed +the evolutionary effect of geographical isolation is due +to "influence exerted by isolation <i>per se</i>." This effect, +he says, is to be ascribed exclusively to the fact that +a geographically isolated portion of a species must +always encounter a change of environment, and therefore +a new set of conditions necessitating a new set of +adaptations at the hands of natural selection<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a>. At +the other end of the series we must place the opinion +of Moritz Wagner, who many years ago published +a masterly essay<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a>, the object of which was to prove +that, in the absence of geographical isolation (including +migration), natural selection would be powerless to +effect any change of specific type. For, he argued, +the initial variations on which the action of this +principle depends would otherwise be inevitably +swamped by free intercrossing. Wagner adduced +a large number of interesting facts in support of this +opinion; but although he thus succeeded in enforcing +the truth that geographical isolation is an +important aid to organic evolution, he failed to establish +his conclusion that it is an indispensable condition.<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span> +Nevertheless he may have been right—and, as I shall +presently show, I believe he was right—in his fundamental +premiss, that in the presence of free intercrossing +natural selection would be powerless to effect +divergent evolution. Where he went wrong was in +not perceiving that geographical isolation is not the +only form of isolation. Had it occurred to him that +there may be other forms quite as effectual for the +prevention of free intercrossing, his essay could hardly +have failed to mark an epoch in the history of Darwinism. +But, on account of this oversight, he really +weakened his main contention, namely, that in the +presence of free intercrossing natural selection must +be powerless to effect divergent evolution. This main +contention I am now about to re-argue. At present, +therefore, we have only to observe that Wagner did it +much more harm than good by neglecting to perceive +that free intercrossing may be prevented in many other +ways besides by migration, and by the intervention of +geographical barriers.</p> + +<p>In order that we may set out with clearer views +upon this matter, I will make one or two preliminary +remarks on the more general facts of isolation as these +are found to occur in nature.</p> + +<p>In the first place, it is obvious that isolation +admits of degrees: it may be either total or partial; +and, if partial, may occur in numberless grades of +efficiency. This is so manifest that I need not wait +to give illustrations. But now, in the second place, +there is another general fact appertaining to isolation +which is not so manifest, and a clear appreciation +of which is so essential to any adequate consideration +of the subject, that I believe the reason why<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span> +evolutionists have hitherto failed to perceive the full +importance of isolation, is because they have failed +to perceive the distinction which has now to be pointed +out. The distinction is, that isolation may be either +discriminate or indiscriminate. If it be discriminate, +the isolation has reference to the resemblance of the +separated individuals to one another; if it be indiscriminate, +it has no such reference. For example, if +a shepherd divides a flock of sheep without regard to +their characters, he is isolating one section from the +other indiscriminately; but if he places all the white +sheep in one field, and all the black sheep in another +field, he is isolating one section from the other +discriminately. Or, if geological subsidence divides +a species into two parts, the isolation will be indiscriminate; +but if the separation be due to one of +the sections developing, for example, a change of +instinct determining migration to another area, or +occupation of a different habitat on the same area, +then the isolation will be discriminate, so far as the +resemblance of instinct is concerned.</p> + +<p>With the exception of Mr. Gulick, I cannot find +that any other writer has hitherto stated this +supremely important distinction between isolation as +discriminate and indiscriminate. But he has fully +as well as independently stated it, and shown in +a masterly way its far-reaching consequences. Indiscriminate +isolation he calls Separate Breeding, while +discriminate isolation he calls Segregate Breeding. +For the sake, however, of securing more descriptive +terms, I will coin the words Apogamy and Homogamy. +Apogamy, of course, answers to indiscriminate isolation, +or separate breeding. Homogamy, on the other<span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span> +hand, answers to discriminate isolation, or segregate +breeding: only individuals belonging to the same +variety or kind are allowed to propagate. Isolation, +then, is a genus, of which Apogamy and Homogamy +are species<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a>.</p> + +<p>Now, in order to appreciate the unsurpassed importance +of isolation as one of the three basal +principles of organic evolution, let us begin by +considering the discriminate species of it, or Homogamy.</p> + +<p>To state the case in the most general terms, we +may say that if the other two basal principles are +given in heredity and variability, the whole theory +of organic evolution becomes neither more nor less +than a theory of homogamy—that is, a theory of +the causes which lead to discriminate isolation, or +the breeding of like with like to the exclusion of +unlike. For the more we believe in heredity and +variability as basal principles of organic evolution, +the stronger must become our persuasion that discriminate +breeding leads to divergence of type, while +indiscriminate breeding leads to uniformity. This, +in fact, is securely based on what we know from the +experience supplied by artificial selection, which consists<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span> +in the intentional mating of like with like to the +exclusion of unlike.</p> + +<p>The point, then, which in the first instance must be +firmly fastened in our minds is this:—so long as there +is free intercrossing, heredity cancels variability, and +makes in favour of fixity of type. Only when assisted +by some form of discriminate isolation, which +determines the exclusive breeding of like with like, +can heredity make in favour of change of type, or +lead to what we understand by organic evolution.</p> + +<p>Now the forms of discriminate isolation, or homogamy, +are very numerous. When, for example, any +section of a species adopts somewhat different habits +of life, or occupies a somewhat different station in +the economy of nature, homogamy arises within that +section. There are forms of homogamy on which +Darwin has laid great stress, as we shall presently +find. Again, when for these or any other reasons a +section of a species becomes in any small degree +modified as to form or colour, if the species happens +to be one where any psychological preference in +pairing can be exercised—as is very generally the +case among the higher animals—exclusive breeding +is apt to ensue as a result of such preference; for +there is abundant evidence to show that, both in birds +and mammals, sexual selection is usually opposed to +the intercrossing of dissimilar varieties. Once more, +in the case of plants, intercrossing of dissimilar +varieties may be prevented by any slight difference in +their seasons of flowering, of topographical stations, +or even, in the case of flowers which depend on +insects for their fertilization, by differences in the +instincts and preferences of their visitors.</p> + +<p><span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span> +But, without at present going into detail with +regard to these different forms of discriminate isolation, +there are still two others, both of which are of much +greater importance than any that I have hitherto +named. Indeed, these two forms are of such immeasurable +importance, that were it not for their +virtually ubiquitous operation, the process of organic +evolution could never have begun, nor, having begun, +continued.</p> + +<p>The first of these two forms is sexual incompatibility—either +partial or absolute—between different +taxonomic groups. If all hares and rabbits, for +example, were as fertile with one another as they +are within their own respective species, there can be +no doubt that sooner or later, and on common areas, +the two types would fuse into one. And similarly, +if the bar of sterility could be thrown down as +between all the species of a genus, or all the genera of +a family, <i>not otherwise prevented from intercrossing</i>, +in time all such species, or all such genera, would +become blended into a single type. As a matter +of fact, complete fertility, both of first crosses and +of their resulting hybrids, is rare, even as between +species of the same genus; while as between genera +of the same family complete fertility does not appear +ever to occur; and, of course, the same applies to +all the higher taxonomic divisions. On the other +hand, some degree of infertility is not unusual as +between different varieties of the same species; and, +wherever this is the case, it must clearly aid the further +differentiation of those varieties. It will be my +endeavour to show that in this latter connexion +sexual incompatibility must be held to have taken<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span> +an immensely important part in the differentiation +of varieties into species. But meanwhile we have +only to observe that <i>wherever</i> such incompatibility is +concerned, it is to be regarded as an isolating agency +of the very first importance. And as it is of a +character purely physiological, I have assigned to it +the name Physiological Isolation; while for the particular +case where this general principle is concerned +in the origination of specific types, I have reserved +the name Physiological Selection.</p> + +<p>The other most important form of discriminate +isolation to which I have alluded is Natural Selection. +To some evolutionists it has seemed paradoxical +thus to regard natural selection as a form of isolation; +but a little thought will suffice to show that +such is really the most accurate way of regarding it. +For, as Mr. Gulick says, "Natural selection is the +exclusive breeding of those better adapted to the environment: +... it is a process in which the fittest are +prevented from crossing with the less fitted, by the +exclusion of the less fitted." Therefore it is, strictly +and accurately, a mode of isolation, where the +isolation has reference to adaptation, and is secured +in the most effectual of possible ways—i.e. by the +destruction of all individuals whose intercrossing would +interfere with the isolation. Indeed, the very term +"natural <i>selection</i>" shows that the principle is tacitly +understood to be one of isolation, because this name +was assigned to the principle by Darwin for the +express purpose of marking the analogy that obtains +between it and the intentional isolation which is +practised by breeders, fanciers, and horticulturists. +The only difference between "natural selection" and<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span> +"artificial selection" consists in this—that under the +former process the excluded individuals must necessarily +perish, while under the latter they need not do +so. But clearly this difference is accidental: it is in +no way essential to the process considered as a process +of discriminate isolation. For, as far as homogamous +breeding is concerned, it can matter nothing whether +the exclusion of the dissimilar individuals is effected +by separation or by death.</p> + +<p>Natural selection, then, is thus unquestionably +a form of isolation of the discriminate kind; and +therefore, notwithstanding its unique importance in +certain respects, considered as a principle of organic +evolution it is less fundamental—and also less extensive—than +the principle of isolation in general. In +other words, it is but a part of a much larger whole. +It is but a particular form of a general principle, +which, as just shown, presents many other forms, not +only of the discriminate, but likewise of the indiscriminate +kind. Or, reverting to the terminology of +logic, it is a sub-species of the species Homogamy, +which in its turn is but a constituent part of the +genus Isolation.</p> + +<p>So much then for homogamy, or isolation of the +discriminate order. Passing on now to apogamy, or +isolation of the indiscriminate kind, we may well be +disposed, at first sight, to conclude that this kind of +isolation can count for nothing in the process of evolution. +For if the fundamental importance of isolation +in the production of organic forms be due to its +segregation of like with like, does it not follow +that any form of isolation which is indiscriminate +must fail to supply the very condition on which all<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span> +the forms of discriminate isolation depend for their +efficacy in the causing of organic evolution? Or, to +return to our concrete example, is it not self-evident +that the farmer who separated his stock into two +or more parts indiscriminately, would not effect any +more change in his stock than if he had left them +all to breed together?</p> + +<p>Well, although at first sight this seems self-evident, +it is in fact untrue. For, unless the individuals which +are indiscriminately isolated happen to be a very +large number, sooner or later their progeny will come +to differ from that of the parent type, or unisolated +portion of the previous stock. And, of course, as +soon as this change of type begins, the isolation +ceases to be indiscriminate: the previous apogamy +has been converted into homogamy, with the usual +result of causing a divergence of type. The reason +why progeny of an indiscriminately isolated section +of an originally uniform stock—e.g. of a species—will +eventually deviate from the original type is, to quote +Mr. Gulick, as follows:—"No two portions of a species +possess exactly the same average character, and, +therefore, the initial differences are for ever reacting +on the environment and on each other in such a way +as to ensure increasing divergence as long as the +individuals of the two groups are kept from intergenerating<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a>." +Or, as I stated this principle in my +essay on <i>Physiological Selection</i>, published but a short +time before Mr. Gulick's invaluable contributions to +these topics:—</p> + +<p><span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span></p> +<blockquote><p>As a matter of fact, we find that no one individual "is like +another all in all"; which is another way of saying that a +specific type may be regarded as the average mean of all its individual +variations, any considerable departure from this average +being, however, checked by intercrossing.... Consequently, if +from any cause a section of a species is prevented from intercrossing +with the rest of its species, we might expect that new +varieties should arise within that section, and that in time these +varieties should pass into new species. And this is just what +we do find<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a>.</p></blockquote> + +<p>The name which I gave to this cause of specific +change was Independent Variability, or variability in +the absence of overwhelming intercrossing. But it +now appears to me that this cause is really identical +with that which was previously enunciated by +Delbœuf. Again, in his important essay on <i>The +Influence of Isolation</i>, Weismann concludes, on the +basis of a large accumulation of facts, that the constancy +of any given specific type "does not arise +suddenly, but gradually, and is established by the +promiscuous intercrossing of all individuals." From +which, he says, it follows, that this constancy must +cease so soon as the condition which maintains it +ceases—i. e. so soon as intercrossing (Panmixia) +between all individuals ceases, or so soon as a portion +of a species is isolated from its parent stock. To +this principle he assigns the name of Amixia. But +Weismann's Amixia differs from my Independent +Variability in several important particulars; and +on this account I have designedly abstained from<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span> +adopting his term. Here it is enough to remark +that it answers to the generic term Isolation, without +reference to the <i>kind</i> of isolation as discriminate +or indiscriminate, homogamous or apogamous. On +the other hand, my Independent Variability is merely +a re-statement of the so-called "Law of Delbœuf," +which, in his own words, is as follows:—</p> + +<blockquote><p>One point, however, is definitely attained. It is that the +proposition, which further back we designated paradoxical, +is rigorously true, A constant cause of variation, however +insignificant it may be, changes the uniformity [of type] +little by little, and diversifies it <i>ad infinitum</i>. From the homogeneous, +left to itself, only the homogeneous can proceed; but +if there be a slight disturbance ["léger ferment"] in the +homogeneous, the homogeneity will be invaded at a single +point, differentiation will penetrate the whole, and, after +a time—it may be an infinite time—the differentiation will +have disintegrated it altogether.</p></blockquote> + +<p>In other words, the "Law," which Delbœuf has +formulated on mathematical grounds, and with express +reference to the question of segregate breeding, +proves that, no matter how infinitesimally small the +difference may be between the average qualities of +an isolated section of a species compared with the +average qualities of the rest of that species, if the +isolation continues sufficiently long, differentiation of +specific type is necessarily bound to ensue. But, to +make this mathematical law biologically complete, it +ought to be added that the time required for the +change of type to supervene (supposing apogamy to +be the only agent of change) will be governed by the +range of individual variability which the species in +question presents. A highly stable species (such as +the Goose) might require an immensely long time for<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span> +apogamy alone to produce any change of type in an +isolated portion of the species, while a highly variable +species (such as the Ruff) would rapidly change in +any portion that might be indiscriminately isolated. +It was in order to recognize this additional and very +important factor that I chose the name Independent +<i>Variability</i> whereby to designate the diversifying +influence of merely indiscriminate isolation, or apogamy. +Later on Mr. Gulick published his elaborate +papers upon the divergence of type under all kinds of +isolation; and retained my term Independent, but +changed Variability into Generation. I point this +out merely for the sake of remarking that his Independent +Generation is exactly the same principle +as my Independent Variability, and Delbœuf's Mathematical +Law.</p> + +<p>Now, while I fully agree with Mons. Giard where +he says, in the introductory lecture of his course on +<i>The Factors of Evolution</i><a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a>, that sufficient attention +has not been hitherto given by naturalists to this +important factor of organic evolution (apogamy), +I think I have shown that among those naturalists +who have considered it there is a sufficient amount of +agreement. <i>Per contra</i>, I have to note the opinion +of Mr. Wallace, who steadily maintains the impossibility +of any cause other than natural selection (i.e. +one of the forms of homogamy) having been concerned +in the evolution of species. But at present it is enough +to remark that even Professor Ray Lankester—whose +leanings of late years have been to the side of ultra-Darwinism, +and who is therefore disposed to agree<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span> +with Mr. Wallace wherever this is logically possible—even +Professor Ray Lankester observes:—</p> + +<blockquote><p>Mr. Wallace does not, in my judgement, give sufficient +grounds for rejecting the proposition which he indicates as +the main point of Mr. Gulick's valuable essay on <i>Divergent +Evolution through Cumulative Segregation</i>. Mr. Gulick's +idea is that ... no two portions of a species possess exactly the +same average character, and the initial differences will, if the +individuals of the two groups are kept from intercrossing, +assert themselves continuously by heredity in such a way as +to ensure an increasing divergence of the forms belonging to +the two groups, amounting to what is recognized as specific +distinction. Mr. Gulick's idea is simply the recognition of +a permanence or persistency in heredity, which, <i>caeteris +paribus</i>, gives a twist or direction to the variations of the +descendants of one individual as compared with the descendants +of another<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a>.</p></blockquote> + +<p>Now we have seen that "Mr. Gulick's idea," +although independently conceived by him, had been +several times propounded before; and it is partly +implicated in more than one passage of the <i>Origin +of Species</i>, where free intercrossing, or the <i>absence</i> of +isolation, is alluded to as maintaining the <i>constancy</i> +of a specific type<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a>. Moreover, it is still more fully +recognized in the last edition of the <i>Variation of +Animals and Plants</i>, where a paragraph is added for +the purpose of sanctioning the principle in the +imperfect form that it was stated by Weismann<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a>. +Nevertheless, to Mr. Gulick belongs the credit, not +only of having been the first to conceive (though the +last to publish) the "idea" in question, and of having +stated it with greater fullness than anybody else; but<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span> +still more of having verified its importance as a factor +of organic evolution.</p> + +<p>For, in point of fact, Mr. Gulick was led to his +recognition of the principle in question, not by any +deductive reasoning from general principles, but by +his own particular and detailed observations of the +land mollusca of the Sandwich Islands. Here there +are an immense number of varieties belonging to +several genera; but every variety is restricted, not +merely to the same island, but actually to the same +valley. Moreover, on tracing this fauna from valley +to valley, it is apparent that a slight variation in the +occupants of valley 2 as compared with those of the +adjacent valley 1, becomes more pronounced in the +next—valley 3, still more so in 4, &c., &c. Thus it +was possible, as Mr. Gulick says, roughly to estimate +the amount of divergence between the occupants of +any two given valleys by measuring the number of +miles between them.</p> + +<p>As already stated, I have myself examined his +wonderful collection of shells, together with a topographical +map of the district; and therefore I am in +a position to testify to the great value of Mr. Gulick's +work in this connexion, as in that of the utility +question previously considered. The variations, which +affect scores of species, and themselves eventually run +into fully specific distinctions, are all more or less +finely graduated as they pass from one isolated +region to the next; and they have reference to +changes of form and colour, which in no one case +presents any appearance of utility. Therefore—and +especially in view of the fact that, as far as he could +ascertain, the environment in the different valleys was<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span> +essentially the same—no one who examines this +collection can wonder that Mr. Gulick attributes the +results which he has observed to the influence of +apogamy alone, without any reference to utility or +natural selection.</p> + +<p>To this solid array of remarkable facts Mr. Wallace +has nothing further to oppose than his customary +appeal to the argument from ignorance, grounded on +the usual assumption that no principle other than +natural selection <i>can</i> be responsible for even the +minutest changes of form or colour. For my own +part, I must confess that I have never been so deeply +impressed by the dominating influence of the <i>a priori</i> +method as I was on reading Mr. Wallace's criticism +of Mr. Gulick's paper, after having seen the material +on which this paper is founded. To argue that every +one of some twenty contiguous valleys in the area of +the same small island must necessarily present such +differences of environment that all the shells in each +are differently modified thereby, while in no one out +of the hundreds of cases of modification in minute +respects of form and colour can any human being +suggest an adaptive reason therefor—to argue thus +is merely to affirm an intrinsically improbable dogma +in the presence of a great and consistent array of +opposing facts.</p> + +<p>I have laid special stress on this particular case of +the Sandwich Islands' mollusca, because the fifteen +years of labour which Mr. Gulick has devoted to their +exhaustive working out have yielded results more +complete and suggestive than any which so far have +been forthcoming with regard to the effects of isolation +in divergent evolution. But, if space permitted, it<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span> +would be easy to present abundance of additional facts +from other sources, all bearing to the same conclusion—namely, +that as a matter of direct observation, no +less than of general reasoning, any unprejudiced mind +will concede to the principle of indiscriminate isolation +an important share in the origination of organic types. +For as indiscriminate isolation is thus seen sooner or +later to become discriminate, and as we have already +seen that discriminate isolation is a necessary condition +to all or any modification, we can only conclude that +isolation in both its kinds takes rank with heredity +and variability as one of the three basal principles +of organic evolution.</p> + +<hr class="tb" /> + +<p>Having got thus far in the way of generalities, we +must next observe sundry further matters of comparative +detail.</p> + +<p>1. In any case of indiscriminate isolation, or +apogamy, the larger the bulk of the isolated section +the more nearly must its average qualities resemble +those of its parent stock; and, therefore, the less +divergence of character will ensue in a given time +from this cause alone. For instance, if one-fourth of +a large species were to be separated from the other +three-fourths (say, by subsidence causing a discontinuity +of area), it would continue the specific characters +unchanged for an indefinitely long time, so far as +the influence of such an indiscriminate isolation is +concerned. But, on the other hand, if only half a +dozen individuals were to be thus separated from +the rest of their species, a comparatively short time +would be needed for their descendants to undergo +some varietal modification at the hands of apogamy.<span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span> +For, in this case, the chances would be infinitely +against the average characters of the original half-dozen +individuals exactly coinciding with those of +all the rest of their species.</p> + +<p>2. In any case of homogamy, however, it is +immaterial what proportional number of individuals +are isolated in the first instance. For the isolation is +here discriminate, or effected by the initial difference +of the average qualities themselves—a difference, +therefore, which presupposes divergence as having +already commenced, and equally bound to proceed +whether the number of intergenerants be large or +small.</p> + +<p>It may here be remarked that, in his essay on +the <i>Influence of Isolation</i>, Professor Weismann fails +to distinguish between the two kinds of isolation. +This essay deals only with one of the many different +forms of isolation—the geographical—and is therefore +throughout concerned with a consideration of diversity +as arising from apogamy alone. But in dealing with +this side of the matter Weismann anticipated both +Gulick and myself in pointing out the law of inverse +proportion, which I have stated in the preceding +paragraph in what appears to me its strictly accurate +form.</p> + +<p>3. Segregate Breeding, or homogamy, which arises +under any of the many forms of discriminate isolation, +must always tend to be <i>cumulative</i>. For, again to +quote Mr. Gulick, who has constituted this fact the +most prominent as it is the most original feature +of his essay, "In the first place, every new form of +Segregation<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a> that now appears depends on, and is<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span> +superimposed upon, forms of Segregation that have +been previously induced; for when Negative Segregation +arises [i. e. isolation due to mutual sterility], +and the varieties of a species become less and less +fertile with one another, the complete infertility that +has existed between them and some other species +does not disappear, nor does the Positive Segregation +cease [i. e. any other form of isolation previously +existing].... In the second place, whenever +Segregation is directly produced by some quality of +the organism, variations that possess the endowment +in a superior degree will have a larger share in producing +the segregated forms of the next generation, +and accordingly the segregative endowment of the +next generation will be greater than that of the +present generation; and so with each successive +generation the segregation will become increasingly +complete." And to this it may be added, in the +third place, that where the segregation (isolation) is +due to the external conditions of life under which +the organism is placed, or where it is due to natural +selection simultaneously operating in divergent lines +of evolution, the same remarks apply. Hence it +follows that discriminate isolation is, in all its forms, +cumulative.</p> + +<p>4. The next point to be noted is, that the cumulative +divergence of type thus induced can take +place only in as many different lines as there are +different <i>cases</i> of isolation. This is a point which +Mr. Gulick has not expressly noticed; but it is one +that ought to be clearly recognized. Seeing that +isolation secures the breeding of similar forms by +exclusion (immediate or eventual) of those which are<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span> +dissimilar, and that only in as far as it does this +can it be a factor in organic evolution, it follows that +the resulting segregation, even though cumulative, +can only lead to divergence of organic types in as +many directions as there are cases of isolation. For +any one group of intergenerants only <i>serial</i> transformation +is possible, even though the transformation +be cumulative through successive generations in the +single line of change. But there is always a probability +that during the course of such <i>serial transformation +in time</i>, some other case of isolation may supervene, +so as to divide the previously isolated group of intergenerants +into two or more further isolated groups. +Then, of course, opportunity will be furnished for +<i>divergent transformation in space</i>—and this in as +many different lines as there are now different +homogamous groups.</p> + +<p>That this must be so is further evident, if we +reflect that the evolutionary power of isolation +depends, not only on the <i>preventing</i> of intercrossing +between the isolated portion of a species and +the rest of that species, but also upon the <i>permitting</i> +of intercrossing between all individuals of the isolated +portion, whereby the peculiar average of qualities which +they as a whole present may be allowed to assert itself +in their progeny—or, if the isolation has been from the +first discriminate, whereby the resulting homogamy +may thus be allowed to assert itself. Hence any +one case of either species of isolation, discriminate or +indiscriminate, can only give rise to what Mr. Gulick +has aptly called "monotypic evolution," or a chain-like +series of types arising successively in time, as +distinguished from what he has called "polytypic<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span> +evolution," or an arborescent multiplication of types +arising simultaneously in space.</p> + +<p>For example, let us again take the geographical +form of isolation. Where a single small intergenerant +group of individuals is separated from the rest of +its species—say, on an oceanic island—<i>monotypic</i> +evolution may take place through a continuous and +cumulative course of independent variation in a +single line of change: all the <i>individuals</i> composing +any one given generation will closely resemble one +another, although the <i>type</i> may be progressively +altering through a long series of generations. But if +the original species had had two small colonies +separated from itself (one on each of two different +islands, so giving rise to two cases of isolation), then +<i>polytypic</i> evolution would have ensued to the extent +of there having been two different lines of evolution +going on simultaneously (one upon each of +the two islands concerned). Similarly, of course, if +there had been three or four such colonies, there +would have been three or four divergent lines of +evolution, and so on.</p> + +<p>5. In the <i>cases</i> of isolation just supposed there +is only one <i>form</i> of isolation; and it is thus shown +that under one form of isolation there may be as +many lines of divergence as there are separate cases +of such isolation. But now suppose that there are +two or more forms of isolation—for instance, that +on the same oceanic island the original colony has +begun to segregate into secondary groups under +the influence of natural selection, sexual selection, +physiological selection, or any of the other forms +of isolation—then there will be as many lines of<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span> +divergent evolution going on at the same time (and +here on the same area) as there are forms of isolation +affecting the oceanic colony. And this because each +of the <i>forms</i> of isolation has given rise to a different +<i>case</i> of isolation.</p> + +<p>Now, inasmuch as different forms of isolation, when +thus superadded one to another, constitute different +cases of isolation, we may lay down the following +general law as applying to all the forms of isolation—namely, +<i>The number of possible directions in +which divergent evolution can occur, is never greater +than, though it may be equal to, the number of cases +of efficient isolation—or the number of efficiently +separated groups of intergenerants.</i></p> + +<p>6. We have now to consider with some care the +particular and highly important form of isolation +that is presented by natural selection. For while +this form of isolation resembles all the other forms +of the discriminate kind in that it secures homogamy, +there are two points in which it differs from +all of them, and one point in which it differs from +most of them.</p> + +<p>Natural selection differs from <i>all</i> the other known +forms of isolation (whether discriminate or indiscriminate) +in that it has exclusive reference to +<i>adaptations</i> on the one hand, and, on the other hand, +necessitates not only the elimination, but the destruction +of the excluded individuals. Again, natural +selection differs from <i>most</i> of the other forms of +isolation in that, unless assisted by some other +form, it can never lead to polytypic, but only +to monotypic evolution. The first two points of +difference are here immaterial; but the last is one<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span> +of the highest importance, as we shall immediately +perceive.</p> + +<p>In nearly all the other forms of isolation, polytypic +or divergent evolution may arise under the influence +of that form alone, or without the necessary co-operation +of any other form. This we have already +seen, for example, in regard to geographical isolation, +under which there may be as many different lines +of transmutation going on simultaneously as there +are different cases of isolation—say, in so many +different oceanic islands. Again, in regard to physiological +isolation the same remark obviously applies; +for it is evident that even upon the same geographical +area there may be as many different lines of transmutation +going on simultaneously as there are cases +of this form of isolation. The bar of mutual sterility, +whenever and wherever it occurs, must always render +polytypic evolution possible. And so it is with almost +all the other forms of isolation: that is to say, one +<i>form</i> does not necessarily require the assistance of +another <i>form</i> in order to create an additional <i>case</i> +of isolation. But it is a peculiarity of natural selection, +considered as a form of isolation, that it does +necessarily require the assistance of some other form +before it can give rise to an additional case of isolation; +and therefore before it can give rise to any +<i>divergence</i> of character in ramifying lines, as distinguished +from <i>transformation</i> of characters in a single +line. Or, in other words, natural selection, when acting +alone, can never induce polytypic evolution, but +only monotypic.</p> + +<p>That this important conclusion is a necessary +deduction from the theory of natural selection itself,<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span> +a very few words will be enough to show. For, +according to the theory, survival of the fittest is +a form of isolation which acts through utility, by +<i>destroying</i> all the individuals whom it fails to isolate. +Hence it follows that survival of the fittest is a form +of isolation which, if acting alone, cannot <i>possibly</i> +effect divergent evolution. For, in the first place, +there is nothing in this form of isolation to ensure +that the fitter individuals should fail to interbreed +with the less fit which are able to survive; and, in +the second place, in all cases where the less fit are +not sufficiently fit to be suffered to breed, they are +exterminated—i. e. not permitted to form a distinct +variety of their own. If it be said that survival of +the fittest may develop simultaneously two or more +lines of <i>useful</i> change, the answer is that it can +only do this if each of the developing varieties is +isolated from the others by some <i>additional form</i> +of isolation; for, if not, there can be no commencement +of utilitarian <i>divergence</i>, since whatever number +of utilitarian changes may be in course of simultaneous +development, they must in this case be all +blended together in a single line of specific transmutation. +Nay, even if specific divergence has +actually been commenced by natural selection when +associated with some other form of homogamy, if +the latter should afterwards be withdrawn, natural +selection would then be unable to maintain even so +much divergence of character as may already have +been attained: free intercrossing between the two +collateral, and no longer isolated branches, would +ensure their eventual blending into a common stock. +Therefore, I repeat, natural selection, when acting<span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span> +alone, can never induce polytypic evolution, but +only monotypic.</p> + +<p>Now I regret to say that here, for the first and +only time throughout the whole course of the +present treatise, I find myself in seeming opposition +to the views of Darwin. For it was the decidedly +expressed opinion of Darwin that natural selection +<i>is</i> competent to effect polytypic, or divergent, evolution. +Nevertheless, I believe that the opposition +is to a large extent only apparent, or due merely to +the fact that Darwin did not explicitly state certain +considerations which throughout his discussion on +"divergence of character" are seemingly implied. +But, be this as it may, I have not even appeared +to desert his leadership on a matter of such high importance +without having duly considered the question +in all its bearings, and to the utmost limit of my +ability. Moreover, about two years after the publication +of my first paper<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> upon the subject, Mr. Gulick +followed, at somewhat greater length, in the same line +of dissent. Like all the rest of his work, this is so +severely logical in statement, as well as profoundly +thought out in substance, that I do not see how it +is possible for any one to read impartially what he +has written, and then continue to hold that natural +selection, if unassisted by any other form of isolation, +can possibly effect divergence of character—or +polytypic as distinguished from monotypic evolution<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a>.</p> + +<p>I may here quote from Mr. Gulick's paper three +propositions, serving to state three large and general<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span> +bodies of observable fact, which severally and collectively +go to verify, with an overwhelming mass of +evidence, the conclusion previously reached on grounds +of general reasoning.</p> + +<blockquote><p>The facts of geographical distribution seem to me to justify +the following statements:—</p> + +<p>(1) A species exposed to different conditions in the different +parts of the area over which it is distributed, is not represented +by divergent forms when free interbreeding exists +between the inhabitants of the different districts. In other +words, Diversity of Natural Selection without Separation does +not produce divergent evolution.</p> + +<p>(2) We find many cases in which areas, corresponding in +the character of the environment, but separated from each +other by important barriers, are the homes of divergent forms +of the same or allied species.</p> + +<p>(3) In cases where the separation has been long continued, +and the external conditions are the most diverse in points +that involve diversity of adaptation, there we find the most +decided divergences in the organic forms. That is, where +Separation and Divergent Selection have long acted, the +results are found to be the greatest.</p> + +<p>The 1st and 3rd of these propositions will probably be +disputed by few, if by any. The proof of the 2nd is +found wherever a set of closely allied organisms is so +distributed over a territory that each species and variety +occupies its own narrow district, within which it is shut by +barriers that restrain its distribution while each species of +the environing types is distributed over the whole territory. +The distribution of terrestrial molluscs on the Sandwich +Islands presents a great body of facts of this kind.</p></blockquote> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></p> + + + + +<h2>CHAPTER II.<br /> +<span class="smcap">Isolation</span> (<i>continued</i>).</h2> + + +<p>I will now recapitulate the main doctrines which +have been set forth in the foregoing chapter, and then +proceed to consider the objections which have been +advanced against them.</p> + +<p>It must be remembered that by isolation I mean +exactly what Mr. Gulick does by "Segregation," +and approximately what Professor Weismann does +by "Amixia "—i. e. the prevention of intercrossing.</p> + +<p>Isolation occurs in very many forms besides the +geographical, as will be more fully shown at the end +of this chapter; and in all its forms it admits of +degrees.</p> + +<p>It also occurs in two very different species or +kinds—namely, discriminate and indiscriminate. These +I have called respectively Homogamy and Apogamy. +This all-important distinction has been clearly recognized +by Mr. Gulick, as a result of his own thought +and observation, independently of anything that I have +published upon the subject.</p> + +<p>In view of this distinction Isolation takes rank with +Heredity and Variability as one of the most fundamental +principles of organic evolution. For, if these<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span> +other two principles be granted, the whole theory +of descent resolves itself into an inquiry touching the +causes, forms, and degrees of Homogamy.</p> + +<p>Save in cases where very large populations are +concerned, apogamy must sooner or later give rise +<i>per se</i> to homogamy, owing to the Law of Delbœuf. +which is the principle that I have called Independent +Variability, and Gulick has called Independent +Generation. But of course this does not hinder that +under apogamy various other causes of homogamy +are likely to arise—in particular natural selection.</p> + +<p>That natural selection differs from most of the other +forms of isolation in not being capable of causing +<i>divergent</i> or <i>polytypic</i> evolution must at once become +evident, if we remember that the only way in which +isolation of any form can cause such evolution is by +partitioning a given group of intergenerants into two +or more groups, each of which is able to survive as +thus separated from the other, and so to carry on the +evolution in divergent lines. But the distinguishing +peculiarity of natural selection, considered as a form +of isolation, is that it effects the isolation <i>by killing +off all the individuals which it fails to isolate</i>: consequently, +this form of isolation differs from other +forms in prohibiting the possibility of any ramification +of a single group of intergenerants into two or more +groups, for the purpose of carrying on the evolution +in divergent lines. Therefore, under this form of +isolation alone, evolution must proceed, palm-like, in +a single line of growth. So to speak, the successive +generations continuously ascend to higher things on +the steps supplied by their own "dead selves"; but +in doing so they must climb a single ladder, no<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span> +rung of which can be allowed to bifurcate in the +presence of the uniformity secured <i>for that generation</i> +by the free intercrossing of the most fit. Even +though beneficial variations may arise in two or more +directions simultaneously, and all be simultaneously +selected by survival of the fittest, the effect of free +intercrossing (in the absence of any other form of +isolation) will be to fuse all these beneficial variations +into one common type, and so to end in <i>monotypic</i> +evolution as before. In order to secure <i>polytypic</i> +evolution, intercrossing between the different beneficial +variants which may arise must be prevented; +and there is nothing to prevent such intercrossing in +the process of natural selection <i>per se</i>. In order that +the original group of intergenerants should be divided +and sub-divided into two or more groups of intergenerants, +some additional form of isolation must +necessarily supervene—when, of course, polytypic +evolution will result. And, as Mr. Gulick has shown, +the conclusion thus established by deductive reasoning +is verified inductively by the facts of geographical +distribution.</p> + +<p>How, then, are we to account for the fact that +Darwin attributed to natural selection the power to +cause divergence of character? The answer is sufficiently +simple. <i>He does so by tacitly invoking the aid +of some other form of homogamy in every case.</i> If we +carefully read pp. 86-97 of the <i>Origin of Species</i>, where +this subject is under consideration, we shall find that +in every one of the arguments and illustrations which +are adduced to prove the power of natural selection to +effect "divergence of character," he either pre-supposes +or actually names some other form of homogamy as<span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span> +the originating cause of the diversity that is afterwards +presented to natural selection for further intensification. +To give only one example. At the starting-point of +the whole discussion the priority of such other forms +of homogamy is assumed in the following words:—</p> + +<blockquote><p>But how, it may be asked, can any analogous principle +[to that of diversity caused by artificial selection] apply in +nature? I believe it can and does apply most efficiently +(though it was a long time before I saw how), from the +simple circumstance that the more diversified the descendants +from any one species become in structure, constitution, and +habits, by so much will they be better enabled to seize on +many and widely diversified places in the polity of nature, +and so be enabled to increase in numbers.</p></blockquote> + +<p>Now, without question, so soon as segregate +breeding in two or more lines of homogamy has been +in any sufficient degree determined by some "change +of structure, constitution, or habits," natural selection +will forthwith proceed to increase the divergence in +as many different lines as there are thus yielded discriminately +isolated sections of the species. And this +fact it must have been that Darwin really had before +his mind when he argued that diversification of character +is caused by natural selection, through the benefit +gained by the diversified forms being thus "enabled +to increase in number." Nevertheless he does not expressly +state the essential point, that although diversification +of character, <i>when once begun</i>, is thus <i>promoted</i> +by natural selection, which forthwith proceeds to cultivate +each of the resulting branches, yet diversification +of character can never be <i>originated</i> by natural +selection. The change of "structure," of "constitution," +of "habits," of "station," of geographical area, of reciprocal<span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span> +fertility, and so on—this change, <i>whatever</i> it +may have been, must clearly have been antecedent to +any operation of natural selection through the benefit +which arose from the change. Therefore the change +must in all cases have been due, in the first instance, +to some other form of isolation than the superadded +form which afterwards arose from superior fitness +in the possession of superior benefit—although, so +long as the prior form of isolation endured, or continued +to furnish the necessary condition to the co-operation +of survival of the fittest, survival of the +fittest would have continued to increase the divergence +of character in as many ramifying lines as there were +thus given to its action separate cases of isolation +by other means.</p> + +<p>In short, as divergence of character must in all cases +be due to a prevention of intercrossing, and as in the +process of natural selection there is, <i>ex hypothesi</i>, +nothing to prevent the intercrossing until the divergence +has already arisen, to suppose that natural +selection alone can have caused the divergence, is to +suppose that natural selection can have caused the +conditions of its own activity, which is absurd.</p> + +<p>Seeing, then, that even in cases where any "benefit" +arises from divergence of character, such benefit can +arise only after the divergence has already commenced, +and seeing that on this as on other accounts previously +mentioned it is plainly impossible to attribute the +origin of such divergence to natural selection, we find +that natural selection must be in all cases assisted +by some other form of isolation, if it is to be concerned +in polytypic as distinguished from monotypic +evolution. But this does not hinder that, when it<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span> +is so assisted, natural selection may become—and, +I believe, does become—the most efficient of all +the forms of isolation in promoting divergence of +character. For, in the first place, of all the forms +of isolation natural selection is probably the most +energetic in promoting monotypic evolution; so that +under the influence of such isolation monotypic +evolution probably advances more rapidly than +it does under any other form of isolation. In the +second place, when polytypic evolution has been +begun by any of these other forms of isolation, and +natural selection then sets to work on each of the +resulting branches, although natural selection is thus +engaged in as many different acts of monotypic evolution +as there are thus separate cases supplied to it by +these other forms of isolation, the joint result of all +these different acts is to hurry on the polytypic +evolution which was originally started by the other +forms of isolation. So to speak, natural selection is +the forcing heat, acting simultaneously on each of the +separate branches which has been induced to sprout +by other means; and in thus rapidly advancing the +growth of all the branches, it is still entitled to be +regarded as the most important <i>single</i> cause of diversification +in organic nature, although we must henceforth +cease to regard it as in any instance the +<i>originating</i> cause—or even so much as the <i>sustaining</i> +cause.</p> + +<p>So much by way of summary and recapitulation. +I will now briefly consider the only objections +which, so far as I can see, admit of being brought +against the foregoing doctrine of Isolation as held +by Mr. Gulick and myself. These possible objections<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span> +are but two in number—although but one of them +has been hitherto adduced. This, therefore, I will +take first.</p> + +<p>Mr. Wallace, with his customary desire to show +that natural selection is everywhere of itself capable +of causing organic evolution, seeks to minimize the +swamping effects of free intercrossing, and the consequent +importance of other forms of isolation. His +argument is as follows.</p> + +<p>Alluding to the researches of Mr. J. A. Allen, +and others, on the amount of variation presented +by individuals of a species in a state of nature, +Mr. Wallace shows that, as regards any given part of +the animal under consideration, there is always to +be found a considerable range of individual variation +round the average mean which goes to constitute the +specific character of the type. Thus, for example, +Mr. Allen says of American birds, "that a variation +of from fifteen to twenty per cent. in general size, +and an equal degree of variation in the relative size +of different parts, may be ordinarily expected among +specimens from the same species and sex, taken at +the same locality, while in some cases the variation +is even greater than this." Now, Mr. Wallace is under +the impression that these facts obviate the difficulty +which arises from the presence of free intercrossing—the +difficulty, that is, against the theory of natural +selection when natural selection is supposed to have +been the exclusive means of modification. For, as +he says, "if less size of body would be beneficial, +then, as half the variations in size are above and +half below the mean or existing standard of the +species, there would be ample beneficial variations";<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span> +and similarly with regard to longer or shorter legs, +wings, tails, &c., darker or lighter colour, and so on +through all the parts of any given organism.</p> + +<p>Well, although I have no wish at all to disparage +the biological value of these actual measurements +of the range of individual variation, I must point +out that they are without any value at all in the +connexion which Mr. Wallace adduces them. We +did not require these measurements to tell us the +broad and patent fact that "no being on this earthly +ball is like another all in all"—or, in less Tennysonian +words, that as regards every specific structure +there is a certain amount of individual variability +round an average mean. Indeed, in my own paper +on <i>Physiological Selection</i>—against which Mr. Wallace +is here specially arguing—I expressly said, as +previously remarked, "that a specific type may +be regarded as <i>the average mean of all individual +variations</i>." The fact of such individual variability +round a specific mean has always been well known +to anatomists; it constitutes one of the basal pillars +of the whole Darwinian theory; and is besides a +matter of universal recognition as regards human +stature, features, and so forth. The value of Mr. +Allen's work consists in accurately measuring the +<i>amount</i> or <i>range</i> of individual variation; but the +question of its amount or range is without relevancy +in the present connexion. For the desirability of +isolation as an aid to natural selection even where +monotypic evolution is concerned, does not arise +with any reference to the amount or range of variation: +it arises with reference to the <i>number</i> of variations +which are—or are not—<i>similar</i> and <i>simultaneous</i>. If<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span> +there be a sufficient number which are both similar +and simultaneous, the desirability of any co-operating +form of isolation is correspondingly removed, because +natural selection may then have sufficient material +wherewith to overcome the adverse influence of free +intercrossing, and so of itself to produce monotypic +evolution. Now, variations may be numerous, similar, +and simultaneous, either on account of some common +cause acting on many individuals at the same time, +or on account of the structures in question being +more or less variable round a specific mean. In +the latter case—which is the only case that Mr. +Allen's measurements have to do with—the law of +averages will of course determine that half the whole +number of variations in any given structure, in any +given generation, will be above the mean line. But, +equally of course, no one has ever denied that where, +for either of these reasons, natural selection is provided +with sufficient material, it is correspondingly +capable of improving the specific type without +the assistance of any other form of homogamy; +so to speak, they protect themselves by their very +numbers, and their superiority over others leads to +their survival and accumulation. But what is the +result? <i>The result can only be monotypic evolution.</i> +No matter how great the number, or how great the +range, of variations round an average specific mean, +out of such material natural selection can never +produce <i>polytypic</i> evolution: it may <i>change</i> the type +to any extent during successive generations, and +in a single line of change; but it cannot <i>branch</i> +the type, unless some other form of homogamy +intervenes. Therefore, when Mr. Wallace adduces<span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span> +the well-known fact that all structures vary more +or less round a specific mean as proof that natural +selection need not be incommoded by free intercrossing, +but can of itself produce all the known +phenomena of specific evolution, he fails to perceive +that his argument refers only to one aspect of such +evolution (viz. the transformation of species in time), +and does not apply to the aspect with which alone +my paper on <i>Physiological Selection</i> was concerned +(viz. the multiplication of species in space).</p> + +<p>The same thing may be shown in this way. It is +perfectly obvious that where the improvement of type +in a linear series is concerned (monotypic evolution), +free intercrossing, far from being a hindrance to the +process, <i>is the very means by which the process is +accomplished</i>. Improvement here ascends by successive +steps, in successive generations, simply <i>because</i> +of the general intercrossing of the generally most fit +with the result that the species, <i>as a whole</i>, gradually +becomes transformed into another species, <i>as a whole</i>. +Therefore, it would be mere fatuity in any one to +adduce free intercrossing as a "difficulty" against +natural selection alone being competent to produce +evolution of this kind. But where the kind of +evolution is that whereby the species is <i>differentiated</i>—where +it is required, for instance, to produce different +structures in different portions of the species, such as +the commencement of a fighting spur on the wing of +a duck, or <i>novel</i> characters of any sort in different +groups of the species—free intercrossing is no longer +a condition to, but an absolute preventive of, the +process; and, therefore, unless checked as between +each portion of the species by some form of homogamy<span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span> +other than natural selection, it must effectually +inhibit any <i>segregation</i> of specific types, or divergence +of character.</p> + +<p>Hence it is that, while no Darwinian has ever +questioned the power of unaided selection to cause +<i>improvement of character in successive generations</i>, in +common now with not a few other Darwinians I have +emphatically denied so much as the abstract possibility +of selection alone causing a <i>divergence of character +in two or more simultaneous lines of change</i>.</p> + +<p>And, although these opposite views cannot be +reconciled, I am under the impression that they do +admit of being explained. For I take them to +indicate a continued failure to perceive the all-important +distinction between evolution as monotypic +and polytypic. Unless one has fully grasped this +distinction, and constantly holds it in mind, he is +not in a position to understand the "difficulty" in +question; nor can he avoid playing fast and loose +with natural selection as possibly the sole cause of +evolution, and as necessarily requiring the co-operation +of some other cause. But if he once clearly perceives +that "evolution" is a logical genus, of which the monotypic +and the polytypic forms are species, he will +immediately escape from his confusion, and find that +while the monotypic form may be caused by natural +selection alone the polytypic form can never be +so caused.</p> + +<hr class="tb" /> + +<p>The second difficulty which I have to mention as at +first sight attaching to the views of Mr. Gulick and +myself on the subject of Isolation is, that in an isolated +section of a species Mr. Francis Galton's law of<span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span> +regression in the average character of offspring to +the typical character of the group through reversion +or atavism (<i>Natural Inheritance</i>, p. 97) must have +the effect of neutralizing the segregative influence of +mere apogamy. That such, however, cannot be the +case has been well shown by Mr. Gulick in his paper +on <i>Intensive Segregation</i>. Without at all disputing +the validity of Mr. Galton's law, he proves that "it can +hold in full force only where there is free crossing, +otherwise no divergent race could ever be formed by +any amount of selection and independent breeding<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a>." +This is so self-evident that I need not quote his demonstration +of the point.</p> + +<hr class="tb" /> + +<p>In conclusion, then, and having regard to the +principle of isolation as a whole, or in all the many and +varied forms in which this principle obtains, I trust that +I have redeemed the promise with which I set out—viz. +to show that in relation to the theory of descent +this principle is of an importance second to no other, +not even excepting heredity, variability, and the +struggle for existence. This has now been fully +shown, inasmuch as we have clearly seen that the importance +of the struggle for existence, and consequent +survival of the fittest, arises just because survival +of the fittest is a form, and a very stringent form, of +isolation; while, as regards both heredity and variability, +we are now in a position to see that the more +fully we recognize their supreme importance as +principles concerned in organic evolution, the more +must we also recognize that any rational theory of +such evolution becomes, in the last resort, a theory<span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span> +of the different modes in which efficient isolation can +be secured. For, in whatever degree the process of +organic evolution has been dependent upon heredity +with variability, in that degree must it also have been +dependent upon the means of securing homogamy, +whereby alone the force of heredity can be made to +expend itself in the innumerable directions of progressive +change, instead of continually neutralizing the +force of variability by promiscuous intercrossing.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></p> + + + + +<h2>CHAPTER III.<br /> +<span class="smcap">Physiological Selection.</span></h2> + + +<p>So far we have been concerned with the principle +of Isolation in general. We have now to consider +that form of isolation which arises in consequence of +mutual infertility between the members of any group +of organisms and those of all other similarly isolated +groups occupying simultaneously the same area.</p> + +<hr class="tb" /> + +<p>Against the view that natural selection is a sufficient +explanation of the origin of species, there are two +fatal difficulties: one, the contrast between natural +species and domesticated varieties in respect of cross-sterility; +the other, the fact that natural selection +cannot possibly give rise to polytypic as distinguished +from monotypic evolution. Now it is my belief that +the theory of physiological selection fully meets both +these difficulties. Indeed I hold this to be undeniable +in a formal or logical sense: the only question is as +to the evidence which can be adduced for the theory +in a practical or biological sense. Therefore in this +chapter, where the theory has first of all to be stated, +I shall restrict the exposition as much as possible +to the former, leaving for subsequent consideration the +biological side.</p> + +<p><span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span> +The following is a brief outline sketch of this +theory<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a>.</p> + +<p>Of all parts of those variable objects which we +call organisms, the most variable is the reproductive +system; and the variations may carry with them functional +changes, which may be either in the direction +of increased or of diminished fertility. Consequently +variations in the way of greater or less fertility frequently +take place, both in plants and animals; and +probably, if we had adequate means of observing this +point, we should find that there is no one variation +more common. But of course where infertility arises—whether +as a result of changed conditions of life, or, +as we say, spontaneously—it immediately becomes +extinguished, seeing that the individuals which it +affects are less able (if able at all) to propagate and +to hand on the variation. If, however, the variant, +while showing some degree of infertility with the parent +form, continues to be as fertile as before when mated +with similar variants, under these circumstances there +is no reason why such differential fertility should not +be perpetuated.</p> + +<p>Stated in another form this suggestion enables us +to regard many, if not most, species as the records of +variations in the reproductive systems of their ancestors. +When variations of a non-useful kind occur in any +of the other systems or parts of organisms, they are, +as a rule, immediately extinguished by intercrossing. +But whenever they arise in the reproductive system +in the way here suggested, they tend to be preserved +as new natural varieties, or incipient species. At +first the difference would only be in respect of the<span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span> +reproductive systems; but eventually, on account of +independent variation, other differences would supervene, +and the variety would take rank as a true +species.</p> + +<p>Now we must remember that physiological isolation +is not like those other forms of isolation (e.g. +geographical) which depend for their occurrence on +accidents of the environment, and which may therefore +take place suddenly in a full degree of completeness +throughout a large section of a species. Physiological +isolation depends upon distinctive characters +belonging to organisms themselves; and it would +be opposed to the whole theory of descent with +progressive modification to imagine that absolute +sterility usually arises, in a single generation between +two sections of a perfectly fertile species. Therefore +evolutionists must believe that in most, if not in all +cases—could we trace the history, say of any two +species, which having sprung from a single parent +stock on a common area, are now absolutely sterile +with one another—we should find that this mutual +sterility had been itself a product of gradual evolution. +Starting from complete fertility within the limits of a +single parent species, the infertility between derivative +or divergent species, <i>at whatever stage in their evolution +this began to occur</i>, must usually at first have been well-nigh +imperceptible, and thenceforth have proceeded +to increase stage by stage.</p> + +<p>But, if it be true that physiological isolation between +genetically allied groups must usually itself have been +the product of a gradual evolution; and if, when +fully evolved, it constitutes a condition of the first +importance to any further differentiation of these<span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span> +groups (by preventing fusion again into one group, +more or less resembling the original parent form), do +we not perceive at least a strong probability that +in the lower stages of its evolution such mutual infertility +must have acted as a segregating influence +between the diverging types, in a degree proportional +to its own development? The importance of mutual +sterility as a condition to divergent evolution is not +denied, <i>when this sterility is already present in an +absolute degree</i>; and we have just seen that, before +it can have attained to this absolute degree <i>it must +presumably, and as a rule, itself have been the subject +of a gradual development</i>. Does it not therefore +become, on merely antecedent grounds, in a high +degree probable, that from the moment of its inception +this isolating agency must have played the +part of a segregating cause, in a degree proportional +to that of its completeness as a physiological +character?</p> + +<p>Whoever answers this question in the affirmative +will have gone most of the way towards accepting, on +merely antecedent grounds, the theory of physiological +selection. And therefore it is that I have begun this +statement of the theory by introducing it upon these +grounds, thereby hoping to show how extremely simple—how +almost self-evident—is the theory which it will +now be my endeavour to substantiate. I may here +add that the theory was foreshadowed by Mr. Belt +in 1874<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a>, clearly enunciated in its main features by +Mr. Catchpool in 1884<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a>, and very fully thought out +by Mr. Gulick during a period of about fifteen years,<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span> +although he did not publish until a year after the +appearance of my own paper in 1886<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a>.</p> + +<p>I must next proceed to state some of the leading +features of physiological selection in further detail.</p> + +<p>It has already been shown that Darwin clearly +perceived that the very general occurrence of some +degree of infertility between allied species cannot +possibly be attributed to the <i>direct</i> agency of natural +selection. His explanation was that the slight structural +modifications entailed by the transformation of +one specific type into another, so react upon the +highly delicate reproductive system of the changing +type as to render it in some degree infertile with +its parent type. Now the theory of physiological +selection begins by traversing this view. It does +not, however, deny that in <i>some</i> cases the morphological +may be the prior change; but it strenuously +denies that this must be so in <i>all</i> cases. Indeed, +according to my statement in 1886, the theory inclines +to the view that, <i>as a rule</i>, the physiological change +is prior. At the same time, the theory, as I have +always stated it, maintains that it is immaterial whether, +"in the majority of instances," the physiological change +has been prior to the morphological, or vice versa; +since in either case the physiological change will +equally make for divergence of character.</p> + +<p><span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span> +To show this clearly the best way will be to consider +the two cases separately, taking first that in which +the physiological change has priority. In this case +our theory regards any morphological changes which +afterwards supervene as due to the independent variability +which will sooner or later arise under the +physiological isolation thus secured. But to whatever +causes the subsequent morphological changes +may be due, the point to notice is that they are as +a general rule, consequent upon the physiological +change. For in whatever <i>degree</i> such infertility arises +between two sections of a species occupying the same +area, in that <i>degree</i> is their interbreeding prevented, +and, therefore, opportunity is given for a subsequent +divergence of type, whether by the influence of independent +variability alone, or also by that of natural +selection, as now acting more or less independently +on each of the partially separated groups. In short, +all that was said in the foregoing chapters with respect +to isolation in general, here applies to physiological +isolation in particular; and by supposing such isolation +to have been the prior change, we can as well understand +the subsequent appearance of morphological +divergence on continuous areas, as in other forms +of isolation we can understand such divergence on +discontinuous areas, seeing that even a moderate +degree of cross-infertility may be as effectual for +purposes of isolation as a high mountain-chain, or +a thousand miles of ocean.</p> + +<p>Here, then, are two sharply-defined theories to +explain the very general fact of there being some +greater or less degree of cross-infertility between allied +species. The older, and hitherto current theory,<span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span> +supposes the cross-infertility to be but an <i>accident</i> +of specific divergence, which, therefore, has nothing +to do with <i>causing</i> the divergence. The newer theory, +on the other hand, supposes the cross-infertility to +have often been a necessary <i>condition</i> to the divergence +having begun at all. Let us now consider which +theory has most evidence in its favour.</p> + +<p>First of all we have to notice the very general +occurrence of the fact in question. For when we +include the infertility of hybrids, as well as first +crosses, the occurrence of some degree of infertility +between allied species is so usual that Mr. Wallace +recommends experiments to ascertain whether careful +observation might not prove, even of species which +hybridize, "that such species, when crossed with their +near allies, do always produce offspring which are +more or less sterile <i>inter se</i><a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a>." This seems going too +far, but nevertheless it is the testimony of a highly +competent naturalist to the very general occurrence of +an association between the morphological differentiation +of species and the fact of a physiological isolation. +Now I regard it as little short of self-evident that this +general association between mutual infertility and +innumerable secondary, or relatively variable morphological +distinctions, is due to the former having +been an original and a necessary condition to the +occurrence of the latter, in cases where intercrossing +has not been otherwise prevented.</p> + +<p>The importance of physiological isolation, <i>when +once fully developed</i>, cannot be denied, for it is evident +that if such isolation could be suddenly destroyed +between two allied species occupying a common area,<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span> +they would sooner or later become fused into a +common type—supposing, of course, no other form +of isolation to be present. The necessity then for +this physiological form of isolation in <i>maintaining</i> +a specific differentiation which has been already <i>attained</i> +cannot be disputed. Yet it has been regarded +as "Darwinian heresy" to suggest that it can have +been of any important service <i>during the process of +attainment</i>, or while the specific differentiation is +being advanced, and this notwithstanding that the +physiological change must presumably have developed +<i>pari passu</i> with the morphological, and notwithstanding +that in countless cases the former is associated +with every conceivable variety of the latter.</p> + +<p>Again, why should the physiological change be +thus associated with <i>every conceivable variety</i> of +morphological change? Throughout the length and +breadth of both vegetable and animal kingdoms we +find this association, in the great majority of cases, +where new species arise. Therefore, on the supposition +that in all such cases the physiological change +has been adventitiously induced by the morphological +changes, we have to face an apparently unanswerable +question—Why should the reproductive +mechanism of all organic beings have been thus +arranged, as it were, to change in immediate response +to the very slightest alteration in the complex harmony +of "somatic" processes, which now more than +ever is recognized as exercising so comparatively +little influence on the <i>hereditary</i> endowments of this +mechanism? Consider the difference between a worm +and the bird that is eating it, an oak tree and the +gall-insect that is piercing it: are we to suppose that<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span> +in all cases, no matter how greatly the types differ, +they must agree in this, that when any parts of +these complex structures change, ever so slightly, +the reproductive system is almost certain to be +adventitiously affected, yet always thus affected in +the same peculiar way?</p> + +<p>If it be answered that the reproductive system is +known to be very sensitive to slight changes in the +external conditions of life, the answer proves too +much. For though this is true, yet our opponents +must acknowledge that the reproductive system is +not so sensitive, <i>in this particular respect</i>, as their +interpretation of the origin of specific infertility +requires. The proof of this point is overwhelming, +for there is the evidence from the entire range of our +domesticated productions, both vegetable and animal. +Here the amount of structural change, which has been +slowly accumulated by artificial selection, is often +much greater in amount, and incomparably more +rapid, than that which has been induced between +allied species by natural selection; and yet there is +scarcely any indication of the reproductive system +having been affected in the particular way that our +opponents' theory requires. There are many instances +of its having been affected in sundry other +ways (chiefly, however, without any accompanying +morphological change); but among all the thousands +of our more or less enormously modified artificial +types, there is scarcely one instance of such a peculiar +sexual relation between the modified descendants of +a common type as so usually obtains between allied +species in nature. Yet in all other respects evolutionists +are bound to believe that the process of<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span> +modification has been in both cases strictly analogous. +Why then this conspicuous difference with respect to +the reproductive system?</p> + +<p>The answer is simple. It has never been the object +of breeders or of horticulturists to select variations +in the direction of cross-infertility, for the swamping +effects of intercrossing are much more easily and +rapidly prevented by artificial isolation. Consequently, +although they have been able to modify natural types +in so many directions and in such high degrees with +regard to <i>morphology</i>, there has been no accompanying +physiological modification of the kind required. But +in nature there is no such thing as artificial, i.e. intentional, +isolation. Consequently, on common areas +it must usually happen that those changes of morphology +which are associated with cross-infertility +are the only ones which can arise. Hence the very +remarkable contrast between our domesticated varieties +and natural species with regard to cross-infertility +is just what the present theory would expect, or, +indeed, require. But on any other theory it has +hitherto remained inexplicable.</p> + +<p>In particular, the contrast in question has constituted +one of the main difficulties with which the theory +of natural selection has hitherto had to contend, not +only in the popular mind, but also in the judgement +of naturalists, including the joint-authors of the theory +themselves. Thus Darwin says:—</p> + +<blockquote><p>The fertility of varieties is, with reference to my theory, +of equal importance with the sterility of species, for it seems +to make a broad and clear distinction between varieties and +species<a name="FNanchor_20_20" id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a>.</p></blockquote> +<p><span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span></p> +<p>And Mr. Wallace says:—</p> + +<blockquote><p>One of the greatest, or perhaps we may say the greatest, of +all the difficulties in the way of accepting the theory of natural +selection as a complete explanation of the origin of species, has +been the remarkable difference between varieties and species in +respect of fertility when crossed<a name="FNanchor_21_21" id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a>.</p></blockquote> + +<p>Now, in view of this conspicuous contrast, Darwin +suggested that species in a state of nature "will have +been exposed during long periods of time to more +uniform conditions than have domesticated varieties, +and [that] this may well make a wide difference in the +result." Now we have to remember that species, living +and extinct, are numbered by millions, and represent +every variety of type, constitution, and habits; is +it probable, then, that this one peculiarity of the +reproductive system should be due, in so many cases, +to some merely incidental effect produced on that +system by uniform conditions of life? Again, <i>ex +hypothesi</i>, at the time when a variety is first forming, +the influence exercised by uniform conditions of life +(whatever in different cases this may happen to be) +cannot be present as regards that variety: yet this is +just the time when its infertility with the parent (or +allied) form is most likely to have arisen; for it is +just then that the nascent variety would otherwise +have been most liable to extinction by free intercrossing—even +supposing that in the presence of such +intercrossing the variety could ever have come into +existence at all.</p> + +<p>Mr. Wallace meets the difficulty by arguing that +sterility between allied species may have been brought +about by the direct influence of natural selection.<span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span> +But, as previously remarked, this view is expressly +opposed to that of Darwin, who held that Wallace's +contention is erroneous.</p> + +<p>It will be seen, then, that both Darwin, and Wallace, +fully recognize the necessity of finding some explanation +of the infertility of allied species, over and above +the mere reaction of morphological differentiation on +the physiology of the reproductive system, and they +both agree in suggesting additional causes, though +they entirely disagree as to what these causes are. +Now, the theory of physiological selection likewise +suggests an additional cause—or, rather, a new explanation—and +one which is surely the most probable. +For what is to be explained? The very general +association of a certain physiological peculiarity with +that amount of morphological change which distinguishes +species from species, of whatever kind the +change may be, and in whatever family of the animal +or vegetable kingdom it may occur. Well, the theory +of physiological selection explains this very general +association by the simple supposition that, at least +in a large number of cases, it was the physiological +peculiarity which first of all led to the morphological +divergence, by interposing the bar of sterility between +two sections of a previously uniform species; and by +thus isolating the two sections one from another, +started each upon a subsequently independent course +of divergent evolution.</p> + +<p>Or, to put it in another way, if the occurrence of +this physiological peculiarity has been often the only +possible means of isolating two sections of a species +occupying a common area, and thus giving rise to +a divergence of specific type (as obviously <i>must</i> have<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span> +been the case wherever there was an absence of any +other form of isolation), it is nothing less than a +necessary consequence that many allied species should +now present the physiological peculiarity in question. +Thus the association between the physiological peculiarity +and the morphological divergence is explained +by the simple hypothesis, that the former has acted +as a necessary condition to the occurrence of the +latter. In the absence of other forms of isolation, +the morphological divergence could not have taken +place at all, had not the physiological peculiarity +arisen; and hence it is that we now meet with so +many cases where such divergence is associated with +this peculiarity.</p> + +<hr class="tb" /> + +<p>So far we have been considering the physiological +change as historically the prior one. Here, at first +sight, it may seem that the segregative power of +physiological selection must end; for it may well +seem impossible that the physiological change can +ever be necessary for the divergence of morphological +varieties into true species in cases where it has <i>not</i> +been the prior change, but has only set in after morphological +changes have proceeded far enough to have +already constituted definite varieties. A little thought, +however, will show that physiological selection is quite +as potent a condition to the differentiation of species +when it occurs after varietal divergence has begun, as +it is when it occurs before the divergence—and hence +that it really makes no difference to the theory of +physiological selection whether, in particular cases, the +cross-infertility arises before or after any structural or +other modifications with which it is associated.</p><p><span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span></p> + +<p>For the theory does not assert that all varieties +have been due to physiological selection. There are +doubtless many other causes of the origin of varieties +besides cross-infertility with parent forms; but, as +a general rule, it does not appear that they are by +themselves capable of carrying divergence beyond +a merely varietal stage. In order to carry divergence +to the stage of producing <i>species</i>, it appears to be +a general condition that, sooner or later, cross-infertility +should arise—seeing that, when varieties do succeed +in becoming species, we almost invariably find that, +as a matter of fact, cross-infertility has arisen. Hence, +if cross-infertility has thus usually been a necessary +condition to a varietal divergence becoming specific, +it can make no material difference when the incipient +infertility arose.</p> + +<p>It may be asked, however, whether I suppose that, +when the physiological change is subsequent, it is +directly <i>caused</i> by change of structure, size, colour, &c., +or that it arises, so to speak, accidentally, from other +causes which may have affected the sexual system in +the required way. To this question I may briefly +reply, that, looking to the absence of any influence +exercised on the reproductive systems of our domesticated +plants and animals by the great and varied +changes which so many of these forms present, it +would seem that among natural varieties such closely +analogous changes are presumably not the usual causes +of the physiological change, even where the latter are +subsequent to the former. Nevertheless, I do not +deny that in some of these cases changes of structure, +size, colour, &c., may be the causes of the physiological +change by reacting on the sexual system in the required<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span> +way. But in such cases free intercrossing will +have prevented the perpetuation of any morphological +changes, save those which have the power of so reacting +on the reproductive system as to produce the +physiological change, and thus to protect themselves +against the full and adverse power of free intercrossing. +We know that slight or initial changes of structure, +colour, &c., frequently occur as varieties, and yet that +on common areas very few of these varieties become +distinct species: free intercrossing prevents any such +further divergence of character. But if in the course +of many such abortive attempts, as it were, to produce +a new species, nature happens to hit upon a structural +or a colour variation which is capable of reacting on +the sexual system in the particular way required, then +this variation will be enabled to protect itself against +free intercrossing in proportion to its own development. +Or, in other words, the more it develops as a morphological +change, the more will it increase the physiological +change; while the more the physiological +change is thus increased, the more will it in turn +promote the morphological. By such action and +reaction the development of each furthers the development +of the other, till from an almost imperceptible +variety, apparently quite fertile with its parent form, +there arises a distinct species absolutely sterile with +its parent form. In such cases, therefore, it is still +the physiological conditions which have <i>selected</i> the +particular morphological changes capable of so reacting +on the reproductive system as to produce cross-infertility, +and thus to protect themselves against the +destructive power of free intercrossing. So to speak, +free intercrossing is always on the watch to level<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span> +down any changes which natural selection, or any +other cause of varietal divergence, may attempt to +produce; and therefore, in order to produce—or to +increase—such divergence in the absence of any other +form of isolation, natural selection must hit upon such +changes of structure, form, or colour, as are so correlated +with the reproductive system as to create the +physiological isolation that is required.</p> + +<p>To show how the principle of selective fertility +may be combined with what apparently is the most +improbable form of isolation for this purpose—the +geographical—I quote the following suggestion made +by Professor Lloyd Morgan in his <i>Animal Life and +Intelligence</i>:—</p> + +<blockquote><p>Suppose two divergent local varieties were to arise in +adjacent areas, and were subsequently (by stress of competition +or by geographical changes) driven together into a single +area.... If their unions be fertile, the isolation will be annulled +by intercrossing—the two varieties will form one mean or +average variety. But if the unions be infertile, the isolation +will be preserved, and the two varieties will continue separate. +Suppose now, and the supposition is by no means an improbable +one, that this has taken place again and again in the +evolution of species; then it is clear that those varietal forms +which had continued to be fertile together would be swamped +by intercrossing; while those varietal forms which had become +infertile would remain isolated. Hence, in the long run, isolated +forms occupying a common area would be infertile, +(p. 107.)</p></blockquote> + +<p>If then cross-sterility may thus arise even in association +with geographical isolation, may it not also +arise in its absence? And may it not thus give rise +to the differentiation of varieties on account of this +physiological isolation alone?</p><p><span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span></p> + +<p>Only two further points need be mentioned to +make this statement of physiological selection as +complete as the present <i>résumé</i> of its main principles +requires.</p> + +<p>The first is, that, as Mr. Wallace remarks, "every +species has come into existence coincident both in +space and time with a pre-existing and closely allied +species." I regard this as important evidence that +physiological selection is one of the natural causes +concerned. For the general fact implied is that every +species has come into existence on an area occupied +by its parent type, and therefore under circumstances +which render it imperative that intercrossing with that +type should be prevented. In the case of monotypic +evolution by natural selection alone, intercrossing +with the parent type is prevented through the gradual +extinction of that type by successive generations of +the developing type. But in the case of polytypic +evolution, intercrossing with the parent type can +only be prevented by some form of isolation other +than natural selection; and here it is evident that +cross-infertility with the parent type must be as +efficient to that end as any other form of isolation +that can be imagined. Consequently we might +almost have expected beforehand that in a large +proportional number of cases cross-infertility should +have been the means employed. And the fact that +this is actually the case so far corroborates the only +theory which is able to explain it.</p> + +<p>The second point is this.</p> + +<p>It appears to be comparatively rare for any cause +of specific divergence to prove effectual on common +areas, unless it sooner or later becomes associated with<span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span> +some degree of cross-infertility. But through this +association, the segregating influence of both the +causes concerned is, as Mr. Gulick has shown, greatly +increased. For instance, if the segregating influence +of some degree of cross-infertility be associated with +that of any other form of isolation, then, not only +will the two segregating influences be added, but +multiplied together. And thus, by their mutual +action and reaction, divergent evolution is promoted +at a rapidly increasing rate.</p> + +<p>I will now summarize the main points of the theory +of physiological isolation in a categorical form.</p> + +<p>1. If no other form of isolation be present, specific +divergence can only take place when some degree of +cross-infertility has previously arisen between two or +more sections of a species.</p> + +<p>2. When such cross-infertility has arisen it may +cause specific divergence, either (<i>a</i>) by allowing independent +variability in each of the physiologically +isolated groups; (<i>b</i>) by becoming associated with any +other cause of differentiation already operating; or +(<i>c</i>) by both these means combined.</p> + +<p>3. As some degree of cross-infertility generally +obtains between allied species, we are justified in +concluding that this has been the most frequent—or, +at any rate, the most effective—kind of isolation +where the origin of species is concerned; and +therefore the kind with which, in the case of +species-formation, natural selection, or any other +cause of specific divergence, has been most usually +associated.</p> + +<p>4. Where varietal divergence has begun in the<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span> +absence of cross-infertility, such divergence seems, as +a general rule, to have been incapable of attaining to a +specific value.</p> + +<p>5. Therefore, in the vast majority of such cases, it +must have been those varietal changes of structure, +size, colour, &c., which happened to have afterwards +been assisted by the reproductive change that were +on this account <i>selected</i> as successful candidates for +specific differentiation.</p> + +<p>6. It follows, that it makes no difference to the +general theory of physiological selection in what proportion +of cases the physiological change has been +the initial change; for, whether prior or subsequent +to the varietal changes with which it becomes associated, +its presence has been equally important as a +condition to specific divergence.</p> + +<p>7. When physiological isolation becomes associated +with natural selection, or any other form of homogamy, +the segregative power of both is augmented. Moreover, +so great is the augmentation that even very +moderate degrees of physiological isolation—themselves +capable of effecting little or nothing—become +very powerful when associated with moderate degrees +of any other kind of homogamy, and vice versa.</p> + +<p>8. The theory of physiological selection effectually +explains the divergent evolution of specific +types and the cross-infertility of such types when +evolved.</p> + +<hr class="tb" /> + +<p>To prevent, if possible, the continuance of certain +misunderstandings with regard to my original statement +of the new theory, let me here disclaim some +views which have been assigned to me. They are:</p> + +<p><span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span> +1. That the theory of physiological selection is +opposed to the theory of natural selection. Far from +this being so, it is—at all events in my own opinion—a +very important aid to it, in preventing free intercrossing +on a common area, and thus allowing divergent +evolution to occur within that area.</p> + +<p>2. That, in advancing the theory of physiological +selection as "an additional suggestion on the origin +of species," I wish to represent it as being the +originating cause of <i>all</i> species. What I hold is, that +all species must have owed their origin to <i>isolation</i>, in +some form or other; but that as physiological selection +is only one among many other forms of isolation (including +natural selection), and as it can only act on +common areas, a large number of species must have +been formed without its aid.</p> + +<p>3. That I imagine physiological varieties always +to arise "sporadically," or as merely individual +"sports" of the reproductive system. On the contrary, +I expressly stated that this is <i>not</i> the way in +which I suppose the "physiological variation" to +arise, when giving origin to a new species; but that +it arises, whenever it is effectual, as a "collective +variation" affecting a number of individuals simultaneously, +and therefore characterizing "a whole race, +or strain."</p> + +<p>4. That I suppose physiological selection always to +act alone. This I have never supposed. The essential +point is, not that the physiological isolation is unassociated +with other forms of isolation, but that +unless associated with some degree of physiological +isolation, no one of the other forms is capable of +originating species on common areas with any approach<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span> +to frequency. This proposition is the essence of +the new theory, and I take it to be proved, not only +by general deductive reasoning which shows that +it <i>must</i> be so, but also by the fact of an otherwise +inexplicable association between specific divergence +on common areas and some more or less considerable +degree of mutual infertility.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></p> + + + + +<h2>CHAPTER IV.<br /> +<span class="smcap">Evidences of Physiological Selection.</span></h2> + + +<p>I will now give an outline sketch of the evidences +in favour of the theory which has been set forth in +the preceding chapter, stating first what is the nature +of the verification which it requires.</p> + +<p>The theory is deduced from a highly general +association between distinctive specific characters +of <i>any</i> kind and a relatively constant specific +character of a <i>particular</i> kind—namely, sexual +exclusiveness. For it is from this highly general +association that the theory infers that this relatively +constant specific character has been at least one of +the needful conditions to the development of the +other specific characters with which it is found +associated. Hence the necessary verification must +begin by showing the strength of the theory on these +merely deductive, or antecedent, grounds. It may +then proceed to show how far the facts of organic +nature corroborate the theory in other and independent +ways.</p> + +<p>First, let it be carefully observed that here we have +to do only with the <i>fact</i> of selective fertility, and with +its <i>consequences</i> as supposed by the theory: we have<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span> +nothing to do either with its <i>causes</i> or its <i>degrees</i>. +Not with its causes, because in this respect the +theory of physiological selection is in just the same +position as that of natural selection: it is enough for +both if the needful variations are provided, without +its being incumbent on either to explain the causes +which produce them. Not with its degrees, because, +in the first place, it can only be those degrees of +variation which in particular cases are supposed +adequate to induce specific divergence, that fall +within the scope of the theory; and because, in the +second place, degrees which are adequate only to +induce—or to assist in inducing, <i>varietal</i> divergence, +must always tend to increase, or pass into higher +degrees.</p> + + +<h3><i>Antecedent Standing of the Theory.</i></h3> + +<p>The antecedent standing or logical basis of the +theory has already been in large measure displayed +in the preceding chapter; for it was impossible to +state the theory without thereby showing in how +considerable a degree it is self-evident. A brief +recapitulation is therefore all that is here necessary.</p> + +<p>It has been shown that divergent or polytypic +evolution on common areas is inexplicable by natural +selection alone. Hence the question arises: What +form of isolation has, under such circumstances, +rendered possible divergent evolution? In answer +to this question the theory of physiological selection +suggests that variations in the reproductive function +occur in such a way as to isolate more or less +perfectly from each other different sections of a +species. While cross-fertility remains unimpaired<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span> +among the members of each section, there is more or +less cross-infertility when members of either section +mate with those of the other. Thus a physiological +barrier is interposed between the two sections; and +any divergences of structure, colouring, or instinct +arising in the members of either section will not in +any way be affected by such divergences as arise +among the members of the other.</p> + +<p>In support of this suggestion, it has been shown in +the preceding chapter that the very general association +of cross-infertility with specific differentiation points +most strongly to the inference that the former has +usually been an indispensable condition to the +occurrence of the latter. It cannot be denied that +in many cases the specific distinction is now maintained +by means of that sexual isolation which cross-infertility +confers: it is therefore probable that such +isolation has been instrumental in securing its initial +attainment.</p> + +<p>This probability is strengthened by the observed +fact that the general association in question is +conspicuously absent in the case of domesticated +varieties, notwithstanding that their multitudinous +and diverse varietal characters usually equal, and +frequently surpass, specific characters in their degrees +of divergence.</p> + +<p>Since, then, it would seem to be impossible for +divergent evolution on common areas to take place +in the absence of some mode of isolation; since +cross-infertility appears to be the only possible mode +under the given circumstances; and since among +domesticated varieties, where isolation is otherwise +secured by artificial means, cross-infertility is usually<span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span> +absent, the logical foundations of the theory of +physiological selection would seem to be securely laid.</p> + +<p>We may therefore pass to more special lines of +evidence.</p> + + +<h3><i>Evidence from Geographical Distribution.</i></h3> + +<p>Darwin has adduced very good evidence to show +that large areas, notwithstanding the disadvantages +which (on his theory) must arise from free intercrossing, +are what he terms better manufactories of +species than smaller areas, such as oceanic islands. +On the other hand, as a matter of fact, oceanic +islands are comparatively rich in peculiar species. +These two statements, however, are not incompatible. +Smaller areas are, as a rule, rich in peculiar species +relatively to the number of their inhabitants; but +it does not follow that they are rich in species as +contrasted with larger areas containing very many +more inhabitants. Therefore, the rules are that +large areas turn out an absolutely greater number +of specific types than small areas; although, relatively +to the number of individuals or amount of population, +the small areas turn out a larger number of species +than the large areas.</p> + +<p>Now, these two complementary rules admit of +being explained as Darwin explains them. Small +and isolated areas are rich in species relatively to +the amount of population, because, as we have before +seen, this population has been permitted to develop +an independent history of its own, shielded from +intercrossing with parent forms, and from competition +with exotic forms; while, at the same time, the +homogamy thus secured, combined with change of<span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span> +environment, will give natural selection an improved +chance of finding new points of departure for its +operation. On the other hand, large and continuous +areas are favourable to the production of numerous +species, first, because they contain a large population, +thus favouring the occurrence of numerous variations; +and, secondly, because the large area furnishes +a diversity of conditions in its different parts, as to +food, climate, attitude, &c., and thus so many +different opportunities for the occurrence of sundry +forms of homogamy. Now, it is obvious that of all +these sundry forms of homogamy, physiological +selection must have what may be termed a first-rate +opportunity of assisting in the manufacture of species +on large areas. For not only is it upon large and +continuous areas that the antagonistic effects of +intercrossing are most pronounced (and, therefore, +that the influence of physiological selection must be +most useful in the work of species-making); but here +also the diversity in the external conditions of life, +which the large area supplies to different parts of +the extensive population, cannot fail to furnish physiological +selection with a greater abundance of that +particular variation in the reproductive system on +which its action depends. Again, and of still more +importance, on large areas there are a greater <i>number</i> +of species already differentiated from one another +as such; thus a greater number of already sexually +differentiated forms are presented for further differentiation +at the hands of physiological selection. For +all these reasons, therefore, we might have expected, +upon the new theory, that large and continuous areas +would be good manufactories of species.</p> + +<p><span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span> +Again, Darwin has shown that not only large +areas, but likewise "dominant" genera within those +areas, are rich in species. By dominant genera he +meant those which are represented by numerous +individuals, as compared with other genera inhabiting +the same area. This general rule he explains by the +consideration that the qualities which first led to the +form being dominant must have been useful; that +these would be transmitted to the otherwise varying +offspring; and, therefore, that when these offspring +had varied sufficiently to become new species, they +would still enjoy their ancestral advantages in the +struggle for existence. And this, doubtless, is in part +a true explanation; but I also think that the reason +why dominant genera are rich in species, is chiefly +because they everywhere present a great number of +individuals exposed to relatively great differences in +their conditions of life: or, in other words, that they +furnish the best raw material for the manufacture of +species by physiological selection, as explained in +the last paragraph. For, if the fact of dominant +genera being rich in species is to be explained <i>only</i> +by natural selection, it appears to me that the useful +qualities which have already led to the dominance +of the ancestral type ought rather to have proved +inimical to its splitting up into a number of subordinate +types. If already so far "in harmony with +its environment" as to have become for this reason +dominant, one would suppose that there is all the +more reason for its not undergoing change by the +process of natural selection. Or, at least, I do not +see why the fact of its being in an unusual degree +of harmony with its environment should in itself<span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span> +constitute any unusual reason for its modification by +survival of the fittest. On the other hand, as just +observed, I do very plainly see why such a reason +is furnished for the modifying influence of physiological +selection.</p> + +<p>Let us next turn to another of Darwin's general +rules with reference to distribution. He took a great +deal of trouble to collect evidence of the two following +facts, namely, (1) that "species of the larger genera +in each country vary more frequently than the species +of the smaller genera"; and (2) that "many of the +species included within the larger genera resemble +varieties in being very closely, but unequally, related +to each other, and in having restricted ranges<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a>." +By larger genera he means genera containing many +species; and he accounts for these general facts by +the principle, "that where many species of a genus +have been formed, on an average many are still +forming." But <i>how</i> forming? If we say by natural +selection alone, we should expect to find the multitudinous +species differing from one another in respect +of features presenting well-marked adaptive meanings; +yet this is precisely what we do not find. For +Darwin's argument here is that "in large genera the +amount of difference between the species is often +exceedingly small, so that in this respect the species +of the larger genera resemble varieties more than do +the species of the smaller genera." Therefore the +argument, while undoubtedly a very forcible one in +favour of the fact of <i>evolution</i>, appears to me scarcely +consistent with the view of this evolution being due +solely to natural selection. On the other hand, the<span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span> +argument tells strongly (though unconsciously) in +favour of physiological selection. For the larger a +genus, or the greater the number of its species, the +greater must be the opportunity for the occurrence +of that particular kind of variation on which the +principle of physiological selection depends. The +species of a genus may be regarded as so many +varieties which have already been separated from one +another physiologically; therefore each of them may +now constitute a new starting-point for a further and +similar separation—particularly as, in virtue of their +previous segregation, many are now exposed to +different conditions of life. Thus, it seems to me, +we can well understand why it is that genera already +rich in species tend to grow richer; while such is not +the case in so great a degree with genera that are +poor in species. Moreover, we can well understand +that, multiplication of species being as a rule, and in +the first instance, determined by changes in the reproductive +system, wherever a large number of new +species are being turned out, the secondary differences +between them should be "often exceedingly small"—a +general correlation which, so far as I can see, we +are not able to understand on the theory of natural +selection.</p> + +<p>The two subsidiary facts, that very closely allied +species have restricted ranges, and that dominant +species are rich in varieties, both seem to tell more +in favour of physiological than of natural selection. +For "very closely allied species" is but another name +for species which scarcely differ from one another +at all except in their reproductive systems; and, +therefore, the more restricted their ranges, the more<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span> +certainly would they have become fused by intercrossing +with one another, had it not been for the +barrier of sterility imposed by the primary distinction. +Or rather, I should say, had it not been +for the original occurrence of this barrier, these now +closely-allied species could never have become species. +Again, that dominant species should be rich in varieties +is what might have been expected; for the +greater the number of individuals in a species, the +greater is the chance of variations taking place in +all parts of the organic type, and particularly in the +reproductive system, seeing that this system is the +most sensitive to small changes in the conditions +of life, and that the greater the number of individuals +composing a specific type, the more certainty +there is of some of them encountering such +changes. Hence, the richness of dominant species +in varieties is, I believe, mainly due to the greater +opportunity which such species afford of some degree +of cross-infertility arising between their constituent +members.</p> + +<p>Here is another general fact, also first noticed by +Darwin, and one which he experiences some difficulty +an explaining on the theory of natural selection. He +says:—</p> + +<blockquote><p>In travelling from north to south over a continent, we generally +meet at successive intervals with closely-allied or representative +species, evidently filling the same place in the economy of the +land. These representative species often meet and interlock, +and as one becomes rarer and rarer, the other becomes more and +more frequent, till the one replaces the other. But if we compare +these species where they intermingle, they are generally as +absolutely distinct from each other in every detail of structure as +are specimens taken from the metropolis of each.... In the<span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span> +intermediate region, having intermediate conditions of life, why +do we not now find closely-linking intermediate varieties? This +difficulty for a long time quite confounded me. But I think it +can in large part be explained<a name="FNanchor_23_23" id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a>.</p></blockquote> + +<div class="figcenter" style="width: 600px;"> +<img src="images/illus-083.png" width="600" height="74" alt="" /> +</div> + +<p>His explanation is that, "as the neutral territory +between two representative species is generally narrow +in comparison with the territory proper to each, +... and as varieties do not essentially differ from +species, the same rule will probably apply to both; and, +therefore, if we take a varying species inhabiting +a very large area, we shall have to adapt two varieties +to two large areas, and a third variety to a narrow +intermediate zone." It is hence argued that this +third or intermediate variety, on account of its existing +in lesser numbers, will probably be soon overrun and +exterminated by the larger populations on either side +of it. But how is it possible "to adapt two varieties +to two large areas, and a third [transitional] variety +to a narrow intermediate zone," in the face of free +intercrossing on a continuous area? Let <i>A</i>, <i>B</i>, and +<i>C</i> represent the three areas in question. According to +the argument, variety <i>A</i> passes first into variety <i>B</i>, +and then into variety <i>C</i>, while variety <i>B</i> eventually +becomes exterminated by the inroads both from +<i>A</i> and <i>C</i>. But how can all this have taken place +with nothing to prevent intercrossing throughout the +entire area <i>A</i>, <i>B</i>, <i>C</i>? I confess that to me it seems this +argument can only hold on the supposition that the +analogy between varieties and species extends to the<span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span> +reproductive system; or, in a sense more absolute +than the argument has in view, that "varieties do +not essentially differ from the species" which they +afterwards form, but from the first show some +degree of infertility towards one another. And, if so, +we have of course to do with the principles of physiological +selection.</p> + +<p>That in all such cases of species-distribution these +principles have played an important part in the +species-formation, appears to be rendered further +probable from the suddenness of transition on the +area occupied by contiguous species, as well as from +the completeness of it—i. e. the absence of connecting +forms. For these facts combine to testify that the +transition was originally due to that particular change +in the reproductive systems of the forms concerned, +which still enables those forms to "interlock" without +intercrossing. On the other hand, neither of these +facts appears to me compatible with the theory of +species-formation by natural selection alone.</p> + +<p>But this leads us to another general fact, also +mentioned by Darwin, and well recognized by all +naturalists, namely, that closely allied species, or +species differing from one another in trivial details, +usually occupy contiguous areas; or, conversely stated, +that contiguity of geographical position is favourable +to the appearance of species closely allied to one +another. Now, the large body of facts to which +I here allude, but need not at present specify, appear +to me to constitute one of the strongest of all my +arguments in favour of physiological selection. Take, +for instance, a large continental area, and follow across +it a chain of species, each link of which differs from<span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span> +those on either side of it by the minute and trivial +distinctions of a secondary kind, but all the links +of which differ from one another in respect of the +primary distinction, so that no one member of the +series is perfectly fertile with any other member. Can +it be supposed that in every case this constant +primary distinction has been superinduced by the +secondary distinctions, distributed as they are over +different parts of all these kindred organisms, and +yet nowhere presenting any but a trifling amount of +morphological change?</p> + +<p>For my own part, I cannot believe—any more +than Darwin could believe—that all these numerous, +diverse, and trivial changes have always had the +accidental effect of inducing the same peculiar change +in the reproductive system, and so producing it without +any reference to the process of specific divergence. +Nor can I believe, as Darwin incidentally and provisionally +suggested, that prolonged exposure to +uniform conditions of life have so generally induced +an equally meaningless result. I can only believe +that all the closely allied species inhabiting our +supposed continent, and differing from one another +in so many and such divers points of small detail, are +merely so many records of the fact that selective +fertility has arisen among their ancestry, and has +thus given as many opportunities for the occurrence +of morphological differentiations as it has furnished +cases of efficient isolation. Of course, I do not deny +that many, or probably most, of these trivial morphological +differentiations have been produced by natural +selection on account of their utility: I merely deny +that they could have been so produced on this<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span> +common area, but for the sexual isolation with which +every distinct set of them is now found to be associated.</p> + + +<h3><i>Evidence from Topographical Distribution of +Species.</i></h3> + +<p>By topographical distribution I mean the distribution +of organisms with reference to comparatively +small areas, as distinguished from larger regions with +reference to which the term geographical distribution +is appropriate.</p> + +<p>It will be at once apparent that a study of the +topographical distribution of organic types is of even +more importance for us than a study of their geographical +distribution. For while the former study is +conducted, as it were, with a low power of our +observing microscope, the latter is conducted with +a high power. The larger facts of geographical +distribution yield, indeed, all the general characters +which we might expect them to yield, on the theory +that divergence of specific types on common areas +has been in chief part determined by physiological +conditions. But for the purpose of testing this +theory in a still more exacting manner, it is of the +first importance to consider the more detailed facts +of topographical distribution, since we here come to +closer quarters with the problem of specific differentiation. +Therefore, as we have already considered +this problem under the most general points of view, +we will now consider it under more special points +of view.</p> + +<hr class="tb" /> + +<p>It is self-evident, as we have seen in the preceding<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span> +section, that the greater the number of individuals +of the same species on a given area, the less must +be the power of natural selection to split that species +into two or more allied types; because, the more +crowded the population, the greater must be the +uniformitarian effect of free intercrossing. This obvious +fact has been insisted upon by several previous +writers on Darwinism; and the only reason why it +has not been recognized by all naturalists is that so +few of them have observed the all-important distinction +between monotypic and polytypic evolution. +The denser the population, and therefore the greater +the intercrossing and the severer the struggle for +existence within the species, the better will it be +for <i>transmutation</i> of the species by natural selection; +but the worse it will be for <i>differentiation</i> of the +species by this form of homogamy. On the other +hand, if physiological selection be entertained as +a form of homogamy, the denser the population, the +better opportunity it will have of differentiating the +species, first, because a greater number of individuals +will be present in which the physiological change +may arise, and, secondly, because, if it does arise, the +severity of the struggle for existence will <i>then</i> give +natural selection a better chance of acting rapidly +and effectually on each of the isolated sections.</p> + +<p>Hence, where the question is whether selective +fertility has played any large or general part in the +differentiation of specific types, the best criterion we +can apply is to ascertain whether it is a general +rule that closely allied species occur in intimate +association, so that their individual members constitute, +as it were, a single population, or, on the<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span> +other hand, whether they occur rather on different +sides of physical barriers. If they occur intimately +associated, the form of homogamy to which their +differentiation was due must have presumably been +the physiological form; whereas, if they are proved +to be correlated with physical barriers, the form of +homogamy which was concerned in their differentiation +must presumably have been the geographical +form.</p> + +<p>Now, at first this consideration was a trouble to +me, because Moritz Wagner had strenuously argued—and +supported his argument by a considerable +wealth of illustration—that allied species are always +found correlated with physical barriers or discontinuous +areas. Weismann's answer, indeed, had +shown that Wagner's statement was much too general: +nevertheless, I was disappointed to find that so +much could be said in favour of the geographical +(or topographical) form of isolation where closely +allied species are concerned. Subsequently, however, +I read the writings of Nägeli on this subject, and +in them I find a very different state of matters +represented.</p> + +<p>Seeing as clearly as Wagner that it is impossible +under any circumstances for natural selection to +cause specific <i>differentiation</i> unless assisted by some +other forms of homogamy, but committing the same +oversight as Wagner and Weismann in supposing +that the only other form of homogamy in nature is +geographical isolation, Nägeli, with great force of +reasoning, and by many examples, founded his argument +against the theory of natural selection on the +ground that in the vegetable kingdom closely allied<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span> +species are most frequently found in intimate association +with one another, not, that is to say, in any +way isolated by means of physical barriers. This +argument is everywhere logically intact; and, as he +sustains it by a large knowledge of topographical +botany, his indictment against natural selection as +a cause of specific <i>differentiation</i> appeared to be +insurmountable. And, in point of fact, it <i>was</i> insurmountable; +so that the whole problem of the +origin of species by <i>differentiation on common areas</i> +has hitherto been left in utter obscurity. Nor is there +now any escape from this obscurity, unless we entertain +the "supplementary factor" of selective fertility. +And, apparently, the only reason why this has not +been universally recognized, is because Darwinians +have hitherto failed to perceive the greatness of the +distinction between the <i>differentiation</i> and the <i>transmutation</i> +of species; and hence have habitually met +such overwhelming difficulties as Nägeli presented by +an illogical confounding of these two totally distinct +things.</p> + +<p>But if the idea of selective fertility had ever +occurred to Nägeli as a form of segregation which +gives rise to specific differentiation, I can have no +doubt that so astute and logical a thinker would +have perceived that his whole indictment against +natural selection was answered. For it is incredible +that he should not have perceived how this physiological +form of homogamy (supposing it to arise <i>before</i> +or <i>during</i>, and not <i>after</i> the specific differentiation) +would perform exactly the same function on a continuous +area, as he allowed that "isolation" does on +a discontinuous one.</p><p><span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span></p> + +<p>However, be this as it may, there cannot be any +question touching the immense value of his facts and +arguments as evidence in favour of physiological +selection—albeit this evidence was given unconsciously, +or, as it were, prophetically. Therefore +I will here quote a few examples of both, from his +paper <i>Du Développement des Espèces Sociales</i><a name="FNanchor_24_24" id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a>.</p> + +<p>After stating the theory of natural selection, he +says that if the theory is (of itself) a true explanation +of the origin (or divergence) of specific forms, it +ought to follow that</p> + +<blockquote><p>two closely allied forms, derived the one from the other, +would necessarily occupy two different geographical areas [or +topographical stations], since otherwise they would soon become +blended. Until they had already become sufficiently consolidated +as distinct species to render mutual intercrossing highly improbable, +they could not be intermingled without disadvantage +[to differentiation]. Had Darwin endeavoured to support his +hypothesis by facts, he would, at least in the vegetable kingdom, +have found little to favour his cause. I can cite many hundreds +of cases, in which species in every stage of development have +been found closely mingling with one another, and not in any +way isolated. Therefore, I do not think that one can rightly +speak of natural selection in the Darwinian sense in the +vegetable kingdom; and, in my estimation, there is a great +difference between the formation of species by nature and the +production of stock by a breeder.... (p. 212).</p> + +<p>Of the two kinds of distribution (i. e. growing apart and +growing together), Synoicy (or growing together) is by far +the most usual in nature. I reckon that out of a hundred +allied vegetable forms, at least ninety-five would be found to be +synoical (p. 219).</p></blockquote> + +<p>This is a most important point. That so enormous<span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span> +a proportion of vegetable species should have originated +in intimate association with their parent or +sister types, is clearly unintelligible on the theory of +natural selection alone; there obviously <i>must</i> be some +other form of homogamy which, whether or not in +all places <i>associated</i> with natural selection, is the +primary condition to the differentiation. Such +I hold with Nägeli, is a logical necessity; and this +whether or not I am right in believing the other +form of homogamy in question to be selective fertility. +But I go further and say, Surely there can be no +rational question that this other form of homogamy +must have been, at any rate as a highly general rule, +the one which I have assigned. For how is it that +in these ninety-five per cent. of cases, where vegetable +species are growing intimately associated with their +nearest allies, there is no hybridizing, or blending +and relapsing to the original undifferentiated types? +We know well the answer. These are fully differentiated +species, and, as such, are protected from mutual +intercrossing by the barrier of mutual sterility. But +now, if this bar is thus necessary for preserving the +specific distinctions when they have been fully +developed, much more must it have been so to admit +of their development; or, otherwise stated, since we +know that this barrier is associated with "synoical" +species, and since we clearly perceive that were it +withdrawn these species would soon cease to exist, +can we reasonably doubt that their existence (or +origin) is due to the previous erection of this +barrier? If synoical species were comparatively +rare, the validity of such reasoning might be open +to question; or, even if we should not doubt it in<span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span> +such cases, at any rate we might well doubt the +importance or extent of selective fertility as a factor +in the origination of species. But the value of +Nägeli's writings on the present subject consists in +showing that synoical species constitute so overwhelming +a majority of the vegetable kingdom, that +here, at all events, it appears impossible to rate too +highly the importance of the principle I have called +physiological selection.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></p> + + + + +<h2>CHAPTER V.<br /> +<span class="smcap">Further Evidences of Physiological +Selection.</span></h2> + + +<h3><i>Evidence from Topographical Distribution of +Varieties.</i></h3> + +<p>In the last section we have considered the topographical +distribution of closely allied <i>species</i>. I now +propose to go still further into matters of detail, by +considering the case of natural <i>varieties</i>. And here +we come upon a branch of our inquiry where we may +well expect to meet with the most crucial tests of +our theory. For if it should appear that these nascent +species more or less resemble fully developed species +in presenting the feature of cross-infertility, the theory +would be verified in the most direct and conclusive +manner possible. These nascent species may be +called embryo species, which are actually in course +of differentiation from their parent-type; and therefore, +if they do not exhibit the feature in relation +to that type which the present theory infers to be +necessary for the purposes of differentiation, the +theory must be abandoned. On the other hand, if +they do exhibit this feature, it is just the feature +which the theory predicted as one that would be +found highly characteristic of such embryo types.<span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span> +Contrariwise, the theory of natural selection can have +no reason to form any such anticipation; or rather +its anticipation would necessarily require to be the +exact opposite. For, according to this theory, the +cross-infertility of allied species is due, either to +correlation with morphological changes which are +being produced by the selection, or else, as Darwin +supposed, to "prolonged exposure to uniform conditions +of life"; and thus, in either case, the sterility +variation ought to be, as a general rule at all events, +subsequent to the specific differentiation, and, according +to Darwin's view, <i>long</i> subsequent. Thus +we ought not to find that the physiological change +is ever, on any large or general scale, the initial +change; nor ought we to find that it is, on any +such scale, even so much as a contemporary change: +there ought, in fact, to be no constant or habitual +association between divergence of embryo-types and +the concurrence of cross-infertility.</p> + +<p>Now, it will be my endeavour to prove that +there is an extraordinarily general association between +<i>varietal</i> divergence and cross-infertility, <i>wherever +common areas are concerned</i>; and in as far as this +can be proved, I take it that the evidence will make +wholly in favour of physiological selection as the +prime condition to specific divergence, while at the +same time they will make no less wholly, <i>and quite +independently</i>, against natural selection as the unaided +cause of such divergence.</p> + +<p>I shall begin with some further quotations from +Nägeli.</p> + +<blockquote><p>Species may be synoical at all stages of relationship. We +come across varieties, scarcely distinguishable from one another,<span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span> +growing in the same locality (as, for example, the <i>Cirsium +heterophyllum</i>, with smooth or jagged leaves, the <i>Hieracium +sylvaticum</i>, with or without caulinary leaves); again, we meet +other varieties more accentuated (as the <i>H. hoppeanum</i>, with +under ligules of white or red, the <i>Campanula</i>, with white or lilac +flowers, &c.), other varieties even more marked, which might +almost be elevated to the rank of species (<i>Hieracium alpinum</i>, +with hairs and glands, and the new form <i>H. holadenium</i>, which +has only glands, <i>Campanula rotundifolia</i> with smooth and hairy +leaves), or forms still more distinct, up to well-defined species. +I could enumerate endless examples at all stages.</p> + +<p>It will be seen that in my definition of synoicy I do not mean +to assert that <i>all</i> allied forms are invariably found together, but +that they are much more often seen in groups than singly. +Take, for instance, nine forms closely related (<i>A</i> to <i>I</i>). <i>A</i>, <i>E</i>, <i>H</i> +will be found side by side at one point, <i>B</i>, <i>D</i> at another, <i>C</i>, <i>F</i> +at a third, &c. These facts are plainly opposed to the theory of +isolation and amixia, and make, on the contrary, in favour of the +social development of species (<i>loc. cit.</i>, p. 221).</p></blockquote> + +<p>Not to multiply quotations to the same general effect, +I will supply but one other, referring to a particular +case.</p> + +<blockquote><p>At one spot (<i>Rothwand</i>) much exposed to the sun, and +difficult of access, I remarked two closely allied forms, so nearly +related to <i>H. villosum</i> that this would seem to be an intermediary +form between the two. One of these (<i>H. villosissimum</i>) +is distinguished by its tongue and thick pubescence, its tolerably +large capitula, and by the lengthened and separated scales of +the involucrum; the other, on the contrary (<i>H. elongatum</i>), is +less pubescent, has smaller capitula, and more compact scales +on the involucrum than <i>H. villosum</i>. Both are finally distinguishable +from the type by their longer stalks, which are more +decidedly aphyllous, and by their later flowering. At the spot +where I found them the two forms were closely intermingled, +and each was represented by a considerable number of plants. +I did not find them anywhere else on the mountain, nor could +I find at the spot where these were growing a single specimen +of the true <i>H. villosum</i>, nor a single hybrid from these two.</p><p><span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span></p> + +<p>I concluded that these two new forms had, by joining their +forces, expelled the <i>H. villosum</i> from its primitive abode, but +had not succeeded in displacing one another. As to their origin, +they had evidently developed in two different directions from +a common point of departure, namely <i>H. villosum</i>. They had +succeeded, not only in separating themselves from the original +form, but also in preventing any intermediary form from interposing. +I thought myself therefore justified in considering this +as a case of varieties which have come into existence subsequently +to the Glacial epoch. The morphological characteristics of the +three forms are sufficiently distinct for them to be designated as +species by a good many writers. They are better defined than +some of MM. Frolich and Fries' weaker species, and as well +defined as some of MM. Koch and Grisebach's (p. 222).</p></blockquote> + +<p>Now it is clear, without comment, that all this is +exactly as it ought to be, if allied species have been +differentiated on common areas by selective fertility. +For if, as Nägeli elsewhere says, "one meets forms +in nature associated with one another, and severally +distinguished by every possible degree of differentiation," +not only as Nägeli adds, does this general +fact lead to the inference that species are (usually) +developed when plants grow intimately associated +together; but as certainly it leads to the further +inference that such development must be due to +a prior development of cross-infertility between the +diverging varietal forms, cross-infertility which is +therefore afterwards so characteristic of the allied +species, when these are found, in their fully differentiated +condition, still occupying the same area +in large and intimately mingled populations.</p> + +<p>To my mind there could not be any inference more +strongly grounded than this, because, with the one +exception of the physiological form, no other form +of homogamy can be conceived which shall account<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span> +for the origin and permanence of these synoical +varieties, in all degrees of differentiation up to well-defined +synoical species. Least of all, as we have +seen, can natural selection alone have had anything +to do with such a state of matters; while, as we have +likewise seen, in all its details it is exactly the state +of matters which the theory of physiological selection +requires.</p> + +<p>Nevertheless, although this inference is so strongly +grounded, we ought to remember that it is only an +inference. In order fully to verify the theory of +physiological selection, we ought to prove by experiment +the fact of cross-infertility between these synoical +varieties, as we learn that it afterwards obtains between +synoical species. It is to be regretted that the theory +of physiological selection did not occur to the mind +of Nägeli, because he would then, no doubt, have +ascertained this by actual experiment. As it is, the +great value of his observations goes no further than +establishing a strong presumption, that it <i>must</i> be +selective fertility which causes the progressive differentiation +of synoical varieties; and also that, if +so, this <i>must</i> be the principal factor in the differentiation +of vegetable species, seeing that some ninety-five +per cent. are of synoical origin.</p> + + +<h3><i>Evidence from Experimental Research.</i></h3> + +<p>My paper on <i>Physiological Selection</i> pointed out that +the whole theory would have to stand or fall with the +experimental proof of the presence or the absence of +cross-infertility between varieties of the same species +growing on common areas. From the facts and +considerations which we have hitherto been dealing<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span> +with, it did indeed appear to me that there was the +strongest conceivable ground for inferring that cross-infertility +between such varieties would be found by +experiment to be a phenomenon of highly general +occurrence—amply sufficient ground to prove +that allied species on common areas for the most part +owed their origin to this character of mutual sterility, +and not vice versa as previously supposed. At that +time I was not aware that any experiments had been +made in this direction. Soon after the paper was +published, however, my attention was directed to a +laborious research which had been directed to this +very point, and carried on for more than thirty years, +by M. Jordan<a name="FNanchor_25_25" id="FNanchor_25_25"></a><a href="#Footnote_25_25" class="fnanchor">[25]</a>. This had not attracted the general +notice which it undoubtedly deserved; and I have +since ascertained that even Darwin began to look +into it only a few months before his death.</p> + +<p>Having devoted his life to closely observing in +divers stations multitudes of different species of plants—annuals +and perennials, bulbous and aquatic, trees +and shrubs—M. Jordan has been able to satisfy himself, +and the French school of botanists to which this +line of observation has given rise, that in most cases +(or "nearly everywhere"), when a Linnean species +is indigenous to a country and is there of common +occurrence, this species within that district is represented +by more or less numerous and perfectly constant +varieties. These varieties are constituted by such +minute differences of morphological character that<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span> +their very existence eluded the observation of botanists, +until M. Jordan began to search specially for them as +the special objects of his scrutiny. Moreover, these +varieties of a Linnean species occupy common areas, +and there grow in intimate association with one +another, or as M. Jordan says, "<i>pêle-mêle</i>." So far, +be it noticed, Jordan was proceeding on exactly the +same lines as Nägeli; only he carried his observations +over a still wider range of species on the one +hand, and into a still minuter search for varieties +on the other. But the all-important point for us is, +that he further proceeded to test by experiment the +physiological relations between these morphological +varieties; and found, in many hundreds of cases, +that they not only came true to seed (i. e. are hereditary +and not merely climatic), but likewise cross-sterile +<i>inter se</i>. For these reasons, M. Jordan, who is +opposed to the theory of evolution, regards all such +varieties as separately created species; and the +inspiring motive of his prolonged investigations has +been a desire to multiply these proofs of creative +energy. But it clearly makes no difference, so far +as evolutionists are concerned with them, whether +all this multitude of sexually isolated forms be denominated +species or varieties.</p> + +<p>The points which are of importance to evolutionists—and +of the first order of importance in the +present connexion—may be briefly summarized as +follows:—</p> + +<p>(1) The research embraces large numbers of species, +belonging to very numerous and very varied orders +of plants; (2) in the majority of cases—although not +all—indigenous species which are of common occurrence<span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span> +present constant varieties; (3) these varieties, +nevertheless, may be morphologically so slight as to +be almost imperceptible; (4) they occupy common +areas and grow in intimate association; (5) although +many of them have undergone so small an amount +of morphological change, they have undergone a surprising +amount of physiological change; for (6) not +only do very many of these varieties come true to +seed; but, (7) when they do, they are always more or +less cross-infertile <i>inter se</i>.</p> + +<p>Now, it is self-evident that every one of these seven +points is exactly what the theory of physiological +selection requires, while there is not one of them +which it does not require. For if the theory be +sound, we should expect to find large numbers of +species belonging to numerous and varied orders +of plants presenting constant varieties on common +areas; we should expect this to be a highly general, +though not a universal, rule; and we should expect +it to apply only to species which are indigenous. Moreover, +we should expect these varieties, although but +slightly differentiated morphologically, to present a +great differentiation physiologically—and this in the +special direction of selective fertility, combined, of +course, with heredity.</p> + +<p>On the other hand, as I have said, this catalogue +of evidences leaves nothing to be supplied. It gives +us all the facts—and no more than all the facts—which +my paper on <i>Physiological Selection</i> anticipated +as the eventual result of a prolonged experimental +research. And if I have to regret my ignorance of +these facts when that paper was published, at any +rate it now furnishes the best proof that my anticipations<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span> +were not guided by the results of a verification +which had already been supplied. These anticipations +were deduced exclusively from the theory itself, as +representing what <i>ought</i> to be the case if the theory +were true; and, I must confess, if I had then been +told that they had already been realized—that it +had actually been found to be a general rule that +endemic species present constant and hereditary +varieties, intimately commingled on common areas, +morphologically almost indistinguishable, but physiologically +isolated by selective fertility—I should +have felt that the theory had been verified in +advance. For there are only two alternatives: +either these things are due to physiological selection, +or else they are due—as M. Jordan himself believes—to +special creation. Which is equivalent to saying +that, for evolutionists, the facts must be held +to verify the former theory in as complete a manner +as it is logically possible for the theory to be +verified.</p> + + +<h3><i>Evidence from Prepotency.</i></h3> + +<p>We have now to consider the bearing of what is +called "prepotency" on the theory of physiological +selection.</p> + +<p>Speaking of the vast number of species of Compositae, +Darwin says:—</p> + +<blockquote><p>There can be no doubt that if the pollen of all these species +could be simultaneously or successively placed on the stigma of +any one species, this one would elect with unerring certainty its +own pollen. This elective capacity is all the more wonderful, as +it must have been acquired since the many species of this great +group of plants branched off from a common progenitor.</p></blockquote><p><span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span></p> + +<p>Darwin is here speaking of elective affinity in +its fully developed form, as absolute cross-sterility +between fully differentiated species. But we meet +with all lower degrees of cross-infertility—sometimes +between "incipient species," or permanent varieties, +and at other times between closely allied species. +It is then known as "prepotency" of the pollen +belonging to the same variety or species over the +pollen of the other variety or species, when both sets +of pollen are applied to the same stigma. Although +in the absence of the prepotent pollen the less potent +will fertilize the seed, yet, such is the appetency for +the more appropriate pollen, that even if this be +applied to the stigma some considerable time after +the other, it will outstrip or overcome the other in +fertilizing the ovules, and therefore produce the +same result on the next generation as if it had been +applied to the mother plant without any admixture +of the less potent pollen, although in some cases such +incipient degrees of cross-infertility are further shown +by the number or quality of the seeds being fewer +or inferior.</p> + +<p>Now, in different varieties and in different allied +species, all degrees of such prepotency have been +noticed by many observers, from the faintest perceptible +amount up to complete impotency of the +alien pollen—when, of course, there is absolute +sterility between the two varieties or allied species. +The inference is obvious. In this graduated scale +of prepotency—beginning with an experimentally +almost imperceptible amount of sexual differentiation +between two varieties, and ending in an absolute +partitioning of two allied species—we have the only<span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span> +remaining fact that is required to complete the case +in favour of the present theory. We are here brought +back to the very earliest stages of physiological differentiation +or to the stages which lie behind Jordan's +"Physiological Species"; and therefore, when taken +in conjunction with his results, the phenomena of +prepotency may be said to give us the complete and +final demonstration of one continuous development, +which, beginning in an almost imperceptible amount +of cross-infertility, ends in absolute cross-sterility. +The "elective capacity" to which Darwin alludes as +having been "acquired" by all the species of Compositae +since they "branched off from a common +progenitor," is thus seen among innumerable other +species actually in process of acquisition; and so +we can perfectly well understand, what is otherwise +unintelligible, that closely allied species of plants +occur, in ninety-five per cent. of cases, intimately associated +on common areas, while exhibiting towards one +another the character of mutual sterility.</p> + +<p>But more than this. The importance of the widespread +phenomena of prepotency to the theory of +physiological selection does not consist merely in +thus supplying the last link in the chain of evidence +touching the origin of species by selective fertility, +or "elective capacity." These phenomena are of +further importance as showing how in plants, at all +events, physiological selection appears to be frequently +capable of differentiating specific types without the +necessary assistance of any other form of homogamy. +In my original statement of the theory, I was careful +to insist upon the great value, as differentiating agents, +of even small degrees of other forms of homogamy<span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span> +when co-operating with physiological selection. But +I also stated my belief that in many cases selective +fertility is presumably of itself capable of splitting +a specific type; and the reason why I still believe +this is, that I do not otherwise understand these phenomena +of prepotency. I cannot believe that in all the +innumerable cases where they arise, they have been +super-induced by some prior morphological changes +going on in some other part of the organism, or by +"prolonged exposure to uniform conditions of life," +on the part of two well-nigh identical forms which +have arisen intimately commingled in exactly the +same environment, and under the operation of a previously +universal intercrossing. Even if such a thing +could be imagined as happening occasionally, I feel +it difficult to imagine that it can happen habitually, +and yet this view must be held by those who would +attribute prepotency to natural selection.</p> + +<p>It must never be forgotten that the relatively +enormous changes as to size, structure, habit, &c., +which are presented by our domesticated plants as +results of artificial selection, do not entail the physiological +character of cross-sterility in any degree, +save possibly in some small number of cases. Although +in wild species any correspondingly small percentage +of cases (where natural selection happens to hit upon +parts of the organism modifications of which produce +the physiological change by way of correlation) would +doubtless be the ones to survive on common areas, +still it is surely incredible that such an accidental +association between natural selection and cross-infertility +is so habitually the means of specific +differentiation as the facts of prepotency (together<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span> +with the observations of Jordan and Nägeli) would +necessarily demand.</p> + +<p>Moreover, this view of the matter is still further +corroborated by certain other facts and considerations. +For example, the phenomena of prepotency (whether +as between varieties or between closely allied species) +are found to occur when the two forms occupy a +common area, i.e. are growing intermingled with +one another. Therefore, but for this physiological +differentiation, there could be absolutely nothing to +prevent free intercrossing. Yet the fact that hybrids +are so comparatively rare in a state of nature—a fact +which Sir Joseph Hooker has pointed out to me as +otherwise inexplicable—proves the efficacy of even +a low degree of such differentiation in preventing +the physiologically-differentiated forms from intercrossing. +Even in cases where there is no difficulty +in producing artificial hybrids or mongrels between +species or varieties growing on common areas, it is +perfectly astonishing what an extremely small percentage +of the hybrid or mongrel forms are found to +occur in nature. And there can be no question that +this is due to the very efficient manner in which +prepotency does its work—efficient, I mean, from +the point of view of the new theory; for upon any +other theory prepotency is a meaningless phenomenon, +which, notwithstanding its frequent occurrence, +plays no part whatever in the process of organic +evolution.</p> + +<p>I attach considerable importance to the phenomena +of prepotency in view of the contrast which is presented +between plants and animals in the relation of +their species to physical barriers. For animals—and<span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span> +especially the higher animals—appear to depend +for their specific differentiations upon such barriers +much more than in the case with plants. This is no +more than we should expect; for, in accordance with +our theory, selective fertility is not so likely to work +alone in the case of the higher animals which mate +together, as in plants which are fertilized through the +agency of wind or insects. In the former case there +is no opportunity given for the first rise of cross-infertility, +in the form of prepotency; and even where +selective fertility has gained a footing in other ways, +the chances against the suitable mating of "physiological +complements" must be much greater than it +is in the latter case. Hence, among the higher animals, +selective fertility ought much more frequently to be +found in association with other forms of homogamy +than it is among plants. And this is exactly what +we find. Thus it seems to me that this contrast +between the comparative absence and presence of +physical barriers, where allied species of plants and of +higher animals are respectively concerned, is entitled +to be taken as a further corroboration of our theory. +For while it displays exactly such a general correlation +as this theory would expect, the correlation is +one which cannot possibly be explained on any other +theory. It is just where physiological selection can +be seen to have the best opportunity of acting (viz. +in the vegetable kingdom) that we find the most +unequivocal evidence of its action; while, on the +other hand, it is just where it can be seen to have +the least opportunity of asserting itself (viz. among +the higher animals) that we find it most associated +with, and therefore assisted by, other forms of homogamy,<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span> +i. e. not only geographical isolation, but also +by sexual preference in pairing, and the several +other forms of homogamy, which Mr. Gulick has +shown to arise in different places as the result of +intelligence.</p> + + +<h3><i>Evidence from Special Cases.</i></h3> + +<p>Hitherto I have been considering, from the most +general point of view, the most widespread facts +and broadest principles which serve to substantiate +the theory of physiological selection. I now pass +to the consideration of one of those special cases in +which the theory appears to have been successfully +applied.</p> + +<p>Professor Le Conte has adduced the fossil snails +of Steinheim as serving to corroborate the theory of +physiological selection<a name="FNanchor_26_26" id="FNanchor_26_26"></a><a href="#Footnote_26_26" class="fnanchor">[26]</a>.</p> + +<p>The facts are these. The snail population of this +lake remain for a long time uniform and unchanged. +Then a small percentage of individuals suddenly began +to vary as regards the form of their shells, and this in +two or three directions at the same time, each affected +individual, however, only presenting one of the variations. +But after all these variations had begun to +affect a proportionally large number of individuals, +some individuals occur in which two or more of the +variations are blended together, evidently, as Weismann +says, by intercrossing of the varieties so blended. +Later still, both the separate varieties and their +blended progeny became more and more numerous, +and eventually a single blended type, comprising +in itself all the initial varieties, supplanted the<span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span> +parent form. Then another long period of stability +ensued until another eruption of new variations took +place; and these variations, after having affected +a greater and greater number of individuals, eventually +blended together by intercrossing and supplanted +their parent form. So the process went on, +comparatively short periods of variation alternating +with comparatively long periods of stability, the +variations, moreover, always occurring suddenly in +crops, then multiplying, blending together, and in +their finally blended type eventually supplanting their +parent form.</p> + +<p>Now, the remarkable fact here is that whenever the +variations arose, they only intercrossed between themselves, +they did not intercross with their parent form; +for, if they had, not only could they never have +survived (having been at first so few in number and +there having been no geographical barriers in the +small lake), but we should have found evidence of +the fact in the half-bred progeny. Moreover, natural +selection can have had nothing to do with the process, +because not only are the variations in the form of the +shells of no imaginable use in themselves; but it +would be preposterous to suppose that at each of these +"variation periods" several different variations should +always have occurred simultaneously, all of which were +of some hidden use, although no one of them ever +occurred during any of the prolonged periods of +stability. How, then, are we to explain the fact that +the individuals composing each crop of varieties, while +able to breed among themselves, never crossed with +their parent form? These varieties, each time that +they arose, were intimately commingled with their<span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span> +parent form, and would certainly have been reabsorbed +into it had intercrossing in that direction +been possible. With Professor Le Conte, therefore, +I conclude that there is only one conceivable answer +to this question. Each crop of varieties must have +been <i>protected from intercrossing with their parent +form</i>.</p> + +<p>They must have been the result of a variation, which +rendered the affected individuals sterile with their +parent form, whilst leaving them fertile amongst themselves. +The progeny of these individuals would then +have dispersed through the lake, physiologically isolated +from the parent population, and especially prone to +develop secondary variations as a direct result of the +primary variation. Thus, as we might expect, two or +three variations arose simultaneously, as expressions +of so many different lines of family descent from the +original or physiological variety; these were everywhere +prevented from intercrossing with their parent +form, yet capable of blending whenever they or their +ever-increasing progeny happened to meet. Thus, +without going into further details, we are able by +the theory of physiological selection to give an explanation +of all these facts, which otherwise remain +inexplicable.</p> + +<hr class="tb" /> + +<p>In view of the evidence which has now been presented, +I will now ask five questions which must be +suitably answered by critics of the theory of physiological +selection.</p> + +<p>1. Can you doubt that the hitherto insoluble problem +of inter-specific sterility would be solved, supposing +cross-infertility were proved to arise before or<span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span> +during the process of specific differentiation, instead +of after that process had been fully completed?</p> + +<p>2. Can you doubt, after duly considering the circumstances +under which allied species of plants have +been differentiated—viz. in ninety-five per cent. of +cases intimately commingled on common areas, and +therefore under identical environments—that cross-infertility +<i>must</i> have arisen before or during the +specific differentiation?</p> + +<p>3. Can you doubt, after duly considering the facts +of prepotency on the one hand and those of Jordan's +physiological varieties on the other, that cross-infertility +<i>does</i> arise before or during the specific differentiation?</p> + +<p>4. If you cannot express a doubt upon any of these +points, can you explain why you refuse to accept the +theory of the origin of species by means of physiological +selection, together with the explanation which +this theory affords of the continued cross-fertility of +domesticated varieties?</p> + +<p>5. Supposing this theory to be true, can you conceive +of any other classes of facts which, either +quantitatively or qualitatively, could more directly or +more effectually prove its truth than those which have +now been adduced?</p> + +<p>On these five heads I entertain no doubt. I am +convinced that the theory of physiological selection is +the only one that can explain the facts of inter-specific +sterility on the one hand, and, on the other hand, the +contrast which these facts display to the unimpaired +fertility of our domesticated varieties.</p> + +<p>In conclusion, it seems desirable once more to insist +that there is no antagonism or rivalry between the<span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span> +theories of natural and of physiological selection. For +which purpose I will quote the final paragraph of my +original paper.</p> + +<blockquote><p>So much, then, for the resemblances and the differences +between the two theories. It only remains to add that the two +are complementary. I have already shown some of the respects +in which the newer theory comes to the assistance of the older, +and this in the places where the older has stood most in need of +assistance. In particular, I have shown that segregation of the +fit entirely relieves survival of the fittest from the difficulty under +which it has hitherto laboured of explaining why it is that sterility +is so constantly found between species, while so rarely found +between varieties which differ from one another even more than +many species; why so many features of specific distinction are +useless to the species presenting them; and why it is that +incipient varieties are not obliterated by intercrossing with parent +forms. Again, we have seen that physiological selection, by +preventing such intercrossing, enables natural selection to +promote diversity of character, and thus to evolve species in +ramifying branches instead of in linear series—a work which I +cannot see how natural selection could possibly perform unless +thus aided by physiological selection. Moreover, we have seen +that although natural selection alone could not induce sterility +between allied types, yet when this sterility is given by physiological +selection, the forms which present it would be favoured in +the struggle for existence; and thus again the two principles are +found playing, as it were, into each other's hands. And here, as +elsewhere, I believe that the co-operation enables the two principles +to effect very much more in the way of species-making +than either of them could effect if working separately. On the +one hand, without the assistance of physiological selection, +natural selection would, I believe, be all but overcome by the +adverse influences of free intercrossing—influences all the more +potent under the very conditions which are required for the +multiplication of species by divergence of character. On the +other hand, without natural selection, physiological selection +would be powerless to create any differences of specific type, +other than those of mutual sterility and trivial details of structure,<span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span> +form, and colour—differences wholly without meaning from a +utilitarian point of view. But in their combination these two +principles appear to me able to accomplish what neither can +accomplish alone—namely, a full and satisfactory explanation of +the origin of species.</p></blockquote> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></p> + + + + +<h2>CHAPTER VI.<br /> +<span class="smcap">A Brief History of Opinions on Isolation +as a Factor of Organic Evolution.</span></h2> + + +<p>This historical sketch must begin with a consideration +of Darwin's opinions on the subject; but as these +were considerably modified from time to time during +a period of thirty years by the publications of other +naturalists, it will be impossible to avoid cross-references +as between his writings and theirs. It +may also be observed that the <i>Life and Letters of +Charles Darwin</i> was not published until the year +1887, so that the various opinions which I shall +quote from the letters, and which show some considerable +approximation in his later years to the +views which have been put forward by Mr. Gulick +and myself, were not before us at the time when our +papers were read.</p> + +<p>The earliest allusion that I can find to geographical +isolation in the writings of Darwin occurs in a +correspondence with Sir Joseph Hooker, as far back +as 1844. He there says:—</p> + +<blockquote><p>I cannot give my reasons in detail; but the most general +conclusion which the geographical distribution of all organic<span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span> +beings appears to me to indicate is, that isolation is the chief +concomitant or cause of the appearance of <i>new</i> forms (I well +know there are some staring exceptions)<a name="FNanchor_27_27" id="FNanchor_27_27"></a><a href="#Footnote_27_27" class="fnanchor">[27]</a>.</p></blockquote> + +<p>And again:—</p> + +<blockquote><p>With respect to original creation or production of new forms, +I have said that isolation appears the chief element<a name="FNanchor_28_28" id="FNanchor_28_28"></a><a href="#Footnote_28_28" class="fnanchor">[28]</a>.</p></blockquote> + +<p>Next, in the earlier editions of the <i>Origin of Species</i> +this view is abandoned, and in its stead we meet +with the opinion that geographical isolation lends +a certain amount of assistance to natural selection, +by preventing free intercrossing. But here we must +note two things. First, the distinction between monotypic +and polytypic evolution is not defined. Secondly, +the levelling effect of free intercrossing in nature, and +hence its antagonism to divergence of character by +natural selection, is not sufficiently recognized; while, +on the other hand, and in consequence of this, the +importance of isolation as a factor of evolution is +underrated—not only in its geographical, but likewise +in all its other forms.</p> + +<p>Taking these two points separately, the only +passages in Darwin's writings, so far at least as I +can find, in which any distinction is drawn between +evolution as monotypic and polytypic, are those in +which he deals with a somewhat analogous distinction +between artificial selection as intentional and unconscious. +He says, for example:—</p> + +<blockquote><p>In the case of methodical selection, a breeder selects for some +definite object, and if the individuals be allowed freely to intercross, +his work will completely fail. But when many men, +without intending to alter the breed, have a nearly common<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span> +standard of perfection, and all try to procure and breed from the +best animals, improvement surely but slowly follows from this +unconscious process of selection, notwithstanding that there is no +separation of selected individuals. Thus it will be under nature<a name="FNanchor_29_29" id="FNanchor_29_29"></a><a href="#Footnote_29_29" class="fnanchor">[29]</a>.</p></blockquote> + +<p>Here we have what may perhaps be regarded as a +glimmering of the distinction between monotypic and +polytypic evolution. But that it is only a glimmering +is proved by the immediately ensuing sentences, which +apply this analogy of unconscious selection <i>not</i> to the +case of monotypic, <i>but</i> to that of polytypic evolution. +So likewise, in the succeeding discussion on "divergence +of character," the analogy is again resorted to for the +purpose of showing how polytypic evolution may occur +in nature.</p> + +<p>Thus far, then, it may be said that we have scarcely +so much as a glimmering of the distinction between +monotypic and polytypic evolution; and as the same +discussion (with but a few verbal alterations) runs +through all the editions of the <i>Origin</i>, it may well be +asked why I should have alluded to such passages in +the present connexion. Well, I have done so because +it is apparent that, during the last years of his life, the +distinction between selection as "methodical" and +"unconscious" enabled Darwin much more clearly to +perceive that between evolution as monotypic and +polytypic. Thus in 1868 he wrote to Moritz Wagner +(who, as we shall presently see, entirely failed to +distinguish between monotypic and polytypic evolution), +expressing his belief—</p> + +<blockquote><p>That in many large areas all the individuals of the same +species have been slowly modified, in the same manner, for +instance, as the English racehorse has been improved, that is,<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span> +by the continued selection of the fleetest individuals, without +any separation. But I admit that by this process two or +more new species could hardly be formed within the same limited +area<a name="FNanchor_30_30" id="FNanchor_30_30"></a><a href="#Footnote_30_30" class="fnanchor">[30]</a>.</p></blockquote> + +<p>Again, in 1876 he wrote another letter to Wagner, +in which the following passage occurs:—</p> + +<blockquote><p>I believe that all the individuals of a species can be slowly +modified within the same district, in nearly the same manner as +man effects by what I have called the process of unconscious +selection. I do not believe that one species will give birth to +two or more new species as long as they are mingled together +within the same district<a name="FNanchor_31_31" id="FNanchor_31_31"></a><a href="#Footnote_31_31" class="fnanchor">[31]</a>.</p></blockquote> + +<p>Two years later he wrote to Professor Semper:—</p> + +<blockquote><p>There are two different classes of cases, it appears to me, +viz. those in which species becomes slowly modified in the +same country, and those cases in which a species splits into two, +or three, or more new species; and, in the latter case, I should +think nearly perfect separation would greatly aid in their +"specification," to coin a new word<a name="FNanchor_32_32" id="FNanchor_32_32"></a><a href="#Footnote_32_32" class="fnanchor">[32]</a>.</p></blockquote> + +<p>Now, these passages show a very much clearer +perception of the all-important distinction between +monotypic and polytypic evolution than any which +occur in the <i>Origin of Species</i>; and they likewise +show that he was led to this perception through what +he supposed to be a somewhat analogous distinction +between "unconscious" and "methodical" selection +by man. The analogy, I need hardly say, is radically +unsound; and it is a curious result of its unsoundness +that, whereas in the <i>Origin of Species</i> it is adduced +to illustrate the process of polytypic evolution, as +previously remarked, in the letters above quoted we<span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span> +find it adduced to illustrate the process of monotypic +evolution. But the fact of this analogy being unsound +does not affect the validity of the distinction between +monotypic and polytypic evolution to which it led +Darwin, in his later years, so clearly to express<a name="FNanchor_33_33" id="FNanchor_33_33"></a><a href="#Footnote_33_33" class="fnanchor">[33]</a>.</p> + +<p>Turning next to the second point which we have to +notice, it is easy to show that in the earlier editions +of his works Darwin did not sufficiently recognize +the levelling effects of free intercrossing, and consequently +failed to perceive the importance of isolation +(in any of its forms) as a factor of organic evolution. +This may be most briefly shown by quoting his own +more matured opinion upon the subject. Thus, with +reference to the swamping effects of intercrossing, he +wrote to Mr. Wallace in 1867 as follows:—</p> + +<blockquote><p>I must have expressed myself atrociously: I meant to say +exactly the reverse of what you have understood. F. Jenkin +argued in the <i>North British Review</i> against single variations +being perpetuated, and has convinced me, though not in quite +so broad a manner as here put. I always thought individual +differences more important; but I was blind, and thought that +single variations might be preserved much oftener than I now +see is possible or probable. I mentioned this in my former<span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span> +note merely because I believed that you had come to a similar +conclusion, and I like much to be in accord with you. I believe +I was mainly deceived by single variations offering such simple +illustrations, as when man selects [i.e. isolates]<a name="FNanchor_34_34" id="FNanchor_34_34"></a><a href="#Footnote_34_34" class="fnanchor">[34]</a>.</p></blockquote> + +<p>Again, somewhere about the same time, he wrote +to Moritz Wagner:—</p> + +<blockquote><p>Although I saw the effects of isolation in the case of islands +and mountain-ranges, and knew of a few instances of rivers, +yet the greater number of your facts were quite unknown to me. +I now see that, from the want of knowledge, I did not make +nearly sufficient use of the views which you advocate<a name="FNanchor_35_35" id="FNanchor_35_35"></a><a href="#Footnote_35_35" class="fnanchor">[35]</a>.</p></blockquote> + +<p>Now it would be easy to show the justice of these +self-criticisms by quoting longer passages from earlier +editions of the <i>Origin of Species</i>; but as this, in view +of the above passages, is unnecessary, we may next +pass on to another point.</p> + +<p>The greatest oversight that Wagner made in his +otherwise valuable essays on geographical isolation, +was in not perceiving that geographical isolation is only +one among a number of other forms of isolation: +and, therefore, that although it is perfectly true, as +he insisted, that polytypic evolution cannot be effected +by natural selection alone, it is very far from true, +as he further insisted, that <i>geographical</i> isolation is +the only means whereby natural selection can be +assisted in this matter. Hence it is that, when +Darwin said he had not himself "made nearly +sufficient use" of geographical isolation as a factor +of specific divergence, he quite reasonably added that +he could not go so far as Wagner did in regarding +such isolation as a condition, <i>sine qua non</i>, to divergent +evolution in all cases. Nevertheless, he adds<span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span> +the important words, "I almost wish I could believe +in its importance to the same extent with you; for +you well show, in a manner which never occurred to +me, that it removes many difficulties and objections." +These words are important, because they show that +Darwin had come to feel the force of the "difficulties +and objections" with regard to divergent evolution +being possible by means of natural selection alone, +and how readily they could be removed by assuming +the assistance of isolation. Hence, it is much to be +deplored that Wagner presented a single kind of +isolation (geographical) as equivalent to the principle +of isolation in general. For he thus failed to present +the complete—and, therefore, the true—philosophy +of the subject to Darwin's mind; and in this, as +in certain other respects which I shall notice later +on, served rather to confuse than to elucidate the +matter as a whole.</p> + +<p>To sum up. Although in his later years, as shown +by his correspondence, Darwin came to recognize +more fully the swamping effects of free intercrossing, +and the consequent importance of "separation" for +the prevention of these effects, and although in this +connexion he likewise came more clearly to distinguish +between the "two cases" of monotypic +and polytypic evolution, it is evident that he never +worked out any of these matters—"thinking it prudent," +as he wrote with reference to them in 1878, +"now I am growing old, to work at easier subjects<a name="FNanchor_36_36" id="FNanchor_36_36"></a><a href="#Footnote_36_36" class="fnanchor">[36]</a>." +Therefore he never clearly saw, on the one hand, +that free intercrossing, far from constituting a "difficulty" +to <i>monotypic</i> evolution by natural selection,<span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span> +is the very means whereby natural selection is in +this case enabled to operate; or, on the other +hand, that, in the case of <i>polytypic</i> evolution, the +"difficulty" in question is so absolute as to render +such evolution, by natural selection alone, absolutely +impossible. Hence, although in one sentence of the +<i>Origin of Species</i> he mentions three forms of isolation +(besides the geographical form) as serving in some +cases to assist natural selection in causing "divergence +of character" (i. e. polytypic evolution<a name="FNanchor_37_37" id="FNanchor_37_37"></a><a href="#Footnote_37_37" class="fnanchor">[37]</a>), on +account of not perceiving how great and how sharp +is the distinction between the two kinds or "cases" +of evolution, he never realized that, where "two or +more new species" are in course of differentiation, +<i>some</i> form of isolation other than natural selection +must <i>necessarily</i> be present, whether or not natural +selection be likewise so. The nearest approach which +he ever made to perceiving this necessity was in one +of his letters to Wagner above quoted, where, after +again appealing to the erroneous analogy between +monotypic evolution and "unconscious selection," he +says:—"But I admit that by this process (i. e. unconscious +selection) two or more new species could +hardly be formed within the same limited area: some +degree of separation, if not indispensable, would be +highly advantageous; and here your facts and views +will be of great value." But even in this passage the +context shows that by "separation" he is thinking +exclusively of <i>geographical</i> separation, which he rightly +enough concludes (as against Wagner) need certainly<span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span> +not be "indispensable." Had he gone a step further, +he must have seen that separation, <i>in some form +or another, is</i> "indispensable" to polytypic evolution. +Instead of taking this further step, however, two years +later he wrote to Semper as follows:—</p> + +<blockquote><p>I went as far as I could, perhaps too far, in agreement with +Wagner [i. e. in the last edition of the <i>Origin of Species</i>]; since +that time I have seen no reason to change my mind; but then +I must add that my attention has been absorbed on other +subjects<a name="FNanchor_38_38" id="FNanchor_38_38"></a><a href="#Footnote_38_38" class="fnanchor">[38]</a>.</p></blockquote> + +<p>And he seems to have ended by still failing to +perceive that the explanation which he gives of +"divergence of character" in the <i>Origin of Species</i>, +can only hold on the unexpressed assumption that +free intercrossing is in some way prevented at the +commencement, and throughout the development, of +each diverging type.</p> + +<p>Lastly, we have to consider Darwin's opinion touching +the important principle of "Independent Variability." +This, it will be remembered, is the principle which +ensures that when a portion (not too large) of a +species is prevented from interbreeding with the rest +of the species, sooner or later a divergence of type +will result, owing to the fact that the average qualities +of the separated portion at the time of its separation +cannot have been exactly the same as the average +qualities of the specific type as a whole. Thus the +state of Amixia, being a state of what Mr. Gulick +calls Independent Generation, will of itself—i.e. even +if unassisted by natural selection—induce divergence +of type, in a ratio that has been mathematically +calculated by Delbœuf.</p> + +<p><span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span> +Darwin wrote thus to Professor Weismann in +1872:—</p> + +<blockquote><p>I have now read your essay with very great interest. Your +view of the origin of local races through "Amixia" is altogether +new to me, and seems to throw an important light on an obscure +question<a name="FNanchor_39_39" id="FNanchor_39_39"></a><a href="#Footnote_39_39" class="fnanchor">[39]</a>.</p></blockquote> + +<p>And in the last edition of the <i>Variation of Animals +and Plants</i> he adds the following paragraph:—</p> + +<blockquote><p>This view may throw some light on the fact that the domestic +animals which formerly inhabited the several districts in Great +Britain, and the half-wild cattle lately kept in several British +parks, differed slightly from one another; for these animals were +prevented from wandering over the whole country and intercrossing, +but would have crossed freely within each district or +park<a name="FNanchor_40_40" id="FNanchor_40_40"></a><a href="#Footnote_40_40" class="fnanchor">[40]</a>.</p></blockquote> + +<p>Now, although I allow that Darwin never attributed +to this principle of Amixia, or Independent +Variability, anything like the degree of importance +to which, in the opinion of Delbœuf, Gulick, Giard, +and myself, it is entitled, the above passage appears +to show that, as soon as the "view" was clearly +"suggested" to his mind, he was so far from being +unfavourably disposed towards it, that he added +a paragraph to the last edition of his <i>Variation</i> for +the express purpose of countenancing it. Nevertheless, +later on the matter appears to have entirely +escaped his memory; for in 1878 he wrote to Semper, +that he did "not see at all more clearly than I did +before, from the numerous cases which he [Wagner] +has brought forward, how and why it is that a long +isolated form should almost always become slightly +modified<a name="FNanchor_41_41" id="FNanchor_41_41"></a><a href="#Footnote_41_41" class="fnanchor">[41]</a>." I think this shows entire forgetfulness<span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span> +of the principle in question, because, if the latter is +good for explaining the <i>initial</i> divergence of type as +between separated stocks of "domesticated animals," +much more must it be competent to explain the +<i>further</i> divergence of type which is "almost always" +observable in the case of "a long isolated form" +under nature. The very essence of the principle +being that, when divergence of type has once begun, +this divergence must <i>ipso facto</i> proceed at an ever-accelerating +pace, it is manifestly inconsistent to entertain +the principle as explaining the first commencement of +divergence, and then to ignore it as explaining the +further progress of divergence. Hence, I can only +conclude that Darwin had forgotten this principle +altogether when he wrote his letter to Semper in 1878—owing, +no doubt, as he says in the sentence which +immediately follows, to his having "not attended +much of late years to such questions."</p> + +<hr class="tb" /> + +<p>So much, then, for Darwin's opinions. Next in +order of time we must consider Moritz Wagner's +essays on what he called the "Law of Migration<a name="FNanchor_42_42" id="FNanchor_42_42"></a><a href="#Footnote_42_42" class="fnanchor">[42]</a>." +The merit of these essays was, first, the firm expression +of opinion upon the swamping effects of free +intercrossing; and, second, the production of a large +body of facts showing the importance of geographical +isolation in the prevention of these effects, and in +the consequent differentiation of specific types. On +the other hand, the defect of these essays was, first, +not distinguishing between evolution as monotypic +and polytypic; and, second, not perceiving that geographical<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span> +isolation is only one among a number of +other forms of isolation. From these two radical +oversights—which, however, were shared by all other +writers of the time, with the partial exception of +Darwin himself, as previously shown—there arose the +following and most lamentable errors.</p> + +<p>Over and over again Moritz Wagner insists, as constituting +the fundamental doctrine of his attempted +reform of Darwinism, that evolution by natural +selection is impossible, unless natural selection be +assisted by geographical isolation, in order to prevent +the swamping effects of intercrossing<a name="FNanchor_43_43" id="FNanchor_43_43"></a><a href="#Footnote_43_43" class="fnanchor">[43]</a>. Now, if instead +of "evolution" he had said "divergence of type," +and if instead of "geographical isolation" he had +said "prevention of intercrossing," he would have +enunciated the general doctrine which it has been the +joint endeavour of Mr. Gulick and myself to set forth. +But by not perceiving that "evolution" is of two +radically different kinds—polytypic and monotypic—he +entirely failed to perceive that, while for one of its +kinds the <i>prevention</i> of intercrossing is an absolute +necessity, for the other of its kinds the <i>permission</i> of +intercrossing is a necessity no less absolute. And, +again, in missing the fact that geographical isolation<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span> +is but one of the many ways whereby intercrossing +may be prevented, he failed to perceive that, even +as regards the case of polytypic evolution, he greatly +erred in representing this one form of isolation as +being universally a necessary condition to the process. +The necessary condition to this process is, indeed, the +prevention of intercrossing <i>by some means or another</i>; +but his unfortunate insistence on geographical separation +as the only possible means to this end—especially +when coupled with his no less unfortunate disregard +of monotypic evolution—caused him to hinder rather +than to advance a generalization which he had only +grasped in part. And this generalization is, as now +so repeatedly stated, that while the form of isolation +which we know as natural selection depends for its +action upon the intercrossing of all the individuals +which it isolates (i. e. selects), when acting alone +it can produce only monotypic evolution; but that +when it is supplemented by any of the other +numerous forms of isolation, it is furnished with +the necessary condition to producing polytypic +evolution—and this in as many lines of divergent +change as there may be cases of this efficient +separation.</p> + +<p>Nevertheless, while we must lament these shortcomings +on the part of Wagner, we ought to remember +that he rendered important services in the +way of calling attention to the swamping effects of +free intercrossing, and, still more, in that of showing +the high importance of geographical isolation as a +factor of organic evolution. Therefore, although in an +elaborate criticism of his views Weismann was easily +able to dispose of his generalizations in the imperfect<span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span> +form that they presented, I do not think it was just in +Weismann to remark, "if Wagner had confined himself +to the statement that geographical isolation materially +assists the process of natural selection, and +thus also promotes the origination of new species, he +would have met with little or no opposition; but then, +of course, in saying this much, he would not have +been saying anything new." No doubt, as I have +just shown, he <i>ought</i> thus (as well as in other and +still more important respects not perceived by Prof. +Weismann) to have limited his statement; but, had +he done so, it does not follow that he would not have +been saying anything new. For, in point of fact, in +as far as he said what was true, he did say a great +deal that was also new. Thus, most of what he said +of the <i>principle of separation</i> (apogamy) was as new +as it was true, although, as we have seen, he said it +to very little purpose on account of his identifying +this principle as a whole with that of but one of its +forms. Again, notwithstanding this great error, or +oversight, he certainly showed of the particular form +in question—viz. geographical isolation—that it was +of considerably <i>more</i> importance than had previously +been acknowledged. And this was so far a valuable +contribution to the general theory of descent.</p> + +<hr class="tb" /> + +<p>Prof. Weismann's essay, to which allusion has just +been made<a name="FNanchor_44_44" id="FNanchor_44_44"></a><a href="#Footnote_44_44" class="fnanchor">[44]</a>, was, however, in all respects a great +advance upon those of Wagner. It was not only +more comprehensive in its view of the whole subject +of geographical isolation, but likewise much more +adequate in its general treatment thereof. Its principal<span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span> +defects, in my judgement, were, first, the inordinately +speculative character of some of its parts, +and, second, the restriction of its analysis to but one +form of isolation—a defect which it shares with the +essays of Wagner, and in quite as high a degree. +Furthermore, although this essay had the great merit +of enunciating the principle of Amixia, it did so in +a very inefficient manner. For not only was this +principle adduced with exclusive reference to <i>geographical</i> +isolation, but even in regard to this one +kind of isolation it was presented in a highly inconsistent +manner, as I will now endeavour to show.</p> + +<p>Weismann was led to perceive the principle in +question by the consideration that new specific characters, +when they first appear, do not all appear +together in the same individuals: they appear one +in one individual, another in another, a third in a +third, &c.; and it is only in the course of successive +generations that they all become blended in +the same individuals by free intercrossing. Hence, the +eventually emerging constant or specific type is the +resultant of all the transitory or varietal types, when +these have been fused together by intercrossing. +From which Weismann deduces what he considers +a general law—namely, that "the constancy of a +specific type does not arise suddenly, but gradually; +and it is established by the promiscuous crossing +of all individuals<a name="FNanchor_45_45" id="FNanchor_45_45"></a><a href="#Footnote_45_45" class="fnanchor">[45]</a>." From which again it follows, +that this constancy must cease so soon as the condition +which maintains it ceases—i. e. so soon as free intercrossing +is prevented by the geographical isolation +of a portion of the species from its parent stock.</p> +<p><span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span></p> +<p>Now, to begin with, this statement of the principle +in question is not a good statement of it. There was +no need while stating the doctrine that separation +induces differentiation, to found the doctrine on any +such highly speculative basis. In point of fact, there +is no real evidence that specific types do attain +their constancy in the way supposed; nor, for the +purposes of the doctrine in question, is it necessary +that there should be. For this doctrine does not +need to show how the constancy has been <i>attained</i>; +it only has to show that the constancy is <i>maintained</i> +by free intercrossing, with the result that when free +intercrossing is <i>by any means</i> prevented, divergence +of character ensues. In short, the correct way of +stating the principle is that which has been adopted +by Delbœuf and Gulick—namely, the average characters +of a separated portion of a species are not +likely to be the same as those of the whole species; +with the result that divergence of type will be set +up in the separated portion by intercrossing within +that portion. Or the principle may be presented +as I presented it under the designation of "Independent +Variability"—namely, "a specific type may +be regarded as the average mean of all individual +variations, any considerable departure from this +average mean being, however, checked by intercrossing," +with the result that when intercrossing +is prevented between a portion of a species and +the rest of the species, "this population is permitted +to develop an independent history of its own, shielded +from intercrossing with its parent form<a name="FNanchor_46_46" id="FNanchor_46_46"></a><a href="#Footnote_46_46" class="fnanchor">[46]</a>."</p> + +<p>Not only, however, is Weismann's principle of<span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span> +"Amixia" thus very differently stated from that +of my "Independent Variability" (apogamy), or +Gulick's "Independent Generation"; but, apparently +owing to this difference of statement, the principle +itself is not the same. In particular, while Weismann +holds with us that when new characters arise in +virtue of the mere prevention of intercrossing with +parent forms these new characters will be of non-utilitarian +kind<a name="FNanchor_47_47" id="FNanchor_47_47"></a><a href="#Footnote_47_47" class="fnanchor">[47]</a>, he appears to think that divergence +of character under such circumstances is not likely to +go on to a <i>specific</i> value. Now, it is of importance +to observe why he arrives at this conclusion, which is +not only so different from that of Delbœuf, Gulick, +and myself, but apparently so inconsistent with his +own recognition of the diversifying effect of "Amixia" +as regards the formation of <i>permanent varieties</i>. For, +as we have already seen while considering Darwin's +views on this same principle of "Amixia," it is highly +inconsistent to recognize its diversifying effect up to +the stage of constituting fixed varieties, and then not +to recognize that, so much divergence of character +having been already secured by the isolation alone, +much more must further divergence continue, and +continue at an ever accelerating pace—as Delbœuf +and Gulick have so well shown. What, then, is the +explanation of this apparent inconsistency on Weismann's +part? The explanation evidently is that, +owing to his erroneous statement of the principle, he +misses the real essence of it. For, in the first place, +he does not perceive that this essence consists in an +initial difference of average characters on the part of +the isolated colony as compared with the rest of their<span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span> +species. On the contrary, he loses himself in a maze +of speculation about all species having had what he +calls "variation-periods," or eruptions of general variability +alternating with periods of repose—both being +as unaccountable in respect of their causation as they +are hypothetical in respect of their occurrence. From +these speculations he concludes, that isolation of a +portion of a species will then only lead to divergence +of character when the isolation happens to coincide +with a "variation-period" on the part of the species +as a whole, and that the divergence will cease so +soon as the "variation-period" ceases. Again, in the +second place as previously remarked, equally with +Wagner whom he is criticizing, he fails to perceive +that <i>geographical</i> isolation is not the only kind of +isolation, or the only possible means to the prevention +of free intercrossing. And the result of this oversight +is, that he thinks amixia can act but comparatively +seldom upon sufficiently small populations to become +a factor of much importance in the differentiation of +species. Lastly, in the third place, owing to his +favourite hypothesis that all species pass through +a "variation-period," he eventually concludes that the +total amount of divergence of type producible by +isolation alone (even in a small population) can never +be greater than that between the extremes of variation +which occur within the whole species at the date +of its partition (p. 75). In other words, the possibility +of change due to amixia alone is taken to be limited +by the range of deviation from the general specific +average, as manifested by different individual variations, +before the species was divided. Thus the +doctrine of amixia fails to recognize the law of<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span> +Delbœuf, or the <i>cumulative</i> nature of divergence of +type when once such divergence begins in a separated +section. Therefore, in this all-important—and, indeed, +essential—respect, amixia differs entirely from the +principle which has been severally stated by Delbœuf, +Gulick, and myself.</p> + +<p>Upon the whole, then, we must say that although +Professor Weismann was the first to recognize the +diversifying influence of merely indiscriminate isolation +<i>per se</i> (apogamy), he did so only in part. He failed +to distinguish the true essence of the principle, and by +overlaying it with a mass of hypothetical speculation, +concealed even more of it than he revealed.</p> + +<hr class="tb" /> + +<p>The general theory of Isolation, as independently +worked out by Mr. Gulick and myself, has already +been so fully explained, that it will here be sufficient +merely to enumerate its more distinguishing features. +These are, first, drawing the sharpest possible line +between evolution as monotypic and polytypic; +second, showing that while for the former the peculiar +kind of isolation which is presented by natural +selection suffices of itself to <i>transform</i> a specific type, +in order to work for the latter, or to <i>branch</i> a specific +type, natural selection must necessarily be assisted by +some other kind of isolation; third, that even in the +absence of natural selection, other kinds of isolation +may be sufficient to effect specific divergence through +independent generation alone; fourth, that, nevertheless, +natural selection, where present, will always +accelerate the process of divergence; fifth, that +monotypic evolution by natural selection depends +upon the <i>presence</i> of intercrossing, quite as much as<span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span> +polytypic evolution (whether with or without natural +selection) depends upon the <i>absence</i> of it; sixth, that, +having regard to the process of evolution throughout +all taxonomic divisions of organic nature, we must +deem the physiological form of isolation as the most +important, with the exception only of natural +selection.</p> + +<p>The only difference between Mr. Gulick's essays +and my own is, that, on the one hand, he has +analyzed much more fully than I have the various +forms of isolation; while, on the other hand, I have +considered much more fully than he has the particular +form of physiological isolation which so frequently +obtains between allied <i>species</i>. This particular form +of physiological isolation I have called "physiological +selection," and claim for it so large a share in the +differentiation of specific types as to find in it a +satisfactory explanation of the contrast between +natural species and artificial varieties in respect of +cross-infertility.</p> + +<hr class="tb" /> + +<p>Mr. Wallace, in his <i>Darwinism</i>, has done good +service by enabling all other naturalists clearly to +perceive how natural selection alone produces monotypic +evolution—namely, through the free intercrossing +of all individuals which have not been eliminated by +the isolating process of natural selection itself. For +he very lucidly shows how the law of averages must +always ensure that in respect of any given specific +character, half the individuals living at the same time +and place will present the character above, and half +below its mean in the population as a whole. Consequently, +if it should ever be of advantage to a species<span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span> +that this character should undergo either increase or +decrease of its average size, form, colour, &c., there +will always be, in each succeeding generation, a sufficient +number of individuals—i. e. half of the whole—which +present variations in the required direction, +and which will therefore furnish natural selection +with abundant material for its action, without the +need of any other form of isolation. It is to be +regretted, however, that while thus so clearly presenting +the fact that free intercrossing is the very +means whereby natural selection is enabled to effect +monotypic evolution, he fails to perceive that such +intercrossing must always and necessarily render it +impossible for natural selection to effect polytypic +evolution. A little thought might have shown him +that the very proof which he gives of the necessity +of intercrossing where the <i>transmutation</i> of species +is concerned, furnishes, measure for measure, as good +a proof of the necessity of its absence where the <i>multiplication</i> +of species is concerned. In justice to him, +however, it may be added, that this distinction between +evolution as monotypic and polytypic (with +the important consequence just mentioned) still continues +to be ignored also by other well-known evolutionists +of the "ultra-Darwinian" school. Professor +Meldola, for example, has more recently said that in +his opinion the "difficulty from intercrossing" has been +in large part—if not altogether—removed by Mr. +Wallace's proof that natural selection alone is capable +of effecting [monotypic] evolution; while he regards +the distinction between monotypic and polytypic +evolution as mere "verbiage<a name="FNanchor_48_48" id="FNanchor_48_48"></a><a href="#Footnote_48_48" class="fnanchor">[48]</a>."</p> +<p><span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span></p> +<p>It is in relation to my presentment of the impossibility +of natural selection alone causing polytypic +evolution, that Mr. Wallace has been at the +pains to show how the permission of intercrossing +(panmixia) is necessary for natural selection in its +work of causing monotypic evolution. And not only +has he thus failed to perceive that the "difficulty" +which intercrossing raises against the view of natural +selection being of itself capable of causing polytypic +evolution in no way applies to the case of monotypic; +but as regards this "difficulty," where it does apply, +he says:—</p> + +<blockquote><p>Professor G. J. Romanes has adduced it as one of the +difficulties which can alone be overcome by his theory of physiological +selection<a name="FNanchor_49_49" id="FNanchor_49_49"></a><a href="#Footnote_49_49" class="fnanchor">[49]</a>.</p></blockquote> + +<p>This, however, is a misapprehension. I have by +no means represented that the difficulty in question +can alone be overcome by this theory. What I have +represented is, that it can be overcome by any of the +numerous forms of isolation which I named, and +of which physiological selection is but one. And +although, <i>where common areas are concerned</i>, I believe +that the physiological form of isolation is the most +important form, this is a very different thing from +entertaining the supposition which Mr. Wallace here +assigns to me.</p> + +<hr class="tb" /> + +<p>I may take this opportunity of correcting a somewhat +similar misunderstanding which has been more +recently published by Professor W. A. Herdman, of +Liverpool; and as the case which he gives is one of<span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span> +considerable interest in itself, I will quote his remarks +in extenso. In his <i>Opening Address to the Liverpool +Biological Society</i>, Professor Herdman said:—</p> + +<blockquote><p>Some of you will doubtless remember that in last year's +address, while discussing Dr. Romanes' theory of physiological +selection, I quoted Professor Flemming Jenkin's imaginary case +of a white man wrecked upon an island inhabited by negroes, +given as an illustration of the supposed swamping effect by +free intercrossing of a marked variety with the parent species. +I then went on to say in criticism of the result at which Jenkin +arrived, viz. that the characteristics of the white man would be +stamped out by intercrossing with the black:—</p> + +<p>"Two influences have, I think, been ignored, viz. atavism, +or reversion to ancestral characters, and the tendency of the +members of a variety to breed with one another. Keeping to +the case described above, I should imagine that the numbers of +intelligent young mulattoes produced in the second, third, fourth, +and few succeeding generations would to a large extent intermarry, +the result of which would be that a more or less white +aristocracy would be formed on the island, including the king +and all the chief people, the most intelligent men and the bravest +warriors. Then atavism might produce every now and then +a much whiter individual—a reversal to the characteristics of +the ancestral European—who, by being highly thought of in +the whitish aristocracy, would have considerable influence +on the colour and other characteristics of the next generation. +Now such a white aristocracy would be in precisely the same +circumstances as a favourable variety competing with its parent +species," &c.</p> + +<p>You may imagine then my pleasure when, a few months after +writing the above, I accidentally found, in a letter<a name="FNanchor_50_50" id="FNanchor_50_50"></a><a href="#Footnote_50_50" class="fnanchor">[50]</a> written by the +celebrated African traveller Dr. David Livingstone to Lord +Granville, and dated "Unyanyembe, July 1st, 1872," the following +passage:—</p> + +<p>"About five generations ago, a white man came to the highlands +of Basañgo, which are in a line east of the watershed.<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span> +He had six attendants, who all died, and eventually their headman, +called Charura, was elected chief by the Basañgo. In +the third generation he had sixty able-bodied spearmen as lineal +descendants. This implies an equal number of the other sex. +They are very light in colour, and easily known, as no one is +allowed to wear coral beads such as Charura brought except the +royal family. A book he brought was lost only lately. The +interest of the case lies in its connexion with Mr. Darwin's +celebrated theory on the 'origin of species,' for it shows that an +improved variety, as we whites modestly call ourselves, is not so +liable to be swamped by numbers as some have thought."</p> + +<p>Here we have a perfect fulfilment of what I last year, in +ignorance of this observation of Livingstone's, predicted as being +likely to occur in such a case. We have the whitish aristocracy +in a dominant condition, and evidently in a fair way to spread +their characteristics over a larger area and give rise to a marked +variety, and it had clearly struck Livingstone fourteen years +before the theory of physiological selection had been heard of, +just as it must strike us now, as an instance telling strongly +against the "swamping" argument as used by Flemming Jenkin +and Romanes.</p></blockquote> + +<p>Here we have a curious example of one writer +supporting the statements of another, while appearing +to be under the impression that he is controverting +those statements. Both Professor Herdman's +imaginary case, and its realization in Livingstone's +account, go to show "the tendency of the members +of a variety to breed with one another." This is +what I have called "psychological selection," and, +far from "ignoring" it, I have always laid stress +upon it as an obviously important form of isolation +or <i>prevention</i> of free intercrossing. But it is a form +of isolation which can only occur in the higher animals, +and, therefore, the whole of Professor Herdman's +criticism is merely a restatement of my own views +as already published in the paper which he is<span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span> +criticizing. For all that his argument goes to prove +is, first, the necessity for <i>some</i> form of isolation if +the overwhelming effects of intercrossing are to be +obviated; and, secondly, the manifest consequence +that where the psychological form is unavailable (as +in many of the lower animals and in all plants), +some other form must be present if divergent evolution +is taking place on a common area.</p> + +<hr class="tb" /> + +<p>Seeing that so much misunderstanding has been +shown with reference to my views on "the swamping +effects of intercrossing," and seeing also that +this misunderstanding extends quite as much to Mr. +Gulick's views as to my own, I will here supply +brief extracts from both our original papers, for the +double purpose of showing our complete agreement, +and of leaving it to be judged whether we can +fairly be held responsible for the misunderstanding +in question. After having supplied these quotations, +I will conclude this historical sketch by considering +what Mr. Wallace has said in reply to the views +therein presented. I will transcribe but a single +passage from our papers, beginning with my own.</p> + +<blockquote><p>Any theory of the origin of species in the way of descent must +be prepared with an answer to the question, Why have species +<i>multiplied</i>? How is it that, in the course of evolution, species +have not simply become transmuted in linear series instead of +ramifying into branches? This question Mr. Darwin seeks to +answer "from the simple circumstance that the more diversified +the descendants from any one species becomes in structure, +constitution, and habits, by so much will they be better enabled +to seize on many and widely diversified places in the economy +of nature, and so be enabled to increase in numbers." And he +proceeds to illustrate this principle by means of a diagram,<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span> +showing the hypothetical divergence of character undergone by +the descendants of seven species. Thus, he attributes divergence +of character exclusively to the influence of natural selection.</p> + +<p>Now, this argument appears to me unassailable in all save +one particular; but this is a most important particular: the +argument wholly ignores the fact of intercrossing with parent +forms. Granting to the argument that intercrossing with parent +forms is prohibited, and nothing can be more satisfactory. The +argument, however, sets out with showing that it is in limited +areas, or in areas already overstocked with the specific form in +question, that the advantages to be derived from diversification +will be most pronounced. It is where they "jostle each other +most closely" that natural selection will set a premium upon +any members of the species which may depart from the common +type. Now, inasmuch as this jostling or overcrowding of +individuals is a needful condition to the agency of natural +selection in the way of diversifying character, must we not feel +that the general difficulty from intercrossing previously considered +is here presented in a special and aggravated form? +At all events, I know that, after having duly and impartially +considered the matter, to me it does appear that unless the +swamping effects of intercrossing with the parent form on an +overcrowded area is in some way prevented to begin with, +natural selection could never have any material supplied by +which to go on with. Let it be observed that I regard Mr. +Darwin's argument as perfectly sound where it treats of the +divergence of <i>species</i>, and of their further divergence into <i>genera</i>; +for in these cases the physiological barrier is known to be +already present. But in applying the argument to explain +the divergence of individuals into varieties, it seems to me that +here, more than anywhere else, Mr. Darwin has strangely lost +sight of the formidable difficulty in question; for in this +particular case so formidable does the difficulty seem to me, +that I cannot believe that natural selection alone could produce +any divergence of specific character, so long as all the individuals +on an overcrowded area occupy that area together. +Yet, if any of them quit that area, and so escape from the +unifying influence of free intercrossing, these individuals also +escape from the conditions which Mr. Darwin names as those<span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span> +that are needed by natural selection in order to produce divergence. +Therefore, it appears to me that, under the circumstances +supposed, natural selection alone could not produce +divergence; the most it could do would be to change the whole +specific type in some one direction, and thus induce transmutation +of species in a linear series, each succeeding member +of which might supplant its parent form. But in order to +secure <i>diversity</i>, <i>multiplication</i>, or <i>ramification</i> of species, +it appears to me obvious that the primary condition required is +that of preventing intercrossing with parent forms at the origin +of each branch, whether the prevention be from the first +absolute, or only partial.</p></blockquote> + +<p>Now for Mr. Gulick, a portion of whose more +lengthy discussion of the subject, however, is all that +I need quote:—</p> + +<blockquote><p>Having found that the evolution of the fitted is secured through +the prevention of crossing between the better fitted and the less +fitted, can we believe that the evolution of a special race, +regularly transmitting a special kind of fitness, can be realized +without any prevention of crossing with other races that have +no power to transmit that special kind of fitness? Can we +suppose that any advantage, derived from new powers that +prevent severe competition with kindred, can be permanently +transmitted through succeeding generations to one small section +of the species while there is free crossing equally distributed +between all the families of the species? Is it not apparent that +the terms of this supposition are inconsistent with the fundamental +laws of heredity? Does not inheritance follow the lines +of consanguinity; and when consanguinity is widely diffused, +can inheritance be closely limited? When there is free crossing +between the families of one species, will not any peculiarity +that appears in one family either be neutralized by crosses +with families possessing the opposite quality, or, being preserved +by natural selection, while the opposite quality is gradually +excluded, will not the new quality gradually extend to all the +branches of the species; so that, in this way or in that, increasing +divergence of form will be prevented?</p> + +<p><span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span> +If the advantage of freedom from competition in any given +variation depends on the possession, in some degree, of new +adaptations to unappropriated resources, there must be some +cause that favours the breeding together of those thus specially +endowed, and interferes in some degree with their crossing +with other variations, or, failing this, the special advantage will +in succeeding generations be lost. As some degree of Independent +Generation is necessary for the continuance of the +advantage, it is evident that the same condition is necessary +for the accumulation through Natural Selection of the powers +on which the advantage depends. The advantage of divergence +of character cannot be retained by those that fail to retain the +divergent character; and divergent character cannot be retained +by those that are constantly crossing with other kinds; and the +prevention of free crossing between those that are equally +successful is in no way secured by Natural Selection.</p></blockquote> + +<p>So much, then, as expressive of Mr. Gulick's +opinion upon this subject. To exactly the same +effect Professor Lloyd Morgan has recently published +his judgement upon it thus:—</p> + +<blockquote><p>That perfectly free intercrossing, between any or all of the +individuals of a given group of animals, is, so long as the +characters of the parents are blended in the offspring, fatal to +divergence of character, is undeniable. Through the elimination +of less favourable variations, the swiftness, strength, and +cunning of a race may be gradually improved. But no form of +elimination can possibly differentiate the group into swift, +strong, and cunning varieties, distinct from each other, so long +as all three varieties freely interbreed, and the characters of +the parents blend in the offspring. Elimination may and does +give rise to progress in any given group, <i>as a group</i>; it does +not and cannot give rise to differentiation and divergence, so +long as interbreeding with consequent interblending of characters +be freely permitted. Whence it inevitably follows, as a matter +of simple logic, that where divergence has occurred, intercrossing +and interbreeding must in some way have been +lessened or prevented. Thus a new factor is introduced, that<span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span> +of <i>isolation</i> or <i>segregation</i>. And there is no questioning the +fact that it is of great importance. Its importance, indeed, can +only be denied by denying the swamping effects of intercrossing, +and such denial implies the tacit assumption that interbreeding +and interblending are held in check by some form of segregation. +The isolation explicitly denied is implicitly assumed<a name="FNanchor_51_51" id="FNanchor_51_51"></a><a href="#Footnote_51_51" class="fnanchor">[51]</a>.</p></blockquote> + +<p>Similarly, and still more recently, Professor +Le Conte writes:—</p> + +<blockquote><p>It is evident, then, as Romanes claims, that natural selection +alone tends to <i>monotypic</i> evolution. Isolation of some sort +seems necessary to <i>polytypic</i> evolution. The tree of evolution +under the influence of natural selection alone grows palm-like +from its terminal bud. Isolation was necessary to the starting +of lateral buds, and thus for the profuse ramification which is its +most conspicuous character<a name="FNanchor_52_52" id="FNanchor_52_52"></a><a href="#Footnote_52_52" class="fnanchor">[52]</a>.</p></blockquote> + +<p>In order to complete this historical review, it only +remains to consider Mr. Wallace's utterances upon the +subject.</p> + +<p>It is needless to say that he stoutly resists the +view of Weismann, Delbœuf, Gulick, and myself, that +specific divergence can ever be due—or, as I understand +him, even so much as assisted—by this principle +of indiscriminate isolation (apogamy). It will be +remembered, however, that Mr. Gulick has adduced +certain general principles and certain special facts +of geographical distribution, in order to prove that +apogamy eventually leads to divergence of character, +provided that the isolated section of the species does +not contain any very large number of individuals. +Now, Mr. Wallace, without making any reference to +this argument of Mr. Gulick, simply states the reverse—namely, +that, as a matter of fact, indiscriminate<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span> +isolation is not found to be associated with divergence +of character. For, he says, "there is an entire +absence of change, where, if this were a <i>vera causa</i>, +we should expect to find it<a name="FNanchor_53_53" id="FNanchor_53_53"></a><a href="#Footnote_53_53" class="fnanchor">[53]</a>." But the only case +which he gives is that of Ireland.</p> + +<p>This, he says, furnishes "an excellent test case, for +we know that it [Ireland] has been separated from +Britain since the end of the glacial epoch: ... yet +hardly one of its mammals, reptiles, or land molluscs +has undergone the slightest change<a name="FNanchor_54_54" id="FNanchor_54_54"></a><a href="#Footnote_54_54" class="fnanchor">[54]</a>." Here, however, +Mr. Wallace shows that he has failed to understand +"the views of those who, like Mr. Gulick, +believe isolation itself to be a cause of modification +of species"; for it belongs to the very essence of these +views that the efficiency of indiscriminate isolation as +a "<i>vera causa</i>" of organic evolution varies inversely +with the number of individuals (i. e. the size of the +species-section) exposed to its influence. Therefore, +far from being "an excellent test case," the case +of Ireland is unsatisfactory. If we are in search of +excellent test cases, in the sense intended by Mr. +Wallace, we ought not to choose a large island, +which from the time of its isolation must have contained +large bulks of each of the geographically +separated species concerned: we ought to choose +cases where as small a number as possible of the +representatives of each species were in the first +instance concerned. And, when we do this, the +answer yielded by any really "excellent test case" is +unequivocal.</p> + +<p>No better test case of this kind has ever been +furnished than that of Mr. Gulick's land-shells,<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span> +which Mr. Wallace is specially considering in the +part of his book where the sentence above quoted +occurs. How, then, does he meet this case? He +meets it by assuming that in all the numerous +adjacent valleys of a small island there must be +as many differences of environment, each of which +is competent to induce slight varietal changes on +the part of its occupants by way of natural selection, +although in no one case can the utility of these +slight changes be surmised. Now, against this explanation +there are three overwhelming considerations. +In the first place, it is purely gratuitous, or offered +merely in order to save the hypothesis that there +<i>can</i> be no other cause of even the most trivial change +in species than that which is furnished by natural +selection. In the second place, as Mr. Gulick writes +to me in a private letter, "if the divergence of +Sandwich Island land molluscs is wholly due to +exposure to different environments, as Mr. Wallace +argues on pages 147-150, then there must be completely +occult influences in the environment that +vary progressively with each successive mile. This +is so violent an assumption that it throws doubt +on any theory that requires such support." In the +third place, the assumption that the changes in +question must have been due to natural selection, +is wholly incompatible with the facts of isolation +elsewhere—namely, in those cases where (as in that +of Ireland) a large section of species, instead of +a small section, has been indiscriminately isolated. +Mr. Wallace, as we have seen, inadvertently alludes +to these "many other cases of isolation" as evidence +against apogamy being <i>per se</i> a cause of specific<span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span> +change. But although, for the reason above stated, +they are without relevancy in this respect, they +appear to me fatal to the explanation which he gives +of specific changes under apogamy where only small +sections of species are concerned. For example, can +it be rationally maintained that there are more +differences of environment between every two of +the many contiguous valleys of a small island, +such as Mr. Gulick describes, than there are in +the incomparably larger area of the whole of +Ireland? But, if not, and if natural selection is +able to work such "occult" wonders in each successive +mile on the Sandwich Islands, why has it so +entirely lost this magic power in the case of Ireland—or +in the "many other cases of isolation" to +which Mr. Wallace refers? On his theory there +is no coherent answer to be given to this question, +while on our theory the answer is given in the +very terms of the theory itself. The facts are +plainly just what the theory requires that they +should be; and therefore, if they were not as they +are, the theory would be deprived of that confirmation +which it now derives from them.</p> + +<p>Thus, in truth, though in an opposite way, the +case of Ireland is, as Mr. Wallace says, "an excellent +test case," when once the theory of apogamy +as a "<i>vera causa</i>" of specific change is understood; +and the effect of applying the test is fully to corroborate +this theory, while at the same time it as +fully negatives the other. For the consideration +whereby Mr. Wallace seeks to explain the inactivity +of natural selection in the case of Ireland is not +"coherent." What he says is, "That changes have<span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span> +not occurred through natural selection, is perhaps +due to the less severe struggle for existence, owing +to the smaller number of competing species<a name="FNanchor_55_55" id="FNanchor_55_55"></a><a href="#Footnote_55_55" class="fnanchor">[55]</a>." But +even with regard to molluscs alone, there is a greatly +larger number of species in Ireland than occurs in +any one valley of the Sandwich Islands; while if we +have regard to all the other classes of animal life, +comparison entirely fails.</p> + +<p>Much more to the point are certain cases which +were adduced long ago by Weismann in his essay +previously considered. Nevertheless, although this +essay was published as far back as 1872, and, +although it expressly deals with the question of +divergence of character through the mere prevention of +intercrossing (Amixia), Mr. Wallace nowhere alludes +to these cases <i>per contra</i>, which are so much more +weighty than his own "test case" of Ireland. Of +such are four species of butterflies, belonging to three +genera<a name="FNanchor_56_56" id="FNanchor_56_56"></a><a href="#Footnote_56_56" class="fnanchor">[56]</a>, which are identical in the polar regions and +in the Alps, notwithstanding that the sparse Alpine +populations have been presumably separated from +their parent stocks since the glacial period; or of +certain species of fresh water crustaceans (<i>Apus</i>), the +representatives of which are compelled habitually to +form small isolated colonies in widely separated +ponds, and nevertheless exhibit no divergence of +character, although apogamy has probably lasted for +centuries. These cases are unquestionably of a very +cogent nature, and appear of themselves to prove +that apogamy alone is not invariably capable of<span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span> +inducing divergence—at any rate, so rapidly as we +might expect. There appears, however, to be +another factor, the presence or absence of which +makes a great difference. This as stated in the text, +is the degree in which a specific type is stable or +unstable—liable or not liable to vary. Thus, for +example, the Goose is what Darwin calls an "inflexible" +type as compared with most other domesticated +birds. Therefore, if a lot of geese were to be indiscriminately +isolated from the rest of their species, the +probability is that in a given time their descendants +would not have diverged from the parent type to such +an extent as would a similar lot of ducks under +similar circumstances: the more stable specific type +would require a longer time to change under the +influence of apogamy alone. Now, the butterflies +and crustaceans quoted by Weismann may be of a +highly stable type, presenting but a small range +of individual variability; and, if so, they would +naturally require a long time to exhibit any change +of type under the influence of apogamy alone. But, +be this as it may, Weismann himself adduces these +cases merely for the sake of showing that there are +cases which seem to tell against the general principle +of modification as due to apogamy alone—i.e. +the general principle which, under the name amixia, +he is engaged in defending. And the conclusion +at which he himself arrives is, that while it would +be wrong to affirm that apogamy <i>must</i> in all +cases produce divergence, we are amply justified +in affirming that in many cases it <i>may</i> have done +so; while there is good evidence to prove that in +not a few cases it <i>has</i> done so, and therefore<span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span> +should be accepted as one of the factors of organic +evolution<a name="FNanchor_57_57" id="FNanchor_57_57"></a><a href="#Footnote_57_57" class="fnanchor">[57]</a>.</p> + +<p>My view from the very first has been that variations +in the way of cross-infertility are of frequent occurrence +(how, indeed, can they be otherwise, looking +to the complex conditions that have to be satisfied +in every case of full fertility?); and, therefore, +however many of such variations are destined to die +out, whenever one arises, "under suitable conditions," +"it must inevitably tend to be preserved as a new +natural variety, or incipient species." Among the +higher animals—which are "comparatively few in +number"—I think it probable that some slight change +of form, colour, habit, &c., must be usually needed +either to "superinduce," or, which is quite a different +thing, to <i>coincide</i> with the physiological change +But in the case of plants and the lower invertebrata. +I see no reason for any frequent concomitance +of this kind; and therefore believe the physiological<span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span> +change to be, "as a general rule," the primordial +change. At the same time, I have always been +careful to insist that this opinion had nothing to do +with "the essence of physiological selection"; seeing +that "it was of no consequence" to the theory in +what proportional number of cases the cross-sterility +had begun <i>per se</i>, had been superinduced by morphological +changes, or only enabled to survive by +happening to coincide with any other form of +homogamy. In short, "the essence of physiological +selection" consists in <i>all</i> cases of the diversifying <i>effect</i> +of cross-infertility, whensoever and howsoever it may +happen in particular cases to have been <i>caused</i>.</p> + +<p>Thus I emphatically reaffirm that "from the first +I have always maintained that it makes no essential +difference to the theory <i>in what proportional +number of cases</i> they [the physiological variations] +have arisen 'alone in an otherwise undifferentiated +species'"; therefore, "even if I am wrong in supposing +that physiological selection can <i>ever</i> act +alone, the <i>principle</i> of physiological selection, as I +have stated it, is not thereby affected. And this +principle is, as Mr. Wallace has re-stated it, 'that +some amount of infertility characterizes the distinct +varieties which are in process of differentiation into +species'—infertility whose absence, 'to obviate the +effects of intercrossing, may be one of the <i>usual</i> +causes of their failure to become developed into +distinct species.'"</p> + +<p>These last sentences are quoted from the correspondence +in <i>Nature</i><a name="FNanchor_58_58" id="FNanchor_58_58"></a><a href="#Footnote_58_58" class="fnanchor">[58]</a>, and to them Mr. Wallace replied +by saying, "if this is not an absolute change of front,<span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span> +words have no meaning"; that "if this is 'the whole +essence of physiological selection,' then physiological +selection is but a re-statement and amplification of +Darwin's views"; that such a "change of front" is +incompatible, not only with my term "physiological +selection," but also with my having "acknowledged +that Mr. Catchpool had 'very clearly put forward the +theory of physiological selection'"; and much more +to the same effect.</p> + +<p>Now, to begin with, it is due to Mr. Catchpool to +state that his only publication upon this subject is +much too brief to justify Mr. Wallace's, inference, that +he supposes variations in the way of cross-infertility +always to arise "alone in an otherwise undifferentiated +species." What Mr. Catchpool's opinion on this +point may be, I have no knowledge; but, whatever it +is, he was unquestionably the first writer who "clearly +stated the leading principles" of physiological selection, +and this fact I am very glad to have "acknowledged." +In my correspondence with Mr. Wallace, +however, I not only named Mr. Catchpool: I also +named—and much more prominently—Mr. Gulick. +For even if I were to grant (which I am far indeed +from doing) that there was any want of clearness in +my own paper touching the point in question, I have +now repeatedly shown that it is simply impossible +for any reader of Mr. Gulick's papers to misunderstand +<i>his</i> views with regard to it. Accordingly, +I replied to Mr. Wallace in <i>Nature</i> by saying:—</p> + +<blockquote><p>Not only have I thus from the first fully recognized the +sundry other causes of specific change with which the physiological +variations may be associated; but Mr. Gulick has gone +into this side of our common theory much more fully, and<span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span> +elaborately calculated out the high ratio in which the differentiating +agency of any of these other causes must be increased +when assisted by—i. e. associated with—even a moderate degree +of the selective fertility, and vice versa. Therefore, it is simply +impossible for Mr. Wallace to show that "our theory" differs +from his in this respect. Yet it is the only respect in which his +reply alleges any difference. (Vol. xliii. p. 127.)</p></blockquote> + +<p>I think it is to be regretted that, in his answer to +this, Mr. Wallace alludes only to Mr. Catchpool, and +entirely ignores Mr. Gulick—whose elaborate calculations +above alluded to were communicated to the +Linnaean Society by Mr. Wallace himself in 1887.</p> + +<p>The time has now come to prove, by means of +quotations, that I have from the first represented +the "principle," or "essence," of physiological selection +to consist in selective fertility furnishing a needful +condition to specific differentiation, in at least +a large proportional number of allied species which +afterwards present the reciprocal character of cross-sterility; +that I have never represented variations +in the way of this selective fertility as necessarily +constituting the initial variations, or as always arising +"alone, in an otherwise undifferentiated species"; +and that, although I have uniformly given it as my +opinion that these variations do <i>in some cases</i> thus +arise (especially among plants and lower invertebrata), +I have as uniformly stated "that it makes no difference +to the theory in what proportional number of +cases they have done so"—or even if, as Mr. Wallace +supposes, they have never done so in any case at all<a name="FNanchor_59_59" id="FNanchor_59_59"></a><a href="#Footnote_59_59" class="fnanchor">[59]</a>.<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span> +These statements (all of which are contradictory +of the only points of difference alleged) have already +been published in my article in the <i>Monist</i> of +October, 1890. And although Mr. Wallace, in his +reply to that article, ignores my references to the +"original paper," it is scarcely necessary to quote the +actual words of the paper itself, since the reader who +is further interested in this controversy can readily +refer to it in the <i>Journal of the Linnaean Society</i> +(vol. xix. pp. 337-411).</p> + +<p>Having arrived at these results with regard to the +theory of Isolation in general and of Physiological +Isolation in particular, I arrive also at the end of this +work. And if, while dealing with the post-Darwinian +period, I have imparted to any general reader the +impression that there is still a great diversity of +expert opinion; I must ask him to note that points +with reference to which disagreement still exists +are but very subordinate to those with regard to +which complete agreement now prevails. The noise +of wrangling disputations which has so filled the +camp of evolutionists since the death of their +captain, is apt to hide from the outside world the +solid unanimity that prevails with regard to all +the larger and more fundamental questions, which +were similarly the subjects of warfare in the past +generation. Indeed, if we take a fair and general<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span> +view of the whole history of Darwinism, what must +strike us as the really significant fact is the astonishing +unanimity which has been so rapidly attained +with regard to matters of such immeasurable importance. +It is now but little more than thirty years +since the publication of the <i>Origin of Species</i>; and +in that period not only have all naturalists unequivocally +embraced the doctrine of descent considered +as a fact; but, in one degree or another, they have +all as unequivocally embraced the theory of natural +selection considered as a method. The only points +with regard to which any difference of opinion still +exist, have reference to the precise causation of that +mighty stream of events which, under the name of +organic evolution, we have now all learnt to accept as +scientifically demonstrated. But it belongs to the +very nature of scientific demonstration that, where +matters of great intricacy as well as of high generality +are concerned, the process of demonstration must be +gradual, even if it be not always slow. It is only by +the labours of many minds working in many directions +that, in such cases, truth admits of being eventually +displayed. Line upon line, precept upon precept, +here a little and there a little—such is the course of +a scientific revelation; and the larger the subject-matter, +the more subtle and the more complex the +causes, the greater must be the room for individual +differences in our reading of the book of Nature. +Now, if all this be true, must we not feel that in the +matter of organic evolution the measure of agreement +which has been attained is out of all proportion to +the differences which still remain—differences which, +although of importance in themselves, are insignificant<span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span> +when compared with those which once divided the +opinions of not a few still living men? And if we are +bound to feel this, are we not bound further to feel +that the very intensity of our disputations over these +residual matters of comparative detail, is really the +best earnest that can be given of the determination +of our quest—determination which, like that of our +fathers, cannot fail to be speedily rewarded by the +discovery of truth?</p> + +<p>Nevertheless, so long as this noise of conflict is +in the Senate, we cannot wonder if the people are +perplexed. Therefore, in conclusion, I may ask it to +be remembered exactly what are the questions—and +the only questions—which still divide the parties.</p> + +<p>Having unanimously agreed that organic evolution +is a fact and that natural selection is a cause, or +a factor in the process, the primary question in debate +is whether natural selection is the only cause, or +whether it has been assisted by the co-operation of +other causes. The school of Weismann maintain that +it is the only cause; and therefore deem it worse +than useless to search for further causes. With this +doctrine Wallace in effect agrees, excepting as regards +the particular case of the human mind. The school +of Darwin, on the other hand—to which I myself +claim to belong—believe that natural selection has +been to a considerable extent supplemented by other +factors; and, therefore, although we further believe +that it has been the "main" factor, we agree with +Darwin himself in strongly reprobating all attempts +to bar <i>a priori</i> the progress of scientific investigation +touching what, if any, these other factors may be. +Lastly, there are several more or less struggling<span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span> +schools, chiefly composed of individual members who +agree with each other only to the extent of holding +that the causal agency of natural selection is not so +great as Darwin supposed. The Duke of Argyll, +Mr. Mivart and Mr. Geddes may be named in this +connexion; together with the self-styled neo-Lamarckians, +who seek to magnify the Lamarckian +principles at the expense of the distinctively Darwinian.</p> + +<p>This primary difference of opinion leads deductively +to certain secondary differences. For if a man starts +with the premiss that natural selection must necessarily +be the "exclusive" cause of organic evolution, +he is likely to draw conclusions which another man +would not draw who starts with the premiss that +natural selection is but the "main" cause. Of these +subordinate differences the most important are those +which relate to the possible transmission of acquired +characters, to the necessary (or only general) utility +of specific characters, and to the problem touching the +inter-sterility of allied species. But we may well +hope that before another ten years shall have passed, +even these still outstanding questions will have been +finally settled; and thus that within the limits of an +ordinary lifetime the theory of organic evolution will +have been founded and completed in all its parts, to +stand for ever in the world of men as at once the +greatest achievement in the history of science, and the +most splendid monument of the nineteenth century.</p> + +<p>In the later chapters of the foregoing treatise I have +sought to indicate certain matters of general principle, +which many years of study specially devoted to this +great movement of contemporary thought have led<span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span> +me to regard as almost certainly sound in themselves, +and no less certainly requisite as complements of the +Darwinian theory. I will now conclude by briefly +summarizing these matters of general principle in the +form of twelve sequent propositions. And, in doing +so, I may ask it to be noticed that the system which +these propositions serve to express may now claim, +at the least, to be a strictly logical system. For the +fact that, not merely in its main outlines, but likewise +in its details, it has been independently constructed +by Mr. Gulick, proves at any rate this much; seeing +that, where matters of such intricacy are concerned, +nothing but accurate reasoning from a common +foundation of <i>data</i> could possibly have yielded so +exact an agreement. The only difference between us +is, that Mr. Gulick has gone into much further detail +than I have ever attempted in the way of classifying +the many and varied forms of isolation; while I have +laid more special stress upon the physiological form, +and found in it what appears to me a satisfactory +solution of "the greatest of all the difficulties in the +way of accepting the theory of natural selection as +a complete explanation of the origin of species"—namely, +"the remarkable difference between varieties +and species when crossed."</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></p> + + + + +<h2>GENERAL CONCLUSIONS.</h2> + +<div class="smcap"> +<p>1. Natural Selection is primarily a theory +of the cumulative development of adaptations +wherever these occur; and therefore +is only incidentally, or likewise, a theory +of the origin of species in cases where allied +species differ from one another in respect of +peculiar characters, which are also adaptive +characters.</p> + +<p>2. Hence, it does not follow from the +theory of natural selection that all +species—much less all specific characters—must +necessarily have owed their origin to +natural selection; since it cannot be proved +deductively from the theory that no "means +of modification" other than natural selection +is competent to produce such slight +degrees of modification as go to constitute +diagnostic distinctions between closely +allied species; while, on the other hand, +there is an overwhelming mass of evidence +to prove the origin of "a large proportional +number of specific characters" by causes of +modification other than natural selection.<span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span></p> + +<p>3. Therefore, and upon the whole, as +Darwin so emphatically held, "Natural +selection has been the main, but not the +exclusive means of modification."</p> + +<p>4. Even if it were true that all species +and all specific characters must necessarily +owe their origin to natural selection, it +would still remain illogical to define the +theory of natural selection as indifferently +a theory of species or a theory of adaptations; +for, even upon this erroneous supposition, +specific characters and adaptive +characters would remain very far indeed +from being conterminous—most of the more +important adaptations which occur in +organic nature being the common property +of many species.</p> + +<p>5. In no case can natural selection have +been the cause of mutual infertility +between allied, or any other, species—<i>i.e.</i> of +the most general of all "specific characters."</p> + +<p>6. Without Isolation, or the prevention of +free intercrossing, organic evolution is in +no case possible. Therefore, it is isolation +that <i>has</i> been "the exclusive means of +modification," or, more correctly, the universal +condition to it. Therefore, also, +Heredity and Variability being given, the +whole theory of organic evolution becomes +a theory of the causes and conditions which +lead to Isolation.<span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span></p> + +<p>7. Isolation may be either discriminate +or indiscriminate. When discriminate, it +has reference to resemblances between individuals +constituting the isolated colony +or group; when indiscriminate, it has no +such reference. In the former case there +arises Homogamy, and in the latter case +there arises Apogamy.</p> + +<p>8. Except where very large populations +are concerned, indiscriminate isolation +always tends to become increasingly discriminate; +and, in the measure that it does so, +apogamy passes into homogamy, by virtue of +Independent Variability.</p> + +<p>9. Natural Selection is one among many +other forms of discriminate isolation, and +presents in this relation the following +peculiarities:—(<i>a</i>) The isolation is with +reference to superiority of fitness; (<i>b</i>) is +effected by death of the excluded individuals; +and (<i>c</i>) unless assisted by some +other form of isolation, can only effect +monotypic as distinguished from polytypic +evolution.</p> + +<p>10. It is a general law of organic evolution +that the number of possible directions in +which divergence may occur can never be +more than equal to the number of cases of +efficient isolation; but, excepting natural +selection, any one form of isolation need +not necessarily require the co-operation<span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span> +of another form in order to create an +additional case of isolation, or to cause +polytypic as distinguished from monotypic +evolution.</p> + +<p>11. Where common areas and polytypic evolution +are concerned, the most general and +most efficient form of isolation has been +the physiological, and this whether the +mutual infertility has been the antecedent +or the consequent of morphological changes +on the part of the organisms concerned, and +whether or not these changes are of an +adaptive character.</p> + +<p>12. This form of isolation—which, in +regard to incipient species, I have called +Physiological Selection—may act either +alone or in conjunction with other forms of +isolation on common areas: in the former +case its agency is of most importance among +plants and the lower classes of animals; +in the latter case its importance consists +in its greatly intensifying the segregative +power of whatever other form of isolation +it may be with which it is associated.</p> +</div> + +<hr class="full" /> +<p class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></p> + + + + +<h2><i>APPENDICES</i></h2> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></p> + + + +<h2>APPENDIX A.<br /> +<span class="smcap">Mr. Gulick's Criticism of Mr. Wallace's Views on +Physiological Selection.</span></h2> + + +<p>I have received from Mr. Gulick the results of his +consideration of Mr. Wallace's criticism. As these results +closely resemble those which I have myself reached, and +as they were independently worked out on the other side of +the globe, I deem it desirable to publish them here for the +sake of comparison.</p> + +<p>In his covering letter Mr. Gulick writes:—</p> + +<blockquote><p>Mr. Wallace has most certainly adopted the fundamental principles +of our theory, and in an arbitrary way attempted to claim +the results produced by these principles as the effects of natural +selection. He takes our principles, which in the previous +chapter he has combated; but he makes such disjointed use of +them that I am not willing to recognize his statement as an +intelligible exposition of our theory.... I have endeavoured to +indicate at what points Mr. Wallace has deserted his own principles, +and at what points he has failed to make the best use of +ours. To bring out these points distinctly has been no easy +task; but if you regard this paper on <i>The Preservation and Accumulation +of Cross-infertility</i> as giving any help in elucidating +the true principles, and in showing Mr. Wallace's position in +regard to them, I shall be satisfied. Please make any use of it +that may seem desirable, and then forward it to Professor +Dana.</p></blockquote> + +<p><span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span> +The following is a general summary of Mr. Gulick's +results:—</p> + +<blockquote><p>Mr. Wallace's criticism of the theory of Physiological Selection +is unsatisfactory; (l) because he has accepted the fundamental principle +of that theory on pages 173-9, in that he +maintains that without the cross-infertility the incipient species +there considered would be swamped; (2) because he assumes +that physiological selection pertains simply to the infertility +of first crosses, and has nothing to do with the infertility of +mongrels and hybrids; (3) because he assumes that infertility +between first crosses is of rare occurrence between species of +the same genus, ignoring the fact that in many species of plants +the pollen of the species is pre-potent on the stigma of the same +species when it has to compete with the pollen of other species +of the same genus; (4) because he not only ignores Mr. Romanes' +statement that cross-infertility often affects "a whole race or +strain," but he gratuitously assumes that the theory of Physiological +Selection excludes this "racial incompatibility" (which +Mr. Romanes maintains is the more probable form), and bases his +computation on the assumption that the cross-infertility is not +associated with any other form of segregation; (5) because he +claims to show that "all infertility not correlated with some +<i>useful</i> variation has a constant tendency to effect its own +elimination," while his computation only shows that, if the cross-infertility +is not associated with some form of <i>positive</i> segregation, +it will disappear<a name="FNanchor_60_60" id="FNanchor_60_60"></a><a href="#Footnote_60_60" class="fnanchor">[60]</a>; and (6) because he does not observe +that the positive segregation may be secured by the very form of +the physiological incompatibility.... Without here entering +into any computation, it is evident that, e.g. the prepotency of +pollen of each kind with its own kind, if only very slight, will +prevent cross-fertilization as effectually as a moderate degree of +instinctive preference in the case of an animal.</p></blockquote> + +<p><span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span> +The paper likewise indicates a point which, in studying +Mr. Wallace's theory, I have missed. It will be remembered +that the only apparent difference between his theory +and mine has been shown to consist in this—that while +I was satisfied to state, in a general way, that natural selection +is probably able to increase a selective fertility which +has already been begun by other causes, Mr. Wallace +has sought to exhibit more in detail the precise conditions +under which it can do so. Now, Mr. Gulick shows that +the particular conditions which Mr. Wallace describes, even +if they do serve to promote an increase of cross-infertility, +are conditions which preclude the possibility of natural selection +coming into play at all. So that if, under these particular +conditions, a further increase of cross-infertility does +take place, it does not take place in virtue of natural selection. +To me it appears that this criticism is sound; and, if so, +it disposes of even the one very subordinate addition to +our theory which Mr. Wallace "claims" as the most +"distinctive" part of his.</p> + +<p>The following is the criticism in question:—</p> + +<blockquote><p>On pages 173-186 Mr. Wallace maintains that "Natural +selection is, in some probable cases at all events, able to +accumulate variations in infertility between incipient species" +(p. 174); but his reasoning does not seem to me conclusive. +Even if we grant that the increase of this character [cross-infertility] +occurs by the steps which he describes, <i>it is not +a process of accumulation by natural selection</i>. In order to be a +means of cumulative modification of varieties, races, or species, +selection, whether artificial or adaptational [i.e. natural], must +preserve certain forms of an intergenerating stock, to the +exclusion of other forms of the same stock. Progressive +change in the size of the occupants of a poultry-yard may be +secured by raising only bantams the first, only common fowls +the second, and only Shanghai fowls the third year; but this +is not the form of selection that has produced the different +races of fowls. So in nature, rats may drive out and supplant<span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span> +mice; but this kind of selection modifies neither rats nor mice. +On the other hand, if certain variations of mice prevail over +others, through their superior success in escaping their pursuers, +then modification begins. Now, turning to page 175, we +find that, in the illustrative case introduced by Mr. Wallace, +the commencement of infertility between the incipient species +is in the relations to each other of two portions of a species +that are locally segregated from the rest of the species, and +partially segregated from each other by different modes of +life. These two local varieties, being by the terms of his +supposition better adapted to the environment than the freely +interbreeding forms in other parts of the general area, increase +till they supplant these original forms. Then, in some limited +portion of the general area, there arise two still more divergent +forms, with greater mutual infertility, and with increased adaptation +to the environment, enabling them to prevail throughout +the whole area. The process here described, if it takes place, +is not modification by natural selection.</p></blockquote> + +<p>On the other hand, it <i>is</i> modification by physiological +selection. For, among the several other forms of isolation +which are called into requisition, the physiological (i.e. +ever accumulating cross-infertility) is supposed to play an +important part. That the modification is not modification +by natural selection may perhaps be rendered more +apparent by observing, that in as far as <i>any</i> other mode +of isolation is involved or supposed, so far is the <i>possible</i> +agency of natural selection eliminated <i>as between the two +or more otherwise isolated sections of a species</i>; and yet it is +modes of isolation other than that furnished by natural selection +(i.e. perishing of the less fit), that Mr. Wallace here +supposes to have been concerned—including, as I have +before shown, the physiological form, to which, indeed, he +really assigns most importance of all. Or, as Mr. Gulick +states the matter in his independent criticism:—</p> + +<blockquote><p>In the supposed case pictured by Mr. Wallace, the principle +by which the two segregating forms are kept from crossing,<span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span> +and so are eventually preserved as permanently distinct forms, +is no other than that which Mr. Romanes and myself have +discussed under the terms Physiological Selection and Segregate +Fecundity. Not only is Mr. Wallace's exposition of the divergence +and the continuance of the same in accord with these +principles which he has elsewhere rejected, but his whole +exposition is at variance with his own principle, which, in the +previous chapter, he vigorously maintains in opposition to my +statement that many varieties and species of Sandwich Island +land molluscs have arisen, while exposed to the same environment, +in the isolated groves of the successive valleys of the same +mountain range. If he adhered to his own theory, "the greater +infertility between the two forms in one portion of the area" +would be attributed to a difference between the <i>environment</i> presented +in that portion and that presented in the other portions; +and the difficulty would be to consistently show how this +greater infertility could continue unabated when the varieties +thus characterized spread beyond the environment on which +the character depends. But, without power to continue, the +process which he describes would not take place. Therefore, in +order to solve the problem of the <i>origin</i> and <i>increase</i> of +infertility between species, he tacitly gives up his own theory, +and adopts not only the theory of Physiological Selection but +that of Intensive Segregation<a name="FNanchor_61_61" id="FNanchor_61_61"></a><a href="#Footnote_61_61" class="fnanchor">[61]</a> through Isolation, though he +still insists on calling the process natural selection; for on +page 183 he says, "No form of infertility or sterility between +the individuals of a species can be increased by natural selection +unless correlated with some useful variation, while all +infertility not so correlated has a constant tendency to effect +its own elimination." Even this claim he seems to unwittingly +abandon when on page 184 he says: "The moment it [a +species] becomes separated either by geographical or selective +isolation, or by diversity of station or of habits, then, while +each portion must be kept fertile <i>inter se</i>, there is nothing +to prevent infertility arising between the two separated +portions."</p></blockquote> + +<p><span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span> +The criticism proceeds to show yet further inconsistencies +and self-contradictions in Mr. Wallace's treatment of this +subject; but it now seems needless to continue. Nor, +indeed, should I have quoted this much but for the sake +of so fully justifying my own criticism by showing the +endorsement which it has received from a completely independent +examination.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></p> + + + + +<h2>APPENDIX B.<br /> +<span class="smcap">An Examination by Mr. Fletcher Moulton of Mr. +Wallace's Calculation touching the Possibility Of +Physiological Selection ever acting alone.</span></h2> + + +<p>We have seen that the only important point of difference +between Mr. Wallace's more recent views and my own on +the problem of inter-specific sterility, has reference to the +question whether variations in the way of cross-infertility can +<i>ever</i> arise and act "alone, in an otherwise undifferentiated +species," or whether they can <i>never</i> so arise and act. It +is Mr. Wallace's opinion that, even if they ever do arise +alone, at all events they can never act in differentiating a +specific type, seeing that the chances against their suitable +mating must be so great: only if they be from the first +associated with some other form of homogamy, which will +have the effect of determining their suitable mating, does +he think that they can act in the way supposed by our +theory of "selective fertility"<a name="FNanchor_62_62" id="FNanchor_62_62"></a><a href="#Footnote_62_62" class="fnanchor">[62]</a>. On the other hand, as<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span> +previously and frequently stated, I have so strong a belief +in the segregating power of physiological selection, or +selective fertility, that I do not think it is necessary for +this principle to be <i>always</i> associated with some other form +of homogamy. From the first, indeed, I have laid great +stress (as, also, has Mr. Gulick) on the re-enforcing influence +which association with any other form of homogamy must +exercise upon the physiological form, and vice versa; but +I have also said that, in my opinion, the physiological form +may in many cases be able to act entirely alone, or without +assistance derived from any other source. The question +here is, as we have already so fully seen, a question of but +secondary importance; since, whether or not the physiological +form of homogamy ever acts alone, even Mr. Wallace +now allows, or rather argues, that it acts in combination—and +this so habitually, as well as with so much effect, that it +constitutes a usual condition to the origination of species. +Nevertheless, although the only relevancy of his numerical +computation of chances—whereby he thinks that he overturns +my theory <i>in toto</i>—is such relevancy as it bears to this +question of secondary importance, I have thought it desirable +to refer the question, together with Mr. Wallace's views upon +it, to the consideration of a trained mathematician.</p> + +<p>As this "subordinate question" depends entirely on +numerical computations involving the doctrine of chances, +I should first of all like to remark, that in reference to +biological problems of the kind now before us, I do not +myself attach much importance to a merely mathematical +analysis. The conditions which such problems involve are +so varied and complex, that it is impossible to be sure about +the validity of the <i>data</i> upon which a mathematical analysis is<span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span> +founded. Nevertheless, for the sake of meeting these +criticisms upon their own ground, I will endeavour to show +that, even as mathematical calculations, they are quite untrustworthy. +And, in order to do this effectually, I will quote +the results of a much more competent, as well as a much more +thorough, inquiry. I applied to Mr. Moulton for this +purpose, not only because he is one of the ablest mathematicians +of my acquaintance; but also because his interest +in biology, and his knowledge of Darwinian literature, +render him well fitted to appreciate exactly, and in all their +bearings, the questions which were submitted to his consideration. +I need only add that his examination was +completely independent, and in no way influenced by me. +Having previously read my paper on <i>Physiological Selection</i>, +Mr. Gulick's paper on <i>Divergent Evolution</i>, and Mr. Wallace's +book on <i>Darwinism</i>, he was in possession of all the materials; +and I merely requested the favour of his opinion upon the +whole case from a mathematical point of view. The +following is his reply; and I give it <i>in extenso</i>, because it +serves to place in another light some of the general considerations +which it has already been my endeavour to present<a name="FNanchor_63_63" id="FNanchor_63_63"></a><a href="#Footnote_63_63" class="fnanchor">[63]</a>.</p> + +<p>After some introductory remarks on Mr. Wallace's +"adoption of the theory of physiological selection pure +and simple," and "the pure caricature of it which he +puts forward as" mine, the letter proceeds thus:—</p> + +<blockquote><p>The reason why it is so easy to attack your theory is that +it is so easy to confuse the survival of an <i>individual</i> with the<span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span> +survival of a <i>peculiarity</i> of <i>type</i>. No one has ever said that +an <i>individual</i> is <i>assisted</i> by the possession of selective fertility: +that is a matter which cannot affect his chance of <i>life</i>. +Nor has any one said that the possession of selective fertility +in an <i>individual</i> will <i>of itself</i> increase the chance of his having +<i>progeny</i> that will survive, and in turn become the progenitors +of others that will survive. Taken by itself, the fact that an +<i>individual</i> is capable of fertility with some only of the opposite +sex lessens the chance of his having progeny. Whether +or not he is more or less favourably situated than his <i>confreres</i> +for the battle of life must be decided by the <i>total sum</i> +of his peculiarities; and the question whether or not this +selective fertility will be a hindrance must be decided by +considerations depending on the other peculiarities associated +with it.</p> + +<p>But when we come to consider the survival or permanence +of a <i>type</i> or <i>peculiarity</i>, the case is quite different. It then +becomes not only a favourable circumstance, but, in my opinion, +almost a necessary condition, that the peculiarity should be +associated with selective fertility<a name="FNanchor_64_64" id="FNanchor_64_64"></a><a href="#Footnote_64_64" class="fnanchor">[64]</a>.</p> + +<p>Take the case of the Jews. I don't think that intermarriage +with other nations would lessen their fertility, or diminish the +number of their progeny; nor is there any reason to think that +this progeny would be unequal to the struggle for existence. +But no one doubts that the abandonment of their voluntary +isolation (which operates so far as this is concerned as a selective +fertility), would lead to the disappearance of the familiar +Jewish type. All the world would get some of it; but as a whole +it would be "swamped."</p> + +<p>Now although no doubt Wallace would admit all this, he +fails to give it the weight it ought to have. In discussing the +question of its operation he considers too exclusively the case +of the individual.</p> + +<p>Of course, a type can only be perpetuated through the medium +of individuals, and all that his argument amounts to is, that<span class="pagenum"><a name="Page_161" id="Page_161">[Pg 161]</a></span> +selective fertility would be so fatal to individuals that <i>no</i> type +which presents it could be formed or perpetuated—a conclusion +which is not only absurd in itself, but contradicted by +his own subsequent adoption of your theory. Besides, apart +from calculations (with which I will deal when I write next), +such reasoning brings its own refutation. Selective fertility is +not in the same category as some of the other influences to +which an important share has been ascribed in the formation +of the existing types. <i>It exists as a recognized phenomenon.</i> +Hence all these numerical proofs that it would lead to extinction, +because it is so disadvantageous to the possessor, prove +too much. They would show that the degree of selective +fertility which so frequently characterizes species is a most +onerous gift; and that, were it not present, there would be +a vastly increased chance of fertility, which would render the +races fitter and lead to their increased survival. Why then +has it not been got rid of?</p> + +<p>The two answers which no doubt would be given seem to +me to support rather than to make against your theory. In +the first place, Wallace might say that this infertility is an +advantage because it keeps pure a type which is specially +fitted to its surroundings, as shown by its continued existence. +But if this be so, and it is necessary to protect the <i>developed</i> +type, how much more necessary to protect the <i>incipient</i> type! +In the second place, he might say that this selective fertility +is not so disadvantageous when the species has been formed, +because the individual can choose his mate from his like; +whereas, when it is beginning to be formed, he must mate +blindly, or without what you call "psychological selection." +But this seems to me to be wholly inapplicable to at least half +the animal, and to all the vegetable kingdom. Moreover, with regard +to the other half of the animal kingdom, it merely raises the +question,—How soon will such an incipient type recognize itself? +Seeing it is probable that many families [broods] will belong to +the same [incipient] type, I should not be surprised if it were +found that this sexual recognition and preference sets in very +early.</p> + +<p>But this leads me to the question of your letter. I understand +you to want me to examine and criticize the attempted<span class="pagenum"><a name="Page_162" id="Page_162">[Pg 162]</a></span> +numerical arguments against or for your theory. Now it seems +to me that it will be best to take, in the first instance, the +vegetable kingdom, and with regard to it I cannot see how +there can be any numerical argument against the theory. For +we often have species side by side with others nearly allied, +but much more numerous. The condition of these is precisely +analogous to that of your incipient species. They are exposed +to fertilization from, say, ten times as numerous individuals of +the allied species. They reject this in favour of that from the +relatively few individuals of their own. Yet the two species +are in competition. I could go through the numerical arguments +of your assailant word for word, applying them to +such a case as this, and they would triumphantly show that +the specific fertility of the rarer kind would lead to its certain +extinction. Yet we know that this is not so.</p> + +<p>Indeed, the too triumphant character of the logic used against +you seems to me to be capable of being turned to your use. +If cross-infertility is so intensely disadvantageous to the individuals +presenting it, it cannot have been <i>that</i> which made +these individuals and their progeny survive. It is therefore +a burden which they have carried. But we find that it is +more or less present in all the closely allied types that occur +on common areas: therefore it must be a necessary feature +in the formation of such types; for it cannot be an accident +that it is present in so many. In other words, it must be +the price which the individual and his progeny pay for their +formation into a type. And this is your theory pure and +simple.</p> + +<p>The more I consider the matter, the more I feel that it is +impossible to decide as to the sufficiency of selective fertility +to explain the formation of species, if we consider merely the +effect it would have on the number of individuals, as contrasted +with what it would be if no such peculiarity had developed +itself. Indeed, I may say that on pondering over +the matter I have come to the conclusion, that mere fertility +is probably a comparatively unimportant factor in the preservation +of the species, after a certain sufficient degree of fertility +is attained. I do not wish to be misunderstood. To a certain +point fertility is not only advantageous but necessary, in<span class="pagenum"><a name="Page_163" id="Page_163">[Pg 163]</a></span> +order to secure survival of the type; but I feel that little +reliance can be placed on calculations based on the numerical +co-efficient of fertility (i. e. the ratio of the number of offspring +to the number of parents) in determining the relative chance of +type-survival.</p> + +<p>Take, for instance, the oak tree. It produces thousands of +acorns, almost the whole of which die without producing any +progeny. Have we any reason to believe that if the number +of acorns borne by oak trees were diminished, even so much +as to one-tenth, the race of oaks would perish? It may +of course be said that, if all other things are equal, the probabilities +of survival must be increased by increased fertility +of this kind; but I feel convinced that when numerical fertility +has attained to a high point in circumstances in which +actual increase of the race cannot take place to any substantial +extent, the numerical value of this fertility sinks down +into a factor of the second or third order of importance—that +is to say, into the position of a factor whose effects are only to +be considered when we have duly allowed for the full effects +of all the main factors. Until we have done that, we gain +little or nothing in the way of accuracy of conclusion by taking +into consideration the minor factors. It may be very well to +neglect the effect of the attraction of Jupiter in our early researches +on the motion of the Moon; and our doing so will +not prevent the results being approximate and having considerable +value, because we are retaining the two main factors that +establish the motion, viz. the effects of the Earth and the Sun. +But if we exclude the effect of one of these main factors, our +results would be worthless; and it would not be rendered substantially +less so by the fact that we had taken Jupiter into +account in arriving at them.</p> + +<p>You must not imagine, however, that I think it wholly profitless +to see whether there would be any substantial effect on +numerical fertility were <i>selective</i> fertility to manifest itself. But +if we want to derive any assistance from calculation, it must +be by applying it with a good deal more precision and definiteness +than anything that Wallace shows. And, in the first +place, it is useless to confuse the vegetable and animal kingdoms. +In the former you have union unaffected by choice; in the latter,<span class="pagenum"><a name="Page_164" id="Page_164">[Pg 164]</a></span> +so far at all events as the higher animals are concerned, you +have "psychological selection." In order to give you a specimen +of what can safely be done by calculation if you take +a problem of sufficient definiteness, I have chosen the case of +a flowering plant in which a certain proportion of the race +have developed the peculiarity of being sterile with the remainder, +while retaining the normal fertility of the race in +unions among themselves. In order to give the greatest advantage +to your critics, I have assumed that such flowers as +possess the peculiarity are not self-fertilizable; for it is clear that +if we suppose that they are self-fertilizable, the fertility need +be very slightly affected.</p> + +<p>As I have excluded self-fertilization, it is necessary, if we are +to get any trustworthy results, that one should consider the +mode in which fertilization will be produced. I have taken +the case of fertilization by insects, and have assumed that each +flower is visited a certain number of times by insects during +the period when fertilization is possible; and, further, that the +insects which visit it have on the average visited a certain +number of flowers of the same species before they came there. +Of course nothing but observation can fix these latter numbers; +but I should not be surprised at finding that they are of +considerable magnitude<a name="FNanchor_65_65" id="FNanchor_65_65"></a><a href="#Footnote_65_65" class="fnanchor">[65]</a>. In order to make the results a little<span class="pagenum"><a name="Page_165" id="Page_165">[Pg 165]</a></span> +more intelligible, I have grouped them under the numbers which +represent the average number of flowers that an insect visits +in a journey. This is a little more than twice as great as the +number which represents the number of flowers he has on the +average visited before coming to the individual whose fertility +we are considering.</p> + +<p>I send you the formula and the calculation on which it is +based in an Appendix; but as I know you have a holy horror +of algebraical formulae, I give you here a few numerical +results.</p> + +<p>The cases I have worked out are those in which the number +of insects visiting each flower is 5, or 10, or 15; and I have +also taken 5, 10, and 15, to represent the number of flowers +which an insect visits each journey. This makes nine cases +in all; and I have applied these to two instances—viz. one +in which one-fifth of the whole race have developed cross-infertility, +and the other in which one-tenth only have done so. +Taking first the instance where one-fifth have developed the +peculiarity, I find that if on the average five insects visit +a flower, and each insect on the average visits five flowers on +a journey, the fertility is diminished by about one-tenth. If, +however, the average number of flowers the insect visits is ten, +the reduction of fertility is less than one per cent. And it +becomes inappreciable if the average number is fifteen. If on +the average ten insects visit each flower, then, if each insect +visits on the average five flowers on a journey, the reduction +of fertility is a little over one per cent.; but if it visits ten or +fifteen the reduction is inappreciable. If fifteen insects visit the +flower on an average, then, if these insects on the average visit<span class="pagenum"><a name="Page_166" id="Page_166">[Pg 166]</a></span> +five or more flowers on a journey, the reduction of fertility +is inappreciable.</p> + +<p>By the term inappreciable I mean that it is not substantially +greater than one-tenth of one per cent.—i.e. not more +than one-thousandth.</p> + +<p>Of course, if the proportion of individuals acquiring the +peculiarity is less, the effect on the fertility under the above +hypothesis will be greater; and it will not be counteracted so +fully unless the number of insect visits is larger, or unless the +insects visit more flowers on a journey. Thus if only one-tenth +of the race have developed the peculiarity, then, if each flower +is visited on the average by five insects who visit five flowers +on each trip, the fertility will be reduced about one-third. +If, however, the insects visit on the average ten flowers per +trip, it will be only diminished about one-tenth; and if they +visit fifteen on each trip, it will be only diminished about +one-fortieth. If in the same case we suppose that each +flower receives ten insect visits, then, if the insects visit on an +average five flowers per trip, the fertility will be diminished +about one-eighth. If they visit ten on a trip, it will be diminished +about one-hundredth, and the diminution is inappreciable +if they visit fifteen on a trip. Similarly, if a flower receives +fifteen insect visits, the diminution is about one-twenty-fifth, +if insects visit on the average five flowers on a trip; and is +inappreciable if they visit ten or fifteen.</p> + +<p>These figures will show you that it is exceedingly possible +that a peculiarity like this, the effect of which at first sight +would seem to be so prejudicial to fertility, may in fact have +little or no influence upon it; and if you set against this the +overwhelming importance of such a peculiarity in segregating +the type so as to give it a chance of becoming a fixed species, +you will, I think, feel that your hypothesis has nothing to +fear from a numerical examination.</p> + +<p>I have not examined the case of fertilization by other means; +nor have I examined the case of fertilization in animals, where +psychological selection can come in. To obtain any useful +results, one would have to consider very carefully the circumstances +of each case; and at present, at all events, I do not +think it would be useful to do so. Nor have I attempted to<span class="pagenum"><a name="Page_167" id="Page_167">[Pg 167]</a></span> +show the converse of the problem—viz. the effect of swamping +where cross-fertilization is possible. I shall be very glad to +examine any one of these cases if you want me to do so; +but I should prefer to leave it until I hear from you again.</p> + +<p>If you contrast the results that I have given above with +those given on pages 181 to 183 of Wallace's book, you will +see the enormous difference. His calculations can only apply +to the animal kingdom in those cases in which there is only +a union between one individual of each sex; and before you +can deal with the question of such animals, you will have to +take into consideration many elements besides that of mere +fertility, if you wish to get any tolerably accurate result<a name="FNanchor_66_66" id="FNanchor_66_66"></a><a href="#Footnote_66_66" class="fnanchor">[66]</a>.</p></blockquote> + +<p>The above analysis leaves nothing to be added by me. +But, in conclusion, I may once more repeat that the particular +point with which it is concerned is a point of very subordinate +importance. For even if Mr. Wallace's computation +of chances had been found by Mr. Moulton to have been an +adequate computation—and, therefore, even if it had been +thus proved that physiological homogamy must always be +associated with some other form of homogamy in order to +produce specific divergence—still the importance of selective +fertility as a factor of organic evolution would not have +been at all diminished. For such a result would merely +have shown that, not only "in many cases" (as I originally +said), but actually in all cases, the selective fertility which +I hold to have been so generally concerned in the differentiation +of species has required for this purpose the co-operation +of some among the numerous other forms of homogamy. +But inasmuch as, by hypothesis, no one of these other or +co-operating factors would of itself have been capable of +effecting specific divergence in any of the cases where its +association with selective fertility is concerned, the mathematical<span class="pagenum"><a name="Page_168" id="Page_168">[Pg 168]</a></span> +proof that such an association is <i>always</i>—and not +merely <i>often</i>—necessary, would not have materially affected +the theory of the origin of species by means of physiological +selection. We have now seen, however, that a competent +mathematical treatment proves the exact opposite; and, therefore, +that Mr. Wallace's criticism fails even as regards the +very subordinate point in question.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_169" id="Page_169">[Pg 169]</a></p> + + + + +<h2>APPENDIX C.<br /> +<span class="smcap">Some Extracts from the Author's Note-books.</span></h2> + + +<p><i>Bearing of Weismannism on Physiological Selection.</i>—If +in view of other considerations I could fully accept Professor +Weismann's theory of heredity, it would appear to me in no +small measure to strengthen my own theory of physiological +selection. For Weismann's theory supposes that all changes +of specific type must have their origin in variations of a +continuous germ-plasm. But <i>the more the origin of species is +referred directly to variations arising in the sexual elements, +the greater is the play given to the principles of physiological +selection</i><a name="FNanchor_67_67" id="FNanchor_67_67"></a><a href="#Footnote_67_67" class="fnanchor">[67]</a>; while, on the other hand, the less standing-ground +is furnished to the theory that cross-infertility between allied +species is due to "external conditions of life," "prolonged +exposure to uniform change of conditions," "structural +modifications re-acting on the sexual functions"; or, in +short, that "somatogenetic" changes of any kind can of +themselves induce the "blastogenetic" change of cross-infertility +between progeny of the same parental stock.</p> + + +<p><i>Cross-infertility and Diversity of Life.</i>—Observe that one +great consequence of duly recognizing the importance of intercrossing +is indefinitely to raise our estimate of the part played +by the principle of cross-infertility in diversifying organic +nature. For whenever in any line of descent the bar of<span class="pagenum"><a name="Page_170" id="Page_170">[Pg 170]</a></span> +sterility arises, there the condition is given for a new crop of +departures (species of a genus); and when genera are formed +by the occurrence of this bar, there natural selection and all +other equilibrating causes are supplied with new material for +carrying on adaptational changes in new directions. Thus, +owing to cross-infertility, all these causes are enabled to +work out numberless adaptations in many directions (i. e. +lines of descent) simultaneously.</p> + + +<p><i>Cross-infertility and Stability.</i>—The importance of sterility +as a diagnostic feature is obvious if we consider that more +than any other feature it serves to give <i>stability</i> to the type; +and unless a type is stable or constant, it cannot be ranked +as a species. That Darwin himself attributes the highest +importance to this feature as diagnostic, see <i>Forms of Flowers</i>, +pp. 58, 64.</p> + + +<p><i>Cross-infertility and Specific Differentiation.</i>—In their +elaborate work on the many species of the genus Hieracium, +Nägeli and Peter are led to the general conclusion that the +best defined species are always those which display absolute +sterility <i>inter se</i>; while the species which present most +difficulty to the systematist are always those which most +easily hybridize. Moreover, they find, as another general +rule applicable to the whole genus, that there is a constant +correlation between inability to hybridize and absence of +intermediate varieties, and, conversely, between ability to +hybridize and the presence of such varieties.</p> + + +<p><i>Cross-infertility in Domesticated Cattle.</i>—Mr. J. W. Crompton, +who has had a large experience as a professional cattle-breeder, +writes to me (March 2, 1887)—</p> + +<blockquote><p>"That form of barrenness, very common in some districts, +which makes heifers become what are called 'bullers'—that +is, irregularly in 'season,' wild, and failing to conceive—is +certainly produced by excess of iron in their drinking-water, +and I suspect also by a deficiency of potash in the soil."</p></blockquote><p><span class="pagenum"><a name="Page_171" id="Page_171">[Pg 171]</a></span></p> + +<p>He also informs me that pure white beasts of either sex +are so well known by experienced breeders to be comparatively +infertile together, that they are never used for breeding +purposes, so that "in some parts of the country, where a +tendency to sterility had become so confirmed in the white +race that they utterly died out," only the coloured breeds are +now to be found. He goes on to say that if "a lot of white +heifers were put to a lot of white bulls, I think you would +probably get a fertile breed of pure white cattle.... I think, +in short, that domestication has produced just what your +theory suggests, a new variety inclined to prove sterile with +its parent stock."</p> + +<p>Commenting on the origin of domesticated cattle, Professor +Oscar Schmidt remarks (<i>Doctrine of Descent</i>, p. 139)—</p> + +<blockquote><p>"Rütimeyer's minute researches on domestic cattle have shown +that, in Europe at least, three well-defined species of the diluvial +period have contributed to their formation—<i>Bos primigenius</i>, +<i>longifrons</i>, and <i>frontosus</i>. These species once lived geographically +separate, but contemporaneously; and they and their +specific peculiarities have perished, to rise again in our domestic +races. These races breed together with unqualified fertility. +In the form of skull and horns they recall one or other of the +extinct species; but collectively they constitute a new main species. +That from their various breeds, the three or any one of the +aboriginal species would ever emerge in a state of pristine +purity, would be an utterly ludicrous assertion."</p></blockquote> + +<p>Now, seeing that these "aboriginal species," although living +"contemporaneously," were "geographically separate," we +can well understand that their divergence of type from a +common ancestor did not require, as a condition to their +divergence, that any cross-sterility should have arisen between +them. The geographical isolation was enough to secure +immunity from mutual intercrossing, and therefore, as our +present theory would have expected as probable, morphological +divergence occurred without any corresponding physiological<span class="pagenum"><a name="Page_172" id="Page_172">[Pg 172]</a></span> +divergence, as must almost certainly have been the +case if such polytypic evolution had occurred on a common +area. Indeed, one of the two lines of experimental verification +of our theory consists in selecting cases where nearly +allied species are separated by geographical barriers, and +proving that, in such cases, there is no cross-sterility.</p> + + +<p><i>Fertility of Domesticated Varieties.</i>—Some writers have +sought to explain the contrast between domesticated varieties +and natural species in respect of fertility when crossed, by +the consideration that it is only those natural species which +have proved themselves so far flexible as to continue fertile +under changed conditions of life that can have ever allowed +themselves to become domesticated. But although this +condition may well serve to explain the unimpaired fertility +under domestication of such species as for this very reason have +ever become domesticated, I fail to see how it explains the +further and altogether different fact, that this fertility continues +unimpaired between all the newly differentiated morphological +types which have been derived from the original specific type. +It is one thing that this type should continue fertile after +domestication: it is quite another thing that fertility should +continue as between all its modified descendants, even +although the amount of modification may extend much +further than that which usually obtains between different +natural species.</p> + + +<p><i>Testing for Cross-infertility</i> among varieties growing on +the same area is a much more crucial line of verification than +testing for unimpaired fertility between allied species which +occupy different areas, because while in the former case we +are dealing with "incipient species" with a view to ascertaining +whether the divergence which they have already undergone +is accompanied by physiological isolation, in the latter case +we can never be sure that two allied species, which are now +widely disconnected geographically, have always been so<span class="pagenum"><a name="Page_173" id="Page_173">[Pg 173]</a></span> +disconnected. They may both have originated on the same +area; or one may have diverged from the other before it +migrated from that area; or even if, when it migrated, it was +unchanged, and if in its new home it afterwards split into two +species by physiological selection, the newer species would +probably prove infertile, not only with its parent type, but +also with its grand-parent in any other part of the world.</p> + + +<p><i>Seebohm on Isolation.</i>—Seebohm is so strongly influenced +by the difficulty from "the swamping effects of free intercrossing," +that he is driven by it to adopt Asa Gray's hypothesis +of variations as teleological. Indeed, he goes as far as +Wagner, for he maintains that in no case can there be +divergence or multiplication of species without isolation. +He makes the important statement that "the more the +geographical distribution of birds is studied, the more doubtful +it seems to be that any species of bird has ever been differentiated +without the aid of geographical isolation" (<i>Charadriidae</i>, +p. 17). If this is true, it makes in favour of physiological +selection by showing the paramount importance of the +swamping effects of intercrossing, and consequent importance +of isolation. But it makes against physiological +selection by showing that the geographical form of isolation +is sufficient to explain all the cases of specific differentiation +in birds. But I must remember that the latter point rests +largely on negative inference, and that birds, owing to +their highly locomotive habits, are the class of animals where +physiological selection is likely to be most handicapped.</p> + + +<p><i>Herbert on Hybridization.</i>—Herbert tells us that when he +first astonished the Horticultural Society by laying before them +the results of his experiments on hybridization, his brother +botanists took serious alarm. For it appeared to them that +this "intermixture of species would confuse the labours +of botanists, and force them to work their way through +a wilderness of uncertainty." Therefore he was bluntly told<span class="pagenum"><a name="Page_174" id="Page_174">[Pg 174]</a></span> +by several of these gentlemen, "I do not thank you for your +mules." Now, although naturalists have travelled far and +learnt much since those days, it appears to me that a modern +evolutionist might still turn to the horticulturist with the same +words. For assuredly he has no reason to thank the +horticulturist for his mules, until he has found a satisfactory +answer to the question why it is that natural species differ so +profoundly as regards their capacity for hybridizing.</p> + + +<p><i>Advance on Herbert's Position.</i>—- If it be said that all my +work amounts to showing what Herbert said long ago—viz. +that the only true or natural distinction between organic types +is the sexual distinction—I answer that my work does much +more than this. For it shows that the principle of sterility +is the main condition to the differentiation, not merely of +species and genera, but also to the evolution of adaptations +everywhere, in higher as well as in lower taxonomic divisions. +Moreover, even though naturalists were everywhere to consent +to abandon specific designations, and, as Herbert advises, to +"entrench themselves behind genera," there would still remain +the facts of what are now called specific differences (of +the secondary or morphological kind), and by whatever name +these are called, they alike demand explanation at the hands +of the evolutionist.</p> + + +<p><i>Fritz Müller on Cross-infertility.</i>—Fritz Müller writes, +"Every plant requires, for the production of the strongest +possible and most prolific progeny, a certain amount of +difference between male and female elements which unite. +Fertility is diminished as well when this degree is too low +(in relatives too closely allied) as when it is too high (in +those too little related)." Then he adds, as a general rule, +"Species which are wholly sterile with pollen of the same +stock, and even with pollen of nearly allied stocks, will +generally be fertilized very readily by the pollen of another +species. The self-sterile species of the genus Abutilon,<span class="pagenum"><a name="Page_175" id="Page_175">[Pg 175]</a></span> +which are, on the other hand, so much inclined to hybridization, +afford a good example of this theory, which appears +to be confirmed also by Lobelia, Passiflora, and Oncidium" +(<i>American Naturalist</i>, vol. viii, pp. 223-4, 1874).</p> + + +<p><i>Different groups of plants exhibit remarkable differences in +the capability of their constituent species to hybridize.</i>—In so +far as these differences have reference only to first crosses, +they have no bearing either for or against my theory. Only +in so far as the differences extend to the production of fertile +hybrids does any question arise for me. First of all, therefore, +I must ascertain whether (or how far) there is any correlation +between groups whose species manifest aptitude to form first +crosses, and groups where first crosses manifest aptitude +to produce fertile hybrids. Next, whatever the result of this +inquiry should be, if I find that certain natural groups of +plants exhibit comparatively well-marked tendencies to form +fertile hybrids, the question will arise, Are these tendencies +correlated with <i>paucity</i> of species? If they are, the fact +would make strongly in favour of physiological selection. +For the fact would mean that in these natural groups, owing +to "the nature of the organisms" included under them, less +opportunity is given to physiological selection in its work of +differentiating specific types than is given by other natural +groups where the nature of the organism renders them more +prone to mutual sterility. But in prosecuting this branch +of verification, I must remember to allow for possibilities of +differential degrees of geographical isolation in the different +groups compared.</p> + +<p>On this subject Focke writes me as follows:—"In a +natural group (family, order, genus) showing considerable +variability in the structure of the flower, we may expect +to find [or do find] a greater number of mules than in +a group whose species are only distinguished by differences +in the shape of the leaves, or in growth, &c. I do not<span class="pagenum"><a name="Page_176" id="Page_176">[Pg 176]</a></span> +know, however, which in this connexion of things is the +cause and which the effect. A useful ancestral structure of +the flower may be conserved by an otherwise varying progeny, +on condition that the progress of diversity be not +disturbed by frequent intercrossings. [Therefore, if this +condition be satisfied, the structure of the flower in different +members of the group will continue constant: here the cause +of <i>constancy</i> in the flower (however much variability there +may be in the leaves, &c.) is its original <i>inability</i> to hybridize.] +On the other hand, in species or groups ready to +hybridize [or capable of hybridizing], the fixation of a new +specific type will require some change in the structure of the +flower, and a change considerable enough to alter the conditions +of fertilization. [Here the reason of the <i>in</i>constancy +of the flower in different members of the group is the +original <i>aptitude</i> of their ancestral forms to hybridize.] +Perhaps there is something in this suggestion, but certainly +there are other efficient physiological relations, which are +at present unknown. Your theory of physiological selection +may serve to explain many difficult facts."</p> + + +<p><i>The Importance of Prepotency.</i>—A. Kerner shows by means +of his own observations on sundry species of plants which +hybridize in the wild state, that they do so very much more +frequently if both, or even if only one of the parent forms be +rare in the neighbourhood. This fact can only be explained +by supposing that, even in species most prone to hybridizing +under Nature, there is some degree of prepotency of pollen +of the same species over that of the other species; so that +where both species are common, it is correspondingly rare +that the foreign pollen gets a chance. But if there were no +prepotency, the two species would blend; and this Kerner +supposes must actually take place wherever two previously +separated species, thus physiologically circumstanced, happen +to be brought together. (Kerner's paper is published in<span class="pagenum"><a name="Page_177" id="Page_177">[Pg 177]</a></span> +<i>Oester. Bot. Zeitschrift</i>, XXI, 1871, where he alludes to +sundry other papers of his own advocating similar views.)</p> + +<p>The relation of these observations to Jordan's <i>espèces affines</i> +is obvious. We have only to suppose that some such slight +and constant difference characterizes the sexual elements of +these allied varieties as demonstrably characterizes their +morphology, and we can understand how pollen-prepotency +would keep the forms distinct—such forms, therefore, being +so many records of such prepotency.</p> + +<p>Both from Kerner's work, and still more from that of +Jordan and Nägeli, I conclude that (at all events in plants) +prepotency is the way in which physiological selection +chiefly acts. That is to say, <i>sudden</i> and <i>extreme</i> variations in +the way of sexual incompatibility are probably rare, as compared +with some degree of prepotency. According as this +degree is small or great so will be the amount of the +corresponding separation. This view would show that in +plants the principle of physiological selection is one of +immensely widespread influence, causing (on the same +areas) more or less permanent varieties much below specific +rank. And when we remember on how delicate a balance +of physiological conditions complete correspondency of pollen +to ovules depends, we may be prepared to expect that the +phenomenon of prepotency is not of uncommon occurrence.</p> + + +<p><i>Self-fertilization and Variability.</i>—It occurred to Count +Berg Sagnitz that, if physiological selection is a true +principle in nature, vegetable species in which self-fertilization +obtains ought to be more rich in constant +varieties than are species in which cross-fertilization rules. +For, although even in the latter case physiological isolation +may occasionally arise, it cannot be of such habitual or +constant occurrence as it must be in the former case. +Acting on this idea, Count Berg Sagnitz applied himself to +ascertain whether there is any general correlation between the<span class="pagenum"><a name="Page_178" id="Page_178">[Pg 178]</a></span> +habit of self-fertilization and the fact of high variability; and +he says that in all the cases which he has hitherto investigated, +the correlation in question is unmistakable.</p> + + +<p><i>Additional Hypothesis concerning Physiological Selection.</i>—In +reciprocal crosses <i>A</i> × <i>B</i> is often more fertile than +<i>B</i> × <i>A</i>. If hybrid <i>AB</i> is more fertile with <i>A</i>, and hybrid +<i>BA</i> with <i>B</i>, than vice versa, there would be given a good +analogy on which to found the following hypothesis.</p> + +<p>Let <i>A</i> and <i>B</i> be two intergenerating groups in which +segregate fecundity is first beginning. Of the hybrids, <i>AB</i> +will be more fertile with <i>A</i>, and <i>BA</i> with <i>B</i>, than vice versa. +The interbreeding of <i>AB</i> with <i>A</i> will eventually modify +sexual characters of <i>A</i> by assimilating it to those of <i>AB</i>, +while the interbreeding of <i>BA</i> with <i>B</i> will similarly modify +sexual characters of <i>B</i> by assimilating it to those of <i>BA</i>. +Consequently, <i>A</i> will become more and more infertile with +<i>B</i>, while <i>B</i> becomes more and more infertile with <i>A</i>. Fewer +and fewer hybrids will thus be produced till mutual sterility +is complete.</p> + +<p>To sustain this hypothesis it would be needful to prove +experimentally, (1) that hybrid forms <i>AB</i> are more fertile +with <i>A</i> than with <i>B</i>, while hybrid forms <i>BA</i> are more fertile +with <i>B</i> than with <i>A</i> [or, it may be possible that the opposite +relations would be found to obtain, viz. that <i>AB</i> would be +more fertile with <i>B</i>, and <i>BA</i> with <i>A</i>]; (2) that, if so, +effect of intercrossing <i>AB</i> with <i>A</i> is to make progeny more +fertile with <i>A</i> than with <i>B</i>, while effect of intercrossing <i>BA</i> +with <i>B</i> is to make progeny more fertile with <i>B</i> than with <i>A</i>.</p> + +<p>Such experiments had best be tried with species where +there is already known to be a difference of fertility between +reciprocal crosses (e.g. Matthiola annua and M. glabra, see +<i>Origin of Species</i>, p. 244).</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_179" id="Page_179">[Pg 179]</a></p> + + + + +<h2>INDEX</h2> + + + +<ul class="index"> +<li class="ifrst">A.</li> + +<li class="indx"><span class="smcap">Allen</span>, Mr. J. A., on variation under nature, <a href="#Page_34">34</a>.</li> + +<li class="indx">Amixia, <a href="#Page_12">12</a>-<a href="#Page_28">28</a>, <a href="#Page_110">110</a>-<a href="#Page_115">115</a>, <a href="#Page_117">117</a>-<a href="#Page_133">133</a>.</li> + +<li class="indx">Apogamy, <a href="#Page_5">5</a>, <a href="#Page_6">6</a>, <a href="#Page_10">10</a>, <a href="#Page_18">18</a>, <a href="#Page_28">28</a>.</li> + + +<li class="ifrst">B.</li> + +<li class="indx"><span class="smcap">Belt</span>, on physiological selection, <a href="#Page_44">44</a>.</li> + +<li class="indx"><span class="smcap">Berg Sagnitz</span>, Count, on self-fertilization and variability, <a href="#Page_177">177</a>.</li> + +<li class="indx">Breeding, separate and segregate, <a href="#Page_5">5</a>.</li> + +<li class="indx">Butterflies of polar regions and Alps, <a href="#Page_133">133</a>.</li> + + +<li class="ifrst">C.</li> + +<li class="indx"><span class="smcap">Catchpool</span>, Mr., on physiological selection, <a href="#Page_44">44</a>, <a href="#Page_137">137</a>.</li> + +<li class="indx">Cross-infertility, <a href="#Page_46">46</a>;</li> +<li class="isub1">and varietal divergence, <a href="#Page_82">82</a>;</li> +<li class="isub1">and diversity of life, <a href="#Page_169">169</a>;</li> +<li class="isub1">and stability, <a href="#Page_170">170</a>;</li> +<li class="isub1">and specific differentiation, <a href="#Page_170">170</a>;</li> +<li class="isub1">in domesticated cattle, <a href="#Page_170">170</a>;</li> +<li class="isub1">testing for, <a href="#Page_172">172</a>;</li> +<li class="isub1">Fritz Müller on, <a href="#Page_174">174</a>.</li> + + +<li class="ifrst">D.</li> + +<li class="indx"><i>Darwin</i>, Charles, on isolation, <a href="#Page_2">2</a>, <a href="#Page_106">106</a>;</li> +<li class="isub1">on diversity under nature, <a href="#Page_31">31</a>;</li> +<li class="isub1">on the fertility of varieties, <a href="#Page_50">50</a>;</li> +<li class="isub1">on the origin of cross-infertility, <a href="#Page_51">51</a>;</li> +<li class="isub1">on distribution, <a href="#Page_68">68</a>;</li> +<li class="isub1">on prepotency, <a href="#Page_89">89</a>;</li> +<li class="isub1">on geographical isolation, <a href="#Page_101">101</a>, <a href="#Page_108">108</a>;</li> +<li class="isub1">on methodical selection, <a href="#Page_102">102</a>;</li> +<li class="isub1">on modification in large areas, <a href="#Page_103">103</a>;</li> +<li class="isub1">on the swamping effects of intercrossing, <a href="#Page_105">105</a>;</li> +<li class="isub1">on independent variability, <a href="#Page_109">109</a>;</li> +<li class="isub1">on domestic animals, <a href="#Page_110">110</a>.</li> + +<li class="indx"><span class="smcap">Delbœuf</span>, law of independent variability, <a href="#Page_13">13</a>.</li> + +<li class="indx">Differentiation under natural selection, <a href="#Page_37">37</a>.</li> + +<li class="indx">Diversity of life and cross-infertility, <a href="#Page_169">169</a>.</li> + +<li class="indx">Domesticated cattle and cross-infertility, <a href="#Page_170">170</a>, <a href="#Page_172">172</a>.</li> + + +<li class="ifrst">E.</li> + +<li class="indx">Evidences of physiological selection, <a href="#Page_62">62</a>.</li> + +<li class="indx">Evolution, monotypic and polytypic, <a href="#Page_21">21</a>, <a href="#Page_75">75</a>, <a href="#Page_102">102</a>, <a href="#Page_107">107</a>, <a href="#Page_112">112</a>, <a href="#Page_129">129</a>.</li> + +<li class="indx">Experimental research in physiological selection, <a href="#Page_85">85</a>.</li> + + +<li class="ifrst">F.</li> + +<li class="indx">Fertility of domesticated varieties, <a href="#Page_172">172</a>.</li> + +<li class="indx"><span class="smcap">Focke</span>, Herr, on hybridization, <a href="#Page_175">175</a>.</li> + + +<li class="ifrst">G.</li> + +<li class="indx"><span class="smcap">Galton</span>, Mr. Francis, law of regression, <a href="#Page_39">39</a>.</li> + +<li class="indx">General conclusions, <a href="#Page_144">144</a>.</li> + +<li class="indx">Geographical distribution and physiological selection, <a href="#Page_65">65</a>.</li> + +<li class="indx"><span class="smcap">Giard</span>, M., on apogamy, <a href="#Page_14">14</a>.</li> + +<li class="indx"><span class="pagenum"><a name="Page_180" id="Page_180">[Pg 180]</a></span><span class="smcap">Grabham</span>, Dr., on mollusca of Madeira, <a href="#Page_135">135</a>.</li> + +<li class="indx"><span class="smcap">Gulick</span>, Rev. J., on natural Selection as a mode of isolation, <a href="#Page_9">9</a>;</li> +<li class="isub1">on divergence, <a href="#Page_11">11</a>;</li> +<li class="isub1">on segregate breeding, <a href="#Page_19">19</a>;</li> +<li class="isub1">on geographical distribution, <a href="#Page_27">27</a>;</li> +<li class="isub1">on the prevention of intercrossing, <a href="#Page_127">127</a>;</li> +<li class="isub1">on Mr. Wallace's criticisms, <a href="#Page_151">151</a>.</li> + + +<li class="ifrst">H.</li> + +<li class="indx"><span class="smcap">Herbert</span>, on hybridization, <a href="#Page_173">173</a>;</li> +<li class="isub1">advance on his position, <a href="#Page_174">174</a>.</li> + +<li class="indx"><span class="smcap">Herdman</span>, Prof., on physiological isolation, <a href="#Page_123">123</a>.</li> + +<li class="indx">Historical sketch of opinions on isolation, <a href="#Page_101">101</a>.</li> + +<li class="indx">Homogamy, <a href="#Page_5">5</a>, <a href="#Page_6">6</a>;</li> +<li class="isub1">forms of, <a href="#Page_7">7</a>, <a href="#Page_19">19</a>, <a href="#Page_29">29</a>.</li> + +<li class="indx">Hybridization, <span class="smcap">Herbert</span> on, <a href="#Page_173">173</a>;</li> +<li class="isub1">in plants, <a href="#Page_175">175</a>.</li> + +<li class="indx">Hypothesis, additional, concerning physiological selection, <a href="#Page_178">178</a>.</li> + + +<li class="ifrst">I.</li> + +<li class="indx">Independent variability, <a href="#Page_12">12</a>-<a href="#Page_29">29</a>.</li> + +<li class="indx">Isolation, defined, <a href="#Page_2">2</a>;</li> +<li class="isub1">forms of, <a href="#Page_3">3</a>, <a href="#Page_6">6</a>;</li> +<li class="isub1">geographical, <a href="#Page_3">3</a>;</li> +<li class="isub1">discriminate and indiscriminate, <a href="#Page_5">5</a>;</li> +<li class="isub1">physiological, <a href="#Page_9">9</a>, <a href="#Page_41">41</a>, <a href="#Page_58">58</a>;</li> +<li class="isub1">its importance, <a href="#Page_39">39</a>;</li> +<li class="isub1">sketch of opinions on, <a href="#Page_101">101</a>;</li> +<li class="isub1">general conclusions, <a href="#Page_144">144</a>;</li> +<li class="isub1"><span class="smcap">Seebohm</span> on, <a href="#Page_173">173</a>.</li> + + +<li class="ifrst">J.</li> + +<li class="indx"><span class="smcap">Jordan</span>, M., on cross sterile varieties of plants, <a href="#Page_86">86</a>;</li> +<li class="isub1">his researches summarized, <a href="#Page_87">87</a>.</li> + + +<li class="ifrst">K.</li> + +<li class="indx"><span class="smcap">Kerner</span>, Prof. A., on prepotency, <a href="#Page_176">176</a>.</li> + + +<li class="ifrst">L.</li> + +<li class="indx"><span class="smcap">Lankester</span>, Prof. Ray, on divergent evolution, <a href="#Page_15">15</a>.</li> + +<li class="indx"><span class="smcap">Le Conte</span>, Prof., on fossil snails of steinheim, <a href="#Page_95">95</a>;</li> +<li class="isub1">on isolation, <a href="#Page_129">129</a>.</li> + +<li class="indx"><span class="smcap">Livingstone</span>, Dr. David, Quoted, <a href="#Page_123">123</a>.</li> + + +<li class="ifrst">M.</li> + +<li class="indx"><span class="smcap">Meldola</span>, Prof., on difficulty from intercrossing, <a href="#Page_121">121</a>.</li> + +<li class="indx">Misunderstandings of Physiological selection, <a href="#Page_59">59</a>.</li> + +<li class="indx">Monotypic evolution, see Evolution.</li> + +<li class="indx"><span class="smcap">Morgan</span>, Prof. Lloyd, on sterility, <a href="#Page_56">56</a>;</li> +<li class="isub1">on isolation, <a href="#Page_128">128</a>.</li> + +<li class="indx"><span class="smcap">Moulton</span>, Mr. Fletcher, an examination of Mr. Wallace's calculations on physiological selection, <a href="#Page_157">157</a>.</li> + +<li class="indx"><span class="smcap">Müller</span>, Fritz, on cross-infertility, <a href="#Page_174">174</a>.</li> + + +<li class="ifrst">N.</li> + +<li class="indx"><span class="smcap">Nägeli</span>, on isolation, <a href="#Page_76">76</a>;</li> +<li class="isub1">on synoicy, <a href="#Page_78">78</a>, <a href="#Page_82">82</a>.</li> + +<li class="indx">Natural selection, a form of discriminate isolation, <a href="#Page_9">9</a>, <a href="#Page_10">10</a>, <a href="#Page_23">23</a>;</li> +<li class="isub1">leads to monotypic evolution, <a href="#Page_24">24</a>-<a href="#Page_29">29</a>;</li> +<li class="isub1">difficulties of, <a href="#Page_41">41</a>, <a href="#Page_51">51</a>.</li> + + +<li class="ifrst">P.</li> + +<li class="indx">Panmixia, <a href="#Page_12">12</a>.</li> + +<li class="indx">Physiological selection, <a href="#Page_9">9</a>, <a href="#Page_41">41</a>;</li> +<li class="isub1">summarized, <a href="#Page_58">58</a>;</li> +<li class="isub1">misunderstandings of, <a href="#Page_59">59</a>;</li> +<li class="isub1">evidences of, <a href="#Page_81">81</a>-<a href="#Page_119">119</a>;</li> +<li class="isub1">and Weismannism, <a href="#Page_169">169</a>;</li> +<li class="isub1">additional hypothesis, <a href="#Page_178">178</a>.</li> + +<li class="indx">Polytypic evolution, see Evolution.</li> + +<li class="indx">Prepotency, <a href="#Page_89">89</a>;</li> +<li class="isub1">importance of, <a href="#Page_176">176</a>.</li> + + +<li class="ifrst">S.</li> + +<li class="indx"><span class="smcap">Schmidt</span>, Prof. Oscar, on domesticated cattle, <a href="#Page_171">171</a>.</li> + +<li class="indx"><span class="smcap">Seebohm</span> on isolation, <a href="#Page_173">173</a>.</li> + +<li class="indx">Segregation, <a href="#Page_28">28</a>.</li> + +<li class="indx">Selection, physiological, see Physiological selection.</li> + +<li class="indx">Self-fertilization and variability, <a href="#Page_177">177</a>.</li> + +<li class="indx">Snails of Sandwich Islands, <a href="#Page_16">16</a>, <a href="#Page_130">130</a>;</li> +<li class="isub1">fossil of Steinheim, <a href="#Page_95">95</a>.</li> + +<li class="indx">Specific differentiation and cross-infertility, <a href="#Page_170">170</a>.</li> + +<li class="indx">Stability and cross-infertility, <a href="#Page_170">170</a>.</li> + +<li class="indx"><span class="pagenum"><a name="Page_181" id="Page_181">[Pg 181]</a></span>Synoicy, <a href="#Page_78">78</a>.</li> + + +<li class="ifrst">T.</li> + +<li class="indx">Topographical distribution and physiological selection, <a href="#Page_74">74</a>;</li> +<li class="isub1">of varieties, <a href="#Page_81">81</a>.</li> + +<li class="indx">Transformation, serial and divergent, <a href="#Page_21">21</a>, <a href="#Page_121">121</a>.</li> + + +<li class="ifrst">V.</li> + +<li class="indx">Variability and self-fertilization, <a href="#Page_177">177</a>.</li> + +<li class="indx">Variation in birds, <a href="#Page_34">34</a>.</li> + +<li class="indx">Varieties, topographical distribution of, <a href="#Page_81">81</a>.</li> + + +<li class="ifrst">W.</li> + +<li class="indx"><span class="smcap">Wagner</span>, Maritz, <a href="#Page_3">3</a>;</li> +<li class="isub1">on geographical isolation, <a href="#Page_76">76</a>;</li> +<li class="isub1">quoted, <a href="#Page_103">103</a>;</li> +<li class="isub1">law of migration, <a href="#Page_111">111</a>.</li> + +<li class="indx"><span class="smcap">Wallace</span>, Mr. A. R., <a href="#Page_3">3</a>, <a href="#Page_17">17</a>;</li> +<li class="isub1">quoted, <a href="#Page_34">34</a>, <a href="#Page_47">47</a>, <a href="#Page_51">51</a>, <a href="#Page_57">57</a>,<a href="#Page_130">130</a>-<a href="#Page_136">136</a>;</li> +<li class="isub1">criticized by Gulick, <a href="#Page_152">152</a>.</li> + +<li class="indx"><span class="smcap">Weismann</span>, Prof., on geographical isolation, <a href="#Page_76">76</a>, <a href="#Page_114">114</a>-<a href="#Page_118">118</a>.</li> + +<li class="indx">Weismannism and physiological selection, <a href="#Page_169">169</a>.</li> +</ul> + +<hr class="full" /> + + + + +<h2>TITLE LIST OF OPEN COURT PUBLICATIONS +ARRANGED ALPHABETICALLY BY AUTHORS</h2> + + +<p>ANESAKI, M.</p> + +<p>345. BUDDHIST AND CHRISTIAN GOSPELS, Being Gospel Parallels +from Pali Texts. Now first compared from the originals +by Albert J. Edmunds. Edited with parallels and notes from +the Chinese Buddhist Triptaka by <i>M. 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Trans., +Stanford, London, 1873).</p></div> + +<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> I may here most conveniently define the senses in which all the +following terms will be used throughout the present discussion:—<i>Species</i> +of isolation are, as above stated, homogamy and apogamy, or isolation +as discriminate and indiscriminate. <i>Forms</i> of isolation are modes of +isolation, such as the geographical, the sexual, the instinctive, or any +other of the numerous means whereby isolation of either species may be +secured. <i>Cases</i> of isolation are the instances in which any of the forms +of isolation may be at work: thus, if a group of <i>n</i> intergenerants be +segregated into five groups, <i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>, <i>e</i>, then, before the segregation there +would have been one case of isolation, but after the segregation there +would be five such cases.</p></div> + +<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> <i>Divergent Evolution through Cumulative Segregation</i> (<i>Zool. Journal, +Linn. Soc.</i>, vol. xx. pp. 189-274).</p></div> + +<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> The passage proceeds to show that in view of this consideration we +have a strong additional reason for rejecting the <i>a priori</i> dogma that all +specific characters must necessarily be useful characters. For it is evident +that any divergence of specific character which is brought about in this +way need not present any utilitarian significance—although, of course, +natural selection will ensure that it shall never be deleterious.</p></div> + +<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> <i>Revue Scientifique</i>, Nov. 23, 1889.</p></div> + +<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> <i>Nature</i>, Oct. 10, 1889, p. 568.</p></div> + +<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> e. g. p. 81.</p></div> + +<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a> See Chapter xxiii. vol. ii. p. 262. (Edition of 1888.)</p></div> + +<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a> This term may here be taken as equivalent to Isolation.</p></div> + +<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a> <i>Zool. Journal Lin. Soc.</i>, vol. xix. pp. 337-411.</p></div> + +<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a> <i>Ibid.</i>, vol. xx. pp. 202-212.</p></div> + +<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a> <i>Zool. Journal Lin. Soc.</i>, vol. xxiii. p. 313.</p></div> + +<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a> <i>See Nineteenth Century</i>, January, 1887, pp. 61, 62.</p></div> + +<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a> <i>Nicaragua</i>, p. 207.</p></div> + +<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a> <i>Nature</i>, vol. xxxi. p. 4.</p></div> + +<div class="footnote"><p><a name="Footnote_18_18" id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a> <i>Zool. Journal, Lin. Soc.</i>, vol. xix. pp. 337-411 (1886); and for +Mr. Gulick's papers, <i>ibid.</i>, vol. xx. pp. 189-274 (1887), vol. xxiii. +pp. 312-380 (1889). Mr. Gulick has recently drawn my attention, in +a private letter, to the fact that as early as 1872 a paper of his was +read at the British Association, bearing the title <i>Diversity of Evolution +under one set of External Conditions</i>, and that here the principle of +physiological segregation is stated. Although it does not appear that +Mr. Gulick then appreciated the great importance of this principle, it +entitles him to claim priority.</p></div> + +<div class="footnote"><p><a name="Footnote_19_19" id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a> <i>Darwinism</i>, p. 169.</p></div> + +<div class="footnote"><p><a name="Footnote_20_20" id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a> <i>Origin of Species</i>, p. 136.</p></div> + +<div class="footnote"><p><a name="Footnote_21_21" id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a> <i>Darwinism</i>, p. 152.</p></div> + +<div class="footnote"><p><a name="Footnote_22_22" id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a> <i>Origin of Species</i>, pp. 44, 45.</p></div> + +<div class="footnote"><p><a name="Footnote_23_23" id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a> <i>Origin of Species</i>, ed. 6, pp. 134, 135.</p></div> + +<div class="footnote"><p><a name="Footnote_24_24" id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a> <i>Archives des Sciences physiques et naturelles</i> (Genève), vol. liii. (1875), +pp. 211-236.</p></div> + +<div class="footnote"><p><a name="Footnote_25_25" id="Footnote_25_25"></a><a href="#FNanchor_25_25"><span class="label">[25]</span></a> <i>Remarques sur le fait de l'existence en société à l'état sauvage des +espèces végétales affines et sur d'autres faits relatifs à la question de +l'espèce</i>, par Alexis Jordan; lues au congrès de l'Association Française +pour l'Avancemeat des Sciences, 2<sup>m</sup>e session, Lyon, séance de 28 Août, +1873.</p></div> + +<div class="footnote"><p><a name="Footnote_26_26" id="Footnote_26_26"></a><a href="#FNanchor_26_26"><span class="label">[26]</span></a> <i>Evolution and its Relations to Religious Thought</i>, &c. pp. 236-7.</p></div> + +<div class="footnote"><p><a name="Footnote_27_27" id="Footnote_27_27"></a><a href="#FNanchor_27_27"><span class="label">[27]</span></a> <i>Life and Letters</i>, vol. ii. p. 28.</p></div> + +<div class="footnote"><p><a name="Footnote_28_28" id="Footnote_28_28"></a><a href="#FNanchor_28_28"><span class="label">[28]</span></a> <i>Ibid.</i></p></div> + +<div class="footnote"><p><a name="Footnote_29_29" id="Footnote_29_29"></a><a href="#FNanchor_29_29"><span class="label">[29]</span></a> <i>Origin of Species</i>, p. 80, 6th ed. (1872).</p></div> + +<div class="footnote"><p><a name="Footnote_30_30" id="Footnote_30_30"></a><a href="#FNanchor_30_30"><span class="label">[30]</span></a> <i>Life and Letters</i>, vol. iii. p. 158.</p></div> + +<div class="footnote"><p><a name="Footnote_31_31" id="Footnote_31_31"></a><a href="#FNanchor_31_31"><span class="label">[31]</span></a> <i>Ibid.</i> p. 159.</p></div> + +<div class="footnote"><p><a name="Footnote_32_32" id="Footnote_32_32"></a><a href="#FNanchor_32_32"><span class="label">[32]</span></a> <i>Ibid.</i> p. 160.</p></div> + +<div class="footnote"><p><a name="Footnote_33_33" id="Footnote_33_33"></a><a href="#FNanchor_33_33"><span class="label">[33]</span></a> The analogy is radically unsound because unconscious selection +differs from methodical selection only in the <i>degree</i> of "separation" +which it effects. These two forms of selection do not necessarily differ +from one another in regard to the <i>number</i> of characters which are being +simultaneously diversified; for while it may be the object of methodical +selection to breed for modification of a single character alone, it may, +on the other hand, be the result of unconscious selection to diversify an +originally uniform stock, as Darwin himself observes with regard to +horse-breeding. The real distinction between monotypic and polytypic +evolution is, not at all with reference to the <i>degree</i> of isolation (i. e. +<i>amount</i> of "separation"), but to the <i>number of cases</i> in which any +efficient degree of it occurs (i. e. whether in but a single case, or in two +or more cases).</p></div> + +<div class="footnote"><p><a name="Footnote_34_34" id="Footnote_34_34"></a><a href="#FNanchor_34_34"><span class="label">[34]</span></a> <i>Life and Letters</i>, vol. iii. pp. 157-8.</p></div> + +<div class="footnote"><p><a name="Footnote_35_35" id="Footnote_35_35"></a><a href="#FNanchor_35_35"><span class="label">[35]</span></a> <i>Ibid.</i> pp. 157-8.</p></div> + +<div class="footnote"><p><a name="Footnote_36_36" id="Footnote_36_36"></a><a href="#FNanchor_36_36"><span class="label">[36]</span></a> <i>Life and Letters</i>, vol. iii. p. 161.</p></div> + +<div class="footnote"><p><a name="Footnote_37_37" id="Footnote_37_37"></a><a href="#FNanchor_37_37"><span class="label">[37]</span></a> Page 81. The three forms of isolation mentioned are, "from +haunting different stations, from breeding at slightly different seasons, or +from the individuals of each variety preferring to pair together."</p></div> + +<div class="footnote"><p><a name="Footnote_38_38" id="Footnote_38_38"></a><a href="#FNanchor_38_38"><span class="label">[38]</span></a> <i>Life and Letters</i>, vol. iii. p. 159.</p></div> + +<div class="footnote"><p><a name="Footnote_39_39" id="Footnote_39_39"></a><a href="#FNanchor_39_39"><span class="label">[39]</span></a> <i>Life and Letters</i>, vol. iii. p. 155.</p></div> + +<div class="footnote"><p><a name="Footnote_40_40" id="Footnote_40_40"></a><a href="#FNanchor_40_40"><span class="label">[40]</span></a> <i>Variation</i>, &c., vol. ii. p. 262.</p></div> + +<div class="footnote"><p><a name="Footnote_41_41" id="Footnote_41_41"></a><a href="#FNanchor_41_41"><span class="label">[41]</span></a> <i>Life and Letters</i>, vol. iii. p. 161.</p></div> + +<div class="footnote"><p><a name="Footnote_42_42" id="Footnote_42_42"></a><a href="#FNanchor_42_42"><span class="label">[42]</span></a> <i>Die Darwin'sche Theorie und das Migrationsgesetz</i> (1868): <i>Ueber +den Einfluss der geographischen Isolirung</i>, &c. (1870).</p></div> + +<div class="footnote"><p><a name="Footnote_43_43" id="Footnote_43_43"></a><a href="#FNanchor_43_43"><span class="label">[43]</span></a> For instance, speaking of common, or continuous areas, he says:—"In +this case a constant variety, or new species, cannot be produced, +because the free crossing of a new variety with the old unaltered stock +will always cause it to revert to the original type; in other words, will +destroy the new form. The formation of a real variety, which Darwin, +as we know, regards as the commencement of a new species, will only +succeed when a few individuals, having crossed the barrier of their +habitat, are able to separate themselves for a long time from the old +stock." And the last sentence, given as a summary of his whole +doctrine, is—"The geographical isolation of the form, a necessary +consequence of migration, is the cause of its typical character."</p></div> + +<div class="footnote"><p><a name="Footnote_44_44" id="Footnote_44_44"></a><a href="#FNanchor_44_44"><span class="label">[44]</span></a> <i>Ueber den Einfluss der Isolirung auf die Artbildung</i> (1872).</p></div> + +<div class="footnote"><p><a name="Footnote_45_45" id="Footnote_45_45"></a><a href="#FNanchor_45_45"><span class="label">[45]</span></a> <i>Loc. cit.</i>, p. 43.</p></div> + +<div class="footnote"><p><a name="Footnote_46_46" id="Footnote_46_46"></a><a href="#FNanchor_46_46"><span class="label">[46]</span></a> <i>Physiological Selection</i>, pp. 348, 389.</p></div> + +<div class="footnote"><p><a name="Footnote_47_47" id="Footnote_47_47"></a><a href="#FNanchor_47_47"><span class="label">[47]</span></a> <i>Loc. cit.</i>, p. 54.</p></div> + +<div class="footnote"><p><a name="Footnote_48_48" id="Footnote_48_48"></a><a href="#FNanchor_48_48"><span class="label">[48]</span></a> <i>Nature</i>, vol. xliii. p. 410, and vol. xliv. p. 29.</p></div> + +<div class="footnote"><p><a name="Footnote_49_49" id="Footnote_49_49"></a><a href="#FNanchor_49_49"><span class="label">[49]</span></a> <i>Darwinism</i>, p. 143.</p></div> + +<div class="footnote"><p><a name="Footnote_50_50" id="Footnote_50_50"></a><a href="#FNanchor_50_50"><span class="label">[50]</span></a> In Appendix to H. M. Stanley's <i>How I found Livingstone</i>, 2nd ed. +London, 1872, p. 715.</p></div> + +<div class="footnote"><p><a name="Footnote_51_51" id="Footnote_51_51"></a><a href="#FNanchor_51_51"><span class="label">[51]</span></a> <i>Animal Life and Intelligence</i>, pp. 98, 99 (1890-1891).</p></div> + +<div class="footnote"><p><a name="Footnote_52_52" id="Footnote_52_52"></a><a href="#FNanchor_52_52"><span class="label">[52]</span></a> <i>The Factors of Evolution</i> (1891).</p></div> + +<div class="footnote"><p><a name="Footnote_53_53" id="Footnote_53_53"></a><a href="#FNanchor_53_53"><span class="label">[53]</span></a> <i>Darwinism</i>, p. 151.</p></div> + +<div class="footnote"><p><a name="Footnote_54_54" id="Footnote_54_54"></a><a href="#FNanchor_54_54"><span class="label">[54]</span></a> <i>Ibid.</i></p></div> + +<div class="footnote"><p><a name="Footnote_55_55" id="Footnote_55_55"></a><a href="#FNanchor_55_55"><span class="label">[55]</span></a> <i>Loc. cit.</i>, p. 151.</p></div> + +<div class="footnote"><p><a name="Footnote_56_56" id="Footnote_56_56"></a><a href="#FNanchor_56_56"><span class="label">[56]</span></a> Namely, <i>Lycaena denzelii</i>, <i>L. pheretes</i>, <i>Argynnis pales</i>, <i>Erebia +mante</i>.</p></div> + +<div class="footnote"><p><a name="Footnote_57_57" id="Footnote_57_57"></a><a href="#FNanchor_57_57"><span class="label">[57]</span></a> Since the above was written, I have heard of some cases which seem +to present greater difficulties to our theory than those above quoted. +These refer to some of the numerous species of land mollusca which +inhabit the isolated rocks near Madeira (Dezertas). My informant is +Dr. Grabham, who has himself investigated the matter, and reports +as follows:— +</p><p> +"It is no uncommon thing to meet with examples of the same species, +sub-fossil, recent, and living upon one spot, and presenting no variation +in the long record of descent." Then, after naming these examples, he +adds, "All seem to vary immediately on attaining new ground, assuming +many aspects in different districts." +</p><p> +Unquestionably these statements support, in a very absolute manner, +Mr. Wallace's opinion, while making directly against my own. It is +but fair, however, to add that the cases are not numerous (some half-dozen +at the most, and all within the limits of a single genus), and that, +even in the opinion of my informant himself, the facts have not hitherto +been sufficiently investigated for any decisive judgement to be formed +upon them.</p></div> + +<div class="footnote"><p><a name="Footnote_58_58" id="Footnote_58_58"></a><a href="#FNanchor_58_58"><span class="label">[58]</span></a> Vol. xliii. p. 127.</p></div> + +<div class="footnote"><p><a name="Footnote_59_59" id="Footnote_59_59"></a><a href="#FNanchor_59_59"><span class="label">[59]</span></a> This refers to what I understand Mr. Wallace to say in the <i>Nature</i> +correspondence is the supposition on which his own theory of the origin +of species by cross-infertility is founded. But in the original statement +of that theory itself, it is everywhere "supposed" that when species are +originated by cross-infertility, the <i>initial</i> change <i>is</i> the physiological +change. In his original statement of that theory, therefore, he literally +went further than I had gone in my "original paper," with reference to +supposing the physiological change to be the initial change. I do not +doubt that this is due to some oversight of expression; but it is curious +that, having made it, he should still continue his endeavour to fix exactly +the same oversight upon me.</p></div> + +<div class="footnote"><p><a name="Footnote_60_60" id="Footnote_60_60"></a><a href="#FNanchor_60_60"><span class="label">[60]</span></a> "Positive segregation" is Mr. Gulick's term for forms of homogamy +other than that which is due to selective fertility. Of these other, +or "positive" forms, natural selection is one; but as it is far from +being the <i>only</i> one, the criticism points out that utility is not the +<i>only</i> conserving principle with which selective fertility may be associated.</p></div> + +<div class="footnote"><p><a name="Footnote_61_61" id="Footnote_61_61"></a><a href="#FNanchor_61_61"><span class="label">[61]</span></a> By Intensive Segregation Mr. Gulick means what I have called Independent +Variability.</p></div> + +<div class="footnote"><p><a name="Footnote_62_62" id="Footnote_62_62"></a><a href="#FNanchor_62_62"><span class="label">[62]</span></a> His sentence, "all fertility not correlated with some <i>useful</i> variation +has a constant tendency to effect its own elimination," still further +restricts the possible action of physiological selection to cases where at +least one of the other forms of homogamy with which it is associated is +natural selection. Or, in other words, it is represented that physiological +selection must always be associated with natural selection, even if it be +likewise associated with any other form of exclusive breeding. But as +this further limitation appears to me self-evidently unjustifiable (seeing +that utility is not the only possible means of securing effective isolation) +I here neglect it, and take the wider ground marked out above. It is +needless to say that this is giving Mr. Wallace every possible advantage, +by not holding him to his still narrower ground.</p></div> + +<div class="footnote"><p><a name="Footnote_63_63" id="Footnote_63_63"></a><a href="#FNanchor_63_63"><span class="label">[63]</span></a> In our <i>Nature</i> correspondence of 1890-1891, Mr. Wallace remarked: +"If Dr. Romanes will carefully work out numerically (as I have +attempted to do) a few cases showing the preservative and accumulative +agency of pure physiological selection within an otherwise undifferentiated +species, he will do more for his theory than volumes of general disquisition +or any number of assertions that it <i>does</i> possess this power." +Several months before this was written I had already in my hands +Mr. Moulton's letter, with its accompanying calculations.</p></div> + +<div class="footnote"><p><a name="Footnote_64_64" id="Footnote_64_64"></a><a href="#FNanchor_64_64"><span class="label">[64]</span></a> As, for example, in the case of sexuality in general. It is not to +the advantage of such individual male Arthropoda as perish after the +performance of the sexual act that they should perform it; but its performance +is necessary for the perpetuation of their species.—G. J. R.</p></div> + +<div class="footnote"><p><a name="Footnote_65_65" id="Footnote_65_65"></a><a href="#FNanchor_65_65"><span class="label">[65]</span></a> In this anticipation Mr. Moulton is right. The well-known botanist, +Mr. Bennett, read a most interesting paper on the subject before the +British Association in 1881. His results have since been corroborated +by other observers. In particular, Mr. R. M. Christy has recorded the +movements of 76 insects while visiting at least 2,400 flowers. (<i>Entomologist</i>, +July 1883, and <i>Zool. Journal Lin. Soc.</i>, August 1883.) The +following is an analysis of his results. In the case of butterflies, in +twelve observations on nearly as many species, there are recorded +altogether 99 visits to fifteen species of flowers; and of these 99 visits 94 +were constant to the same species, leaving only 5 visits to any other, +or second species. In the case of the hive-bee, there were 8 individuals +observed: these visited altogether 258 flowers, and all the visits paid by +the same individual were paid to the same species in each of the eight +cases. Lastly, as regards bumble-bees, there were altogether observed +55 individuals belonging to four species. These paid altogether 1751 +visits to 94 species of flowers. Of these 1751 visits, 1605 were paid to +one species, 131 to two species, 16 to three, 6 to four, and 1 to five. +Adding all these results together, we find that 75 insects (butterflies +and bees) visited 117 species of flowers: of these visits, 1957 were +constant to one species of flower; 136 were paid also to a second +species, 16 also to a third, 6 also to a fourth, and 1 also to a fifth. Or, +otherwise stated, while 1957 were absolutely constant, from such absolute +constancy there were only 159 deviations. Moreover, if we eliminate +three individual humble bees, which paid nearly an equal number of visits +to two species (and, therefore, would have ministered to the work of +physiological selection almost as well as the others), the 159 deviations +become reduced to 72, or about four per cent. of the whole.—G. J. R.</p></div> + +<div class="footnote"><p><a name="Footnote_66_66" id="Footnote_66_66"></a><a href="#FNanchor_66_66"><span class="label">[66]</span></a> Here follows the Appendix presenting the calculations on which the +above results are founded; but it seems unnecessary to reproduce it +on the present occasion.—G. J. R.</p></div> + +<div class="footnote"><p><a name="Footnote_67_67" id="Footnote_67_67"></a><a href="#FNanchor_67_67"><span class="label">[67]</span></a> <i>Doctrine of Descent and Darwinism</i>, Eng. trans. p. 139.</p></div> + +<hr class="full" /> + +<div class="transnote"> +<h3>Transcriber's Notes</h3> + +<p>In paragraph 4 of page 171 "peculiarites" has been corrected to +"peculiarities"</p> + +<p>Variable spacing in the following abbreviation was left as it was in +the original: "i. e." (22 instances) and "i.e." (14 instances).</p> + +<p>Different hyphenation patterns were left as in the original text:</p> + + +<div class="center"> +<table border="1" cellpadding="4" cellspacing="0" summary=""> +<tr><td align="left">prepotent (1 instance)</td><td align="left">pre-potent (1 instance)</td></tr> +<tr><td align="left">presupposes (1)</td><td align="left">pre-supposes (1)</td></tr> +<tr><td align="left">reacting(5)</td><td align="left">re-acting (1)</td></tr> +<tr><td align="left">restatement (1)</td><td align="left">re-statement (2)</td></tr> +<tr><td align="left">superinduced (2)</td><td align="left">super-induced (1)</td></tr> +</table></div> +</div> + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of Darwin, and After Darwin (Vol 3 of 3), by +George John Romanes + +*** END OF THIS PROJECT GUTENBERG EBOOK DARWIN, AFTER DARWIN (VOL 3 OF 3) *** + +***** This file should be named 37777-h.htm or 37777-h.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/3/7/7/7/37777/ + +Produced by Marilynda Fraser-Cunliffe, LN Yaddanapudi and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Darwin, and After Darwin (Vol 3 of 3) + Post-Darwinian Questions: Isolation and Physiological Selection + +Author: George John Romanes + +Release Date: October 17, 2011 [EBook #37777] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK DARWIN, AFTER DARWIN (VOL 3 OF 3) *** + + + + +Produced by Marilynda Fraser-Cunliffe, LN Yaddanapudi and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + + + + + + + + +DARWIN, AND AFTER DARWIN + +III + +POST-DARWINIAN QUESTIONS + +ISOLATION AND PHYSIOLOGICAL SELECTION + + +BY THE SAME AUTHOR. + +DARWIN, AND AFTER DARWIN. An Exposition of the Darwinian Theory and a +Discussion of Post-Darwinian Questions. + + 1. THE DARWINIAN THEORY. With portrait of Darwin. 460 pages. 125 + illustrations. Cloth, $2.00. + + 2. POST-DARWINIAN QUESTIONS. Edited by Prof. C. Lloyd Morgan. With + portrait of G. J. Romanes. 338 pages. Cloth, $1.50. + + 3. POST-DARWINIAN QUESTIONS. ISOLATION AND PHYSIOLOGICAL SELECTION. + Edited by Prof. C. Lloyd Morgan. With portrait of Mr. J. T. + Gulick. 181 pages. Cloth, $1.00. + + All three volumes together, $4.00 net. + +AN EXAMINATION OF WEISMANNISM. With portrait of Weismann. 236 pages. +Cloth, $1.00. + +THOUGHTS ON RELIGION. Edited by Charles Gore, M.A., Canon of +Westminster. Third Edition. 184 pages. Cloth, gilt top, $1.25. + +THE OPEN COURT PUBLISHING COMPANY, +324 DEARBORN STREET, CHICAGO. + +[Illustration: Frontispiece--John J. Gulick] + + + + +DARWIN, AND AFTER DARWIN + +_AN EXPOSITION OF THE DARWINIAN THEORY +AND A DISCUSSION OF +POST-DARWINIAN QUESTIONS_ + +BY THE LATE + +GEORGE JOHN ROMANES, M.A., LL.D., F.R.S. + +_Honorary Fellow of Gonville and Caius College, Cambridge_ + + +III + +POST-DARWINIAN QUESTIONS + +ISOLATION +AND PHYSIOLOGICAL SELECTION + + +Chicago +THE OPEN COURT PUBLISHING COMPANY +1906 + +CHAPTER 1. COPYRIGHTED BY +THE OPEN COURT PUBLISHING CO. +1897. + + + +The Lakeside Press +R. R. DONNELLEY & SONS CO., CHICAGO + + + + +PREFACE + + +Of the six chapters which constitute this concluding volume of G. J. +Romanes' _Darwin, and after Darwin_, three, the first two and the last, +were in type at the time of his death. I have not considered myself at +liberty to make any alterations of moment in these chapters. For the +selection and arrangement of all that is contained in the other three +chapters I am wholly responsible. + +Two long controversial Appendices have been omitted. Those marked A and +B remain in accordance with the author's expressed injunctions. In a +third, marked C, a few passages from the author's note-books or MSS. +have been printed. + +The portrait of the Rev. J. Gulick, which forms the frontispiece, was +prepared for this volume before the author's death. Mr. Gulick's chief +contributions to the theory of physiological selection are to be found +in the Linnean Society's _Journal_ (_Zoology_, vols. xx and xxiii), and +in four letters to _Nature_ (vol. xli. p. 536; vol. xlii. pp. 28 and +369; and vol. xliv. p. 29). + +I have to thank Mr. Francis Galton, D.C.L., F.R.S. and Mr. F. Howard +Collins for valuable assistance generously rendered for the sake of one +whom all who knew him held dear. For he was, if I may echo the words of +Huxley, "a friend endeared to me, as to so many others, by his kindly +nature, and justly valued by all his colleagues for his powers of +investigation and his zeal for the advancement of science." + +C. LLOYD MORGAN. + +BRISTOL, _May 1897_. + + + + +CONTENTS + + +CHAPTER I. +ISOLATION 1 + +CHAPTER II. +ISOLATION (_continued_) 28 + +CHAPTER III. +PHYSIOLOGICAL SELECTION 41 + +CHAPTER IV. +EVIDENCES OF PHYSIOLOGICAL SELECTION 62 + +CHAPTER V. +FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION 81 + +CHAPTER VI. +A BRIEF HISTORY OF ISOLATION AS A FACTOR IN +ORGANIC EVOLUTION 101 + +GENERAL CONCLUSIONS 144 + +APPENDIX A. MR. GULICK'S CRITICISM OF MR. WALLACE'S +VIEWS ON PHYSIOLOGICAL SELECTION 151 + +APPENDIX B. AN EXAMINATION BY MR. FLETCHER +MOULTON OF MR. WALLACE'S CALCULATION TOUCHING +THE POSSIBILITY OF PHYSIOLOGICAL SELECTION +EVER ACTING ALONE 157 + +APPENDIX C. SOME EXTRACTS FROM THE AUTHOR'S +NOTE-BOOKS 169 + + + + +_ISOLATION_ + + + + +CHAPTER I. + +ISOLATION. + + +This treatise will now draw to a close by considering what, in my +opinion, is one of the most important principles that are concerned in +the process of organic evolution--namely, Isolation. I say in _my_ +opinion such is the case, because, although the importance of isolation +is more or less recognized by every naturalist, I know of only one other +who has perceived all that the principle involves. This naturalist is +the Rev. J. Gulick, and to his essays on the subject I attribute a +higher value than to any other work in the field of Darwinian thought +since the date of Darwin's death[1]. For it is now my matured conviction +that a new point of departure has here been taken in the philosophy of +Darwinism, and one which opens up new territories for scientific +exploration of an endlessly wide and varied character. Indeed I believe, +with Mr. Gulick, that in the principle of Isolation we have a principle +so fundamental and so universal, that even the great principle of +Natural Selection lies less deep, and pervades a region of smaller +extent. Equalled only in its importance by the two basal principles of +Heredity and Variation, this principle of Isolation constitutes the +third pillar of a tripod on which is reared the whole superstructure of +organic evolution. + + [1] It will be remembered that I regard Weismann's theory of + heredity, with all its deductive consequences, as still _sub + judice_. + +By isolation I mean simply the prevention of intercrossing between a +separated section of a species or kind and the rest of that species or +kind. Whether such a separation be due to geographical barriers, to +migration, or to any other state of matters leading to exclusive +breeding within the separated group, I shall indifferently employ the +term isolation for the purpose of designating what in all cases is the +same result--namely, a prevention of intercrossing between A and B, +where A is the separated portion and B the rest of the species or kind. + +The importance of isolation as against dissimilar forms has always been +fully appreciated by breeders, fanciers, horticulturists, &c., who are +therefore most careful to prevent their pedigree productions from +intercrossing with any other stock. Isolation is indeed, as Darwin has +observed, "the corner-stone of the breeder's art." And similarly with +plants and animals in a state of nature: unless intercrossing with +allied (i.e. dissimilar) forms is prevented, the principle of heredity +is bound to work for uniformity, by blending the dissimilar types in +one: only when there is exclusive breeding of similarly modified forms +can the principle of heredity work in the direction of change--i.e. of +evolution. + +Now, the forms of isolation--or the conditions which may lead to +exclusive breeding--are manifold. One of the most important, as well as +the most obvious, is geographical isolation; and no one questions that +this has been an important factor in the process of evolution, although +opinions still vary greatly as to the degree of its importance in this +respect. At one end of the series we may place the opinion of Mr. +Wallace, who denies that any of what may be termed the evolutionary +effect of geographical isolation is due to "influence exerted by +isolation _per se_." This effect, he says, is to be ascribed exclusively +to the fact that a geographically isolated portion of a species must +always encounter a change of environment, and therefore a new set of +conditions necessitating a new set of adaptations at the hands of +natural selection[2]. At the other end of the series we must place the +opinion of Moritz Wagner, who many years ago published a masterly +essay[3], the object of which was to prove that, in the absence of +geographical isolation (including migration), natural selection would be +powerless to effect any change of specific type. For, he argued, the +initial variations on which the action of this principle depends would +otherwise be inevitably swamped by free intercrossing. Wagner adduced a +large number of interesting facts in support of this opinion; but +although he thus succeeded in enforcing the truth that geographical +isolation is an important aid to organic evolution, he failed to +establish his conclusion that it is an indispensable condition. +Nevertheless he may have been right--and, as I shall presently show, I +believe he was right--in his fundamental premiss, that in the presence +of free intercrossing natural selection would be powerless to effect +divergent evolution. Where he went wrong was in not perceiving that +geographical isolation is not the only form of isolation. Had it +occurred to him that there may be other forms quite as effectual for the +prevention of free intercrossing, his essay could hardly have failed to +mark an epoch in the history of Darwinism. But, on account of this +oversight, he really weakened his main contention, namely, that in the +presence of free intercrossing natural selection must be powerless to +effect divergent evolution. This main contention I am now about to +re-argue. At present, therefore, we have only to observe that Wagner did +it much more harm than good by neglecting to perceive that free +intercrossing may be prevented in many other ways besides by migration, +and by the intervention of geographical barriers. + + [2] _Darwinism_, p. 150. + + [3] _The Darwinian Theory, and the Law of Migration_ (Eng. Trans., + Stanford, London, 1873). + +In order that we may set out with clearer views upon this matter, I will +make one or two preliminary remarks on the more general facts of +isolation as these are found to occur in nature. + +In the first place, it is obvious that isolation admits of degrees: it +may be either total or partial; and, if partial, may occur in numberless +grades of efficiency. This is so manifest that I need not wait to give +illustrations. But now, in the second place, there is another general +fact appertaining to isolation which is not so manifest, and a clear +appreciation of which is so essential to any adequate consideration of +the subject, that I believe the reason why evolutionists have hitherto +failed to perceive the full importance of isolation, is because they +have failed to perceive the distinction which has now to be pointed out. +The distinction is, that isolation may be either discriminate or +indiscriminate. If it be discriminate, the isolation has reference to +the resemblance of the separated individuals to one another; if it be +indiscriminate, it has no such reference. For example, if a shepherd +divides a flock of sheep without regard to their characters, he is +isolating one section from the other indiscriminately; but if he places +all the white sheep in one field, and all the black sheep in another +field, he is isolating one section from the other discriminately. Or, if +geological subsidence divides a species into two parts, the isolation +will be indiscriminate; but if the separation be due to one of the +sections developing, for example, a change of instinct determining +migration to another area, or occupation of a different habitat on the +same area, then the isolation will be discriminate, so far as the +resemblance of instinct is concerned. + +With the exception of Mr. Gulick, I cannot find that any other writer +has hitherto stated this supremely important distinction between +isolation as discriminate and indiscriminate. But he has fully as well +as independently stated it, and shown in a masterly way its far-reaching +consequences. Indiscriminate isolation he calls Separate Breeding, while +discriminate isolation he calls Segregate Breeding. For the sake, +however, of securing more descriptive terms, I will coin the words +Apogamy and Homogamy. Apogamy, of course, answers to indiscriminate +isolation, or separate breeding. Homogamy, on the other hand, answers +to discriminate isolation, or segregate breeding: only individuals +belonging to the same variety or kind are allowed to propagate. +Isolation, then, is a genus, of which Apogamy and Homogamy are +species[4]. + + [4] I may here most conveniently define the senses in which all the + following terms will be used throughout the present + discussion:--_Species_ of isolation are, as above stated, homogamy + and apogamy, or isolation as discriminate and indiscriminate. + _Forms_ of isolation are modes of isolation, such as the + geographical, the sexual, the instinctive, or any other of the + numerous means whereby isolation of either species may be secured. + _Cases_ of isolation are the instances in which any of the forms of + isolation may be at work: thus, if a group of _n_ intergenerants be + segregated into five groups, _a_, _b_, _c_, _d_, _e_, then, before + the segregation there would have been one case of isolation, but + after the segregation there would be five such cases. + +Now, in order to appreciate the unsurpassed importance of isolation as +one of the three basal principles of organic evolution, let us begin by +considering the discriminate species of it, or Homogamy. + +To state the case in the most general terms, we may say that if the +other two basal principles are given in heredity and variability, the +whole theory of organic evolution becomes neither more nor less than a +theory of homogamy--that is, a theory of the causes which lead to +discriminate isolation, or the breeding of like with like to the +exclusion of unlike. For the more we believe in heredity and variability +as basal principles of organic evolution, the stronger must become our +persuasion that discriminate breeding leads to divergence of type, while +indiscriminate breeding leads to uniformity. This, in fact, is securely +based on what we know from the experience supplied by artificial +selection, which consists in the intentional mating of like with like +to the exclusion of unlike. + +The point, then, which in the first instance must be firmly fastened in +our minds is this:--so long as there is free intercrossing, heredity +cancels variability, and makes in favour of fixity of type. Only when +assisted by some form of discriminate isolation, which determines the +exclusive breeding of like with like, can heredity make in favour of +change of type, or lead to what we understand by organic evolution. + +Now the forms of discriminate isolation, or homogamy, are very numerous. +When, for example, any section of a species adopts somewhat different +habits of life, or occupies a somewhat different station in the economy +of nature, homogamy arises within that section. There are forms of +homogamy on which Darwin has laid great stress, as we shall presently +find. Again, when for these or any other reasons a section of a species +becomes in any small degree modified as to form or colour, if the +species happens to be one where any psychological preference in pairing +can be exercised--as is very generally the case among the higher +animals--exclusive breeding is apt to ensue as a result of such +preference; for there is abundant evidence to show that, both in birds +and mammals, sexual selection is usually opposed to the intercrossing of +dissimilar varieties. Once more, in the case of plants, intercrossing of +dissimilar varieties may be prevented by any slight difference in their +seasons of flowering, of topographical stations, or even, in the case of +flowers which depend on insects for their fertilization, by differences +in the instincts and preferences of their visitors. + +But, without at present going into detail with regard to these different +forms of discriminate isolation, there are still two others, both of +which are of much greater importance than any that I have hitherto +named. Indeed, these two forms are of such immeasurable importance, that +were it not for their virtually ubiquitous operation, the process of +organic evolution could never have begun, nor, having begun, continued. + +The first of these two forms is sexual incompatibility--either partial +or absolute--between different taxonomic groups. If all hares and +rabbits, for example, were as fertile with one another as they are +within their own respective species, there can be no doubt that sooner +or later, and on common areas, the two types would fuse into one. And +similarly, if the bar of sterility could be thrown down as between all +the species of a genus, or all the genera of a family, _not otherwise +prevented from intercrossing_, in time all such species, or all such +genera, would become blended into a single type. As a matter of fact, +complete fertility, both of first crosses and of their resulting +hybrids, is rare, even as between species of the same genus; while as +between genera of the same family complete fertility does not appear +ever to occur; and, of course, the same applies to all the higher +taxonomic divisions. On the other hand, some degree of infertility is +not unusual as between different varieties of the same species; and, +wherever this is the case, it must clearly aid the further +differentiation of those varieties. It will be my endeavour to show that +in this latter connexion sexual incompatibility must be held to have +taken an immensely important part in the differentiation of varieties +into species. But meanwhile we have only to observe that _wherever_ such +incompatibility is concerned, it is to be regarded as an isolating +agency of the very first importance. And as it is of a character purely +physiological, I have assigned to it the name Physiological Isolation; +while for the particular case where this general principle is concerned +in the origination of specific types, I have reserved the name +Physiological Selection. + +The other most important form of discriminate isolation to which I have +alluded is Natural Selection. To some evolutionists it has seemed +paradoxical thus to regard natural selection as a form of isolation; but +a little thought will suffice to show that such is really the most +accurate way of regarding it. For, as Mr. Gulick says, "Natural +selection is the exclusive breeding of those better adapted to the +environment: ... it is a process in which the fittest are prevented from +crossing with the less fitted, by the exclusion of the less fitted." +Therefore it is, strictly and accurately, a mode of isolation, where the +isolation has reference to adaptation, and is secured in the most +effectual of possible ways--i.e. by the destruction of all individuals +whose intercrossing would interfere with the isolation. Indeed, the very +term "natural _selection_" shows that the principle is tacitly +understood to be one of isolation, because this name was assigned to the +principle by Darwin for the express purpose of marking the analogy that +obtains between it and the intentional isolation which is practised by +breeders, fanciers, and horticulturists. The only difference between +"natural selection" and "artificial selection" consists in this--that +under the former process the excluded individuals must necessarily +perish, while under the latter they need not do so. But clearly this +difference is accidental: it is in no way essential to the process +considered as a process of discriminate isolation. For, as far as +homogamous breeding is concerned, it can matter nothing whether the +exclusion of the dissimilar individuals is effected by separation or by +death. + +Natural selection, then, is thus unquestionably a form of isolation of +the discriminate kind; and therefore, notwithstanding its unique +importance in certain respects, considered as a principle of organic +evolution it is less fundamental--and also less extensive--than the +principle of isolation in general. In other words, it is but a part of a +much larger whole. It is but a particular form of a general principle, +which, as just shown, presents many other forms, not only of the +discriminate, but likewise of the indiscriminate kind. Or, reverting to +the terminology of logic, it is a sub-species of the species Homogamy, +which in its turn is but a constituent part of the genus Isolation. + +So much then for homogamy, or isolation of the discriminate order. +Passing on now to apogamy, or isolation of the indiscriminate kind, we +may well be disposed, at first sight, to conclude that this kind of +isolation can count for nothing in the process of evolution. For if the +fundamental importance of isolation in the production of organic forms +be due to its segregation of like with like, does it not follow that any +form of isolation which is indiscriminate must fail to supply the very +condition on which all the forms of discriminate isolation depend for +their efficacy in the causing of organic evolution? Or, to return to our +concrete example, is it not self-evident that the farmer who separated +his stock into two or more parts indiscriminately, would not effect any +more change in his stock than if he had left them all to breed together? + +Well, although at first sight this seems self-evident, it is in fact +untrue. For, unless the individuals which are indiscriminately isolated +happen to be a very large number, sooner or later their progeny will +come to differ from that of the parent type, or unisolated portion of +the previous stock. And, of course, as soon as this change of type +begins, the isolation ceases to be indiscriminate: the previous apogamy +has been converted into homogamy, with the usual result of causing a +divergence of type. The reason why progeny of an indiscriminately +isolated section of an originally uniform stock--e.g. of a species--will +eventually deviate from the original type is, to quote Mr. Gulick, as +follows:--"No two portions of a species possess exactly the same average +character, and, therefore, the initial differences are for ever reacting +on the environment and on each other in such a way as to ensure +increasing divergence as long as the individuals of the two groups are +kept from intergenerating[5]." Or, as I stated this principle in my +essay on _Physiological Selection_, published but a short time before +Mr. Gulick's invaluable contributions to these topics:-- + + [5] _Divergent Evolution through Cumulative Segregation_ (_Zool. + Journal, Linn. Soc._, vol. xx. pp. 189-274). + + As a matter of fact, we find that no one individual "is like + another all in all"; which is another way of saying that a specific + type may be regarded as the average mean of all its individual + variations, any considerable departure from this average being, + however, checked by intercrossing.... Consequently, if from any + cause a section of a species is prevented from intercrossing with + the rest of its species, we might expect that new varieties should + arise within that section, and that in time these varieties should + pass into new species. And this is just what we do find[6]. + + [6] The passage proceeds to show that in view of this consideration + we have a strong additional reason for rejecting the _a priori_ + dogma that all specific characters must necessarily be useful + characters. For it is evident that any divergence of specific + character which is brought about in this way need not present any + utilitarian significance--although, of course, natural selection + will ensure that it shall never be deleterious. + +The name which I gave to this cause of specific change was Independent +Variability, or variability in the absence of overwhelming +intercrossing. But it now appears to me that this cause is really +identical with that which was previously enunciated by Delboeuf. +Again, in his important essay on _The Influence of Isolation_, Weismann +concludes, on the basis of a large accumulation of facts, that the +constancy of any given specific type "does not arise suddenly, but +gradually, and is established by the promiscuous intercrossing of all +individuals." From which, he says, it follows, that this constancy must +cease so soon as the condition which maintains it ceases--i. e. so soon +as intercrossing (Panmixia) between all individuals ceases, or so soon +as a portion of a species is isolated from its parent stock. To this +principle he assigns the name of Amixia. But Weismann's Amixia differs +from my Independent Variability in several important particulars; and on +this account I have designedly abstained from adopting his term. Here +it is enough to remark that it answers to the generic term Isolation, +without reference to the _kind_ of isolation as discriminate or +indiscriminate, homogamous or apogamous. On the other hand, my +Independent Variability is merely a re-statement of the so-called "Law +of Delboeuf," which, in his own words, is as follows:-- + + One point, however, is definitely attained. It is that the + proposition, which further back we designated paradoxical, is + rigorously true, A constant cause of variation, however + insignificant it may be, changes the uniformity [of type] little by + little, and diversifies it _ad infinitum_. From the homogeneous, + left to itself, only the homogeneous can proceed; but if there be a + slight disturbance ["leger ferment"] in the homogeneous, the + homogeneity will be invaded at a single point, differentiation will + penetrate the whole, and, after a time--it may be an infinite + time--the differentiation will have disintegrated it altogether. + +In other words, the "Law," which Delboeuf has formulated on +mathematical grounds, and with express reference to the question of +segregate breeding, proves that, no matter how infinitesimally small the +difference may be between the average qualities of an isolated section +of a species compared with the average qualities of the rest of that +species, if the isolation continues sufficiently long, differentiation +of specific type is necessarily bound to ensue. But, to make this +mathematical law biologically complete, it ought to be added that the +time required for the change of type to supervene (supposing apogamy to +be the only agent of change) will be governed by the range of individual +variability which the species in question presents. A highly stable +species (such as the Goose) might require an immensely long time for +apogamy alone to produce any change of type in an isolated portion of +the species, while a highly variable species (such as the Ruff) would +rapidly change in any portion that might be indiscriminately isolated. +It was in order to recognize this additional and very important factor +that I chose the name Independent _Variability_ whereby to designate the +diversifying influence of merely indiscriminate isolation, or apogamy. +Later on Mr. Gulick published his elaborate papers upon the divergence +of type under all kinds of isolation; and retained my term Independent, +but changed Variability into Generation. I point this out merely for the +sake of remarking that his Independent Generation is exactly the same +principle as my Independent Variability, and Delboeuf's Mathematical +Law. + +Now, while I fully agree with Mons. Giard where he says, in the +introductory lecture of his course on _The Factors of Evolution_[7], +that sufficient attention has not been hitherto given by naturalists to +this important factor of organic evolution (apogamy), I think I have +shown that among those naturalists who have considered it there is a +sufficient amount of agreement. _Per contra_, I have to note the opinion +of Mr. Wallace, who steadily maintains the impossibility of any cause +other than natural selection (i.e. one of the forms of homogamy) having +been concerned in the evolution of species. But at present it is enough +to remark that even Professor Ray Lankester--whose leanings of late +years have been to the side of ultra-Darwinism, and who is therefore +disposed to agree with Mr. Wallace wherever this is logically +possible--even Professor Ray Lankester observes:-- + + [7] _Revue Scientifique_, Nov. 23, 1889. + + Mr. Wallace does not, in my judgement, give sufficient grounds for + rejecting the proposition which he indicates as the main point of + Mr. Gulick's valuable essay on _Divergent Evolution through + Cumulative Segregation_. Mr. Gulick's idea is that ... no two + portions of a species possess exactly the same average character, + and the initial differences will, if the individuals of the two + groups are kept from intercrossing, assert themselves continuously + by heredity in such a way as to ensure an increasing divergence of + the forms belonging to the two groups, amounting to what is + recognized as specific distinction. Mr. Gulick's idea is simply the + recognition of a permanence or persistency in heredity, which, + _caeteris paribus_, gives a twist or direction to the variations of + the descendants of one individual as compared with the descendants + of another[8]. + + [8] _Nature_, Oct. 10, 1889, p. 568. + +Now we have seen that "Mr. Gulick's idea," although independently +conceived by him, had been several times propounded before; and it is +partly implicated in more than one passage of the _Origin of Species_, +where free intercrossing, or the _absence_ of isolation, is alluded to +as maintaining the _constancy_ of a specific type[9]. Moreover, it is +still more fully recognized in the last edition of the _Variation of +Animals and Plants_, where a paragraph is added for the purpose of +sanctioning the principle in the imperfect form that it was stated by +Weismann[10]. Nevertheless, to Mr. Gulick belongs the credit, not only of +having been the first to conceive (though the last to publish) the +"idea" in question, and of having stated it with greater fullness than +anybody else; but still more of having verified its importance as a +factor of organic evolution. + + [9] e. g. p. 81. + + [10] See Chapter xxiii. vol. ii. p. 262. (Edition of 1888.) + +For, in point of fact, Mr. Gulick was led to his recognition of the +principle in question, not by any deductive reasoning from general +principles, but by his own particular and detailed observations of the +land mollusca of the Sandwich Islands. Here there are an immense number +of varieties belonging to several genera; but every variety is +restricted, not merely to the same island, but actually to the same +valley. Moreover, on tracing this fauna from valley to valley, it is +apparent that a slight variation in the occupants of valley 2 as +compared with those of the adjacent valley 1, becomes more pronounced in +the next--valley 3, still more so in 4, &c., &c. Thus it was possible, +as Mr. Gulick says, roughly to estimate the amount of divergence between +the occupants of any two given valleys by measuring the number of miles +between them. + +As already stated, I have myself examined his wonderful collection of +shells, together with a topographical map of the district; and therefore +I am in a position to testify to the great value of Mr. Gulick's work in +this connexion, as in that of the utility question previously +considered. The variations, which affect scores of species, and +themselves eventually run into fully specific distinctions, are all more +or less finely graduated as they pass from one isolated region to the +next; and they have reference to changes of form and colour, which in no +one case presents any appearance of utility. Therefore--and especially +in view of the fact that, as far as he could ascertain, the environment +in the different valleys was essentially the same--no one who examines +this collection can wonder that Mr. Gulick attributes the results which +he has observed to the influence of apogamy alone, without any reference +to utility or natural selection. + +To this solid array of remarkable facts Mr. Wallace has nothing further +to oppose than his customary appeal to the argument from ignorance, +grounded on the usual assumption that no principle other than natural +selection _can_ be responsible for even the minutest changes of form or +colour. For my own part, I must confess that I have never been so deeply +impressed by the dominating influence of the _a priori_ method as I was +on reading Mr. Wallace's criticism of Mr. Gulick's paper, after having +seen the material on which this paper is founded. To argue that every +one of some twenty contiguous valleys in the area of the same small +island must necessarily present such differences of environment that all +the shells in each are differently modified thereby, while in no one out +of the hundreds of cases of modification in minute respects of form and +colour can any human being suggest an adaptive reason therefor--to argue +thus is merely to affirm an intrinsically improbable dogma in the +presence of a great and consistent array of opposing facts. + +I have laid special stress on this particular case of the Sandwich +Islands' mollusca, because the fifteen years of labour which Mr. Gulick +has devoted to their exhaustive working out have yielded results more +complete and suggestive than any which so far have been forthcoming with +regard to the effects of isolation in divergent evolution. But, if space +permitted, it would be easy to present abundance of additional facts +from other sources, all bearing to the same conclusion--namely, that as +a matter of direct observation, no less than of general reasoning, any +unprejudiced mind will concede to the principle of indiscriminate +isolation an important share in the origination of organic types. For as +indiscriminate isolation is thus seen sooner or later to become +discriminate, and as we have already seen that discriminate isolation is +a necessary condition to all or any modification, we can only conclude +that isolation in both its kinds takes rank with heredity and +variability as one of the three basal principles of organic evolution. + + * * * * * + +Having got thus far in the way of generalities, we must next observe +sundry further matters of comparative detail. + +1. In any case of indiscriminate isolation, or apogamy, the larger the +bulk of the isolated section the more nearly must its average qualities +resemble those of its parent stock; and, therefore, the less divergence +of character will ensue in a given time from this cause alone. For +instance, if one-fourth of a large species were to be separated from the +other three-fourths (say, by subsidence causing a discontinuity of +area), it would continue the specific characters unchanged for an +indefinitely long time, so far as the influence of such an +indiscriminate isolation is concerned. But, on the other hand, if only +half a dozen individuals were to be thus separated from the rest of +their species, a comparatively short time would be needed for their +descendants to undergo some varietal modification at the hands of +apogamy. For, in this case, the chances would be infinitely against the +average characters of the original half-dozen individuals exactly +coinciding with those of all the rest of their species. + +2. In any case of homogamy, however, it is immaterial what proportional +number of individuals are isolated in the first instance. For the +isolation is here discriminate, or effected by the initial difference of +the average qualities themselves--a difference, therefore, which +presupposes divergence as having already commenced, and equally bound to +proceed whether the number of intergenerants be large or small. + +It may here be remarked that, in his essay on the _Influence of +Isolation_, Professor Weismann fails to distinguish between the two +kinds of isolation. This essay deals only with one of the many different +forms of isolation--the geographical--and is therefore throughout +concerned with a consideration of diversity as arising from apogamy +alone. But in dealing with this side of the matter Weismann anticipated +both Gulick and myself in pointing out the law of inverse proportion, +which I have stated in the preceding paragraph in what appears to me its +strictly accurate form. + +3. Segregate Breeding, or homogamy, which arises under any of the many +forms of discriminate isolation, must always tend to be _cumulative_. +For, again to quote Mr. Gulick, who has constituted this fact the most +prominent as it is the most original feature of his essay, "In the first +place, every new form of Segregation[11] that now appears depends on, and +is superimposed upon, forms of Segregation that have been previously +induced; for when Negative Segregation arises [i. e. isolation due to +mutual sterility], and the varieties of a species become less and less +fertile with one another, the complete infertility that has existed +between them and some other species does not disappear, nor does the +Positive Segregation cease [i. e. any other form of isolation previously +existing].... In the second place, whenever Segregation is directly +produced by some quality of the organism, variations that possess the +endowment in a superior degree will have a larger share in producing the +segregated forms of the next generation, and accordingly the segregative +endowment of the next generation will be greater than that of the +present generation; and so with each successive generation the +segregation will become increasingly complete." And to this it may be +added, in the third place, that where the segregation (isolation) is due +to the external conditions of life under which the organism is placed, +or where it is due to natural selection simultaneously operating in +divergent lines of evolution, the same remarks apply. Hence it follows +that discriminate isolation is, in all its forms, cumulative. + + [11] This term may here be taken as equivalent to Isolation. + +4. The next point to be noted is, that the cumulative divergence of type +thus induced can take place only in as many different lines as there are +different _cases_ of isolation. This is a point which Mr. Gulick has not +expressly noticed; but it is one that ought to be clearly recognized. +Seeing that isolation secures the breeding of similar forms by exclusion +(immediate or eventual) of those which are dissimilar, and that only in +as far as it does this can it be a factor in organic evolution, it +follows that the resulting segregation, even though cumulative, can only +lead to divergence of organic types in as many directions as there are +cases of isolation. For any one group of intergenerants only _serial_ +transformation is possible, even though the transformation be cumulative +through successive generations in the single line of change. But there +is always a probability that during the course of such _serial +transformation in time_, some other case of isolation may supervene, so +as to divide the previously isolated group of intergenerants into two or +more further isolated groups. Then, of course, opportunity will be +furnished for _divergent transformation in space_--and this in as many +different lines as there are now different homogamous groups. + +That this must be so is further evident, if we reflect that the +evolutionary power of isolation depends, not only on the _preventing_ of +intercrossing between the isolated portion of a species and the rest of +that species, but also upon the _permitting_ of intercrossing between +all individuals of the isolated portion, whereby the peculiar average of +qualities which they as a whole present may be allowed to assert itself +in their progeny--or, if the isolation has been from the first +discriminate, whereby the resulting homogamy may thus be allowed to +assert itself. Hence any one case of either species of isolation, +discriminate or indiscriminate, can only give rise to what Mr. Gulick +has aptly called "monotypic evolution," or a chain-like series of types +arising successively in time, as distinguished from what he has called +"polytypic evolution," or an arborescent multiplication of types +arising simultaneously in space. + +For example, let us again take the geographical form of isolation. Where +a single small intergenerant group of individuals is separated from the +rest of its species--say, on an oceanic island--_monotypic_ evolution +may take place through a continuous and cumulative course of independent +variation in a single line of change: all the _individuals_ composing +any one given generation will closely resemble one another, although the +_type_ may be progressively altering through a long series of +generations. But if the original species had had two small colonies +separated from itself (one on each of two different islands, so giving +rise to two cases of isolation), then _polytypic_ evolution would have +ensued to the extent of there having been two different lines of +evolution going on simultaneously (one upon each of the two islands +concerned). Similarly, of course, if there had been three or four such +colonies, there would have been three or four divergent lines of +evolution, and so on. + +5. In the _cases_ of isolation just supposed there is only one _form_ of +isolation; and it is thus shown that under one form of isolation there +may be as many lines of divergence as there are separate cases of such +isolation. But now suppose that there are two or more forms of +isolation--for instance, that on the same oceanic island the original +colony has begun to segregate into secondary groups under the influence +of natural selection, sexual selection, physiological selection, or any +of the other forms of isolation--then there will be as many lines of +divergent evolution going on at the same time (and here on the same +area) as there are forms of isolation affecting the oceanic colony. And +this because each of the _forms_ of isolation has given rise to a +different _case_ of isolation. + +Now, inasmuch as different forms of isolation, when thus superadded one +to another, constitute different cases of isolation, we may lay down the +following general law as applying to all the forms of isolation--namely, +_The number of possible directions in which divergent evolution can +occur, is never greater than, though it may be equal to, the number of +cases of efficient isolation--or the number of efficiently separated +groups of intergenerants._ + +6. We have now to consider with some care the particular and highly +important form of isolation that is presented by natural selection. For +while this form of isolation resembles all the other forms of the +discriminate kind in that it secures homogamy, there are two points in +which it differs from all of them, and one point in which it differs +from most of them. + +Natural selection differs from _all_ the other known forms of isolation +(whether discriminate or indiscriminate) in that it has exclusive +reference to _adaptations_ on the one hand, and, on the other hand, +necessitates not only the elimination, but the destruction of the +excluded individuals. Again, natural selection differs from _most_ of +the other forms of isolation in that, unless assisted by some other +form, it can never lead to polytypic, but only to monotypic evolution. +The first two points of difference are here immaterial; but the last is +one of the highest importance, as we shall immediately perceive. + +In nearly all the other forms of isolation, polytypic or divergent +evolution may arise under the influence of that form alone, or without +the necessary co-operation of any other form. This we have already seen, +for example, in regard to geographical isolation, under which there may +be as many different lines of transmutation going on simultaneously as +there are different cases of isolation--say, in so many different +oceanic islands. Again, in regard to physiological isolation the same +remark obviously applies; for it is evident that even upon the same +geographical area there may be as many different lines of transmutation +going on simultaneously as there are cases of this form of isolation. +The bar of mutual sterility, whenever and wherever it occurs, must +always render polytypic evolution possible. And so it is with almost all +the other forms of isolation: that is to say, one _form_ does not +necessarily require the assistance of another _form_ in order to create +an additional _case_ of isolation. But it is a peculiarity of natural +selection, considered as a form of isolation, that it does necessarily +require the assistance of some other form before it can give rise to an +additional case of isolation; and therefore before it can give rise to +any _divergence_ of character in ramifying lines, as distinguished from +_transformation_ of characters in a single line. Or, in other words, +natural selection, when acting alone, can never induce polytypic +evolution, but only monotypic. + +That this important conclusion is a necessary deduction from the theory +of natural selection itself, a very few words will be enough to show. +For, according to the theory, survival of the fittest is a form of +isolation which acts through utility, by _destroying_ all the +individuals whom it fails to isolate. Hence it follows that survival of +the fittest is a form of isolation which, if acting alone, cannot +_possibly_ effect divergent evolution. For, in the first place, there is +nothing in this form of isolation to ensure that the fitter individuals +should fail to interbreed with the less fit which are able to survive; +and, in the second place, in all cases where the less fit are not +sufficiently fit to be suffered to breed, they are exterminated--i. e. +not permitted to form a distinct variety of their own. If it be said +that survival of the fittest may develop simultaneously two or more +lines of _useful_ change, the answer is that it can only do this if each +of the developing varieties is isolated from the others by some +_additional form_ of isolation; for, if not, there can be no +commencement of utilitarian _divergence_, since whatever number of +utilitarian changes may be in course of simultaneous development, they +must in this case be all blended together in a single line of specific +transmutation. Nay, even if specific divergence has actually been +commenced by natural selection when associated with some other form of +homogamy, if the latter should afterwards be withdrawn, natural +selection would then be unable to maintain even so much divergence of +character as may already have been attained: free intercrossing between +the two collateral, and no longer isolated branches, would ensure their +eventual blending into a common stock. Therefore, I repeat, natural +selection, when acting alone, can never induce polytypic evolution, but +only monotypic. + +Now I regret to say that here, for the first and only time throughout +the whole course of the present treatise, I find myself in seeming +opposition to the views of Darwin. For it was the decidedly expressed +opinion of Darwin that natural selection _is_ competent to effect +polytypic, or divergent, evolution. Nevertheless, I believe that the +opposition is to a large extent only apparent, or due merely to the fact +that Darwin did not explicitly state certain considerations which +throughout his discussion on "divergence of character" are seemingly +implied. But, be this as it may, I have not even appeared to desert his +leadership on a matter of such high importance without having duly +considered the question in all its bearings, and to the utmost limit of +my ability. Moreover, about two years after the publication of my first +paper[12] upon the subject, Mr. Gulick followed, at somewhat greater +length, in the same line of dissent. Like all the rest of his work, this +is so severely logical in statement, as well as profoundly thought out +in substance, that I do not see how it is possible for any one to read +impartially what he has written, and then continue to hold that natural +selection, if unassisted by any other form of isolation, can possibly +effect divergence of character--or polytypic as distinguished from +monotypic evolution[13]. + + [12] _Zool. Journal Lin. Soc._, vol. xix. pp. 337-411. + + [13] _Ibid._, vol. xx. pp. 202-212. + +I may here quote from Mr. Gulick's paper three propositions, serving to +state three large and general bodies of observable fact, which +severally and collectively go to verify, with an overwhelming mass of +evidence, the conclusion previously reached on grounds of general +reasoning. + + The facts of geographical distribution seem to me to justify the + following statements:-- + + (1) A species exposed to different conditions in the different + parts of the area over which it is distributed, is not represented + by divergent forms when free interbreeding exists between the + inhabitants of the different districts. In other words, Diversity + of Natural Selection without Separation does not produce divergent + evolution. + + (2) We find many cases in which areas, corresponding in the + character of the environment, but separated from each other by + important barriers, are the homes of divergent forms of the same or + allied species. + + (3) In cases where the separation has been long continued, and the + external conditions are the most diverse in points that involve + diversity of adaptation, there we find the most decided divergences + in the organic forms. That is, where Separation and Divergent + Selection have long acted, the results are found to be the + greatest. + + The 1st and 3rd of these propositions will probably be disputed by + few, if by any. The proof of the 2nd is found wherever a set of + closely allied organisms is so distributed over a territory that + each species and variety occupies its own narrow district, within + which it is shut by barriers that restrain its distribution while + each species of the environing types is distributed over the whole + territory. The distribution of terrestrial molluscs on the Sandwich + Islands presents a great body of facts of this kind. + + + + +CHAPTER II. + +ISOLATION (_continued_). + + +I will now recapitulate the main doctrines which have been set forth in +the foregoing chapter, and then proceed to consider the objections which +have been advanced against them. + +It must be remembered that by isolation I mean exactly what Mr. Gulick +does by "Segregation," and approximately what Professor Weismann does by +"Amixia "--i. e. the prevention of intercrossing. + +Isolation occurs in very many forms besides the geographical, as will be +more fully shown at the end of this chapter; and in all its forms it +admits of degrees. + +It also occurs in two very different species or kinds--namely, +discriminate and indiscriminate. These I have called respectively +Homogamy and Apogamy. This all-important distinction has been clearly +recognized by Mr. Gulick, as a result of his own thought and +observation, independently of anything that I have published upon the +subject. + +In view of this distinction Isolation takes rank with Heredity and +Variability as one of the most fundamental principles of organic +evolution. For, if these other two principles be granted, the whole +theory of descent resolves itself into an inquiry touching the causes, +forms, and degrees of Homogamy. + +Save in cases where very large populations are concerned, apogamy must +sooner or later give rise _per se_ to homogamy, owing to the Law of +Delboeuf, which is the principle that I have called Independent +Variability, and Gulick has called Independent Generation. But of course +this does not hinder that under apogamy various other causes of homogamy +are likely to arise--in particular natural selection. + +That natural selection differs from most of the other forms of isolation +in not being capable of causing _divergent_ or _polytypic_ evolution +must at once become evident, if we remember that the only way in which +isolation of any form can cause such evolution is by partitioning a +given group of intergenerants into two or more groups, each of which is +able to survive as thus separated from the other, and so to carry on the +evolution in divergent lines. But the distinguishing peculiarity of +natural selection, considered as a form of isolation, is that it effects +the isolation _by killing off all the individuals which it fails to +isolate_: consequently, this form of isolation differs from other forms +in prohibiting the possibility of any ramification of a single group of +intergenerants into two or more groups, for the purpose of carrying on +the evolution in divergent lines. Therefore, under this form of +isolation alone, evolution must proceed, palm-like, in a single line of +growth. So to speak, the successive generations continuously ascend to +higher things on the steps supplied by their own "dead selves"; but in +doing so they must climb a single ladder, no rung of which can be +allowed to bifurcate in the presence of the uniformity secured _for that +generation_ by the free intercrossing of the most fit. Even though +beneficial variations may arise in two or more directions +simultaneously, and all be simultaneously selected by survival of the +fittest, the effect of free intercrossing (in the absence of any other +form of isolation) will be to fuse all these beneficial variations into +one common type, and so to end in _monotypic_ evolution as before. In +order to secure _polytypic_ evolution, intercrossing between the +different beneficial variants which may arise must be prevented; and +there is nothing to prevent such intercrossing in the process of natural +selection _per se_. In order that the original group of intergenerants +should be divided and sub-divided into two or more groups of +intergenerants, some additional form of isolation must necessarily +supervene--when, of course, polytypic evolution will result. And, as Mr. +Gulick has shown, the conclusion thus established by deductive reasoning +is verified inductively by the facts of geographical distribution. + +How, then, are we to account for the fact that Darwin attributed to +natural selection the power to cause divergence of character? The answer +is sufficiently simple. _He does so by tacitly invoking the aid of some +other form of homogamy in every case._ If we carefully read pp. 86-97 of +the _Origin of Species_, where this subject is under consideration, we +shall find that in every one of the arguments and illustrations which +are adduced to prove the power of natural selection to effect +"divergence of character," he either pre-supposes or actually names some +other form of homogamy as the originating cause of the diversity that +is afterwards presented to natural selection for further +intensification. To give only one example. At the starting-point of the +whole discussion the priority of such other forms of homogamy is assumed +in the following words:-- + + But how, it may be asked, can any analogous principle [to that of + diversity caused by artificial selection] apply in nature? I + believe it can and does apply most efficiently (though it was a + long time before I saw how), from the simple circumstance that the + more diversified the descendants from any one species become in + structure, constitution, and habits, by so much will they be better + enabled to seize on many and widely diversified places in the + polity of nature, and so be enabled to increase in numbers. + +Now, without question, so soon as segregate breeding in two or more +lines of homogamy has been in any sufficient degree determined by some +"change of structure, constitution, or habits," natural selection will +forthwith proceed to increase the divergence in as many different lines +as there are thus yielded discriminately isolated sections of the +species. And this fact it must have been that Darwin really had before +his mind when he argued that diversification of character is caused by +natural selection, through the benefit gained by the diversified forms +being thus "enabled to increase in number." Nevertheless he does not +expressly state the essential point, that although diversification of +character, _when once begun_, is thus _promoted_ by natural selection, +which forthwith proceeds to cultivate each of the resulting branches, +yet diversification of character can never be _originated_ by natural +selection. The change of "structure," of "constitution," of "habits," of +"station," of geographical area, of reciprocal fertility, and so +on--this change, _whatever_ it may have been, must clearly have been +antecedent to any operation of natural selection through the benefit +which arose from the change. Therefore the change must in all cases have +been due, in the first instance, to some other form of isolation than +the superadded form which afterwards arose from superior fitness in the +possession of superior benefit--although, so long as the prior form of +isolation endured, or continued to furnish the necessary condition to +the co-operation of survival of the fittest, survival of the fittest +would have continued to increase the divergence of character in as many +ramifying lines as there were thus given to its action separate cases of +isolation by other means. + +In short, as divergence of character must in all cases be due to a +prevention of intercrossing, and as in the process of natural selection +there is, _ex hypothesi_, nothing to prevent the intercrossing until the +divergence has already arisen, to suppose that natural selection alone +can have caused the divergence, is to suppose that natural selection can +have caused the conditions of its own activity, which is absurd. + +Seeing, then, that even in cases where any "benefit" arises from +divergence of character, such benefit can arise only after the +divergence has already commenced, and seeing that on this as on other +accounts previously mentioned it is plainly impossible to attribute the +origin of such divergence to natural selection, we find that natural +selection must be in all cases assisted by some other form of isolation, +if it is to be concerned in polytypic as distinguished from monotypic +evolution. But this does not hinder that, when it is so assisted, +natural selection may become--and, I believe, does become--the most +efficient of all the forms of isolation in promoting divergence of +character. For, in the first place, of all the forms of isolation +natural selection is probably the most energetic in promoting monotypic +evolution; so that under the influence of such isolation monotypic +evolution probably advances more rapidly than it does under any other +form of isolation. In the second place, when polytypic evolution has +been begun by any of these other forms of isolation, and natural +selection then sets to work on each of the resulting branches, although +natural selection is thus engaged in as many different acts of monotypic +evolution as there are thus separate cases supplied to it by these other +forms of isolation, the joint result of all these different acts is to +hurry on the polytypic evolution which was originally started by the +other forms of isolation. So to speak, natural selection is the forcing +heat, acting simultaneously on each of the separate branches which has +been induced to sprout by other means; and in thus rapidly advancing the +growth of all the branches, it is still entitled to be regarded as the +most important _single_ cause of diversification in organic nature, +although we must henceforth cease to regard it as in any instance the +_originating_ cause--or even so much as the _sustaining_ cause. + +So much by way of summary and recapitulation. I will now briefly +consider the only objections which, so far as I can see, admit of being +brought against the foregoing doctrine of Isolation as held by Mr. +Gulick and myself. These possible objections are but two in +number--although but one of them has been hitherto adduced. This, +therefore, I will take first. + +Mr. Wallace, with his customary desire to show that natural selection is +everywhere of itself capable of causing organic evolution, seeks to +minimize the swamping effects of free intercrossing, and the consequent +importance of other forms of isolation. His argument is as follows. + +Alluding to the researches of Mr. J. A. Allen, and others, on the amount +of variation presented by individuals of a species in a state of nature, +Mr. Wallace shows that, as regards any given part of the animal under +consideration, there is always to be found a considerable range of +individual variation round the average mean which goes to constitute the +specific character of the type. Thus, for example, Mr. Allen says of +American birds, "that a variation of from fifteen to twenty per cent. in +general size, and an equal degree of variation in the relative size of +different parts, may be ordinarily expected among specimens from the +same species and sex, taken at the same locality, while in some cases +the variation is even greater than this." Now, Mr. Wallace is under the +impression that these facts obviate the difficulty which arises from the +presence of free intercrossing--the difficulty, that is, against the +theory of natural selection when natural selection is supposed to have +been the exclusive means of modification. For, as he says, "if less size +of body would be beneficial, then, as half the variations in size are +above and half below the mean or existing standard of the species, there +would be ample beneficial variations"; and similarly with regard to +longer or shorter legs, wings, tails, &c., darker or lighter colour, and +so on through all the parts of any given organism. + +Well, although I have no wish at all to disparage the biological value +of these actual measurements of the range of individual variation, I +must point out that they are without any value at all in the connexion +which Mr. Wallace adduces them. We did not require these measurements to +tell us the broad and patent fact that "no being on this earthly ball is +like another all in all"--or, in less Tennysonian words, that as regards +every specific structure there is a certain amount of individual +variability round an average mean. Indeed, in my own paper on +_Physiological Selection_--against which Mr. Wallace is here specially +arguing--I expressly said, as previously remarked, "that a specific type +may be regarded as _the average mean of all individual variations_." The +fact of such individual variability round a specific mean has always +been well known to anatomists; it constitutes one of the basal pillars +of the whole Darwinian theory; and is besides a matter of universal +recognition as regards human stature, features, and so forth. The value +of Mr. Allen's work consists in accurately measuring the _amount_ or +_range_ of individual variation; but the question of its amount or range +is without relevancy in the present connexion. For the desirability of +isolation as an aid to natural selection even where monotypic evolution +is concerned, does not arise with any reference to the amount or range +of variation: it arises with reference to the _number_ of variations +which are--or are not--_similar_ and _simultaneous_. If there be a +sufficient number which are both similar and simultaneous, the +desirability of any co-operating form of isolation is correspondingly +removed, because natural selection may then have sufficient material +wherewith to overcome the adverse influence of free intercrossing, and +so of itself to produce monotypic evolution. Now, variations may be +numerous, similar, and simultaneous, either on account of some common +cause acting on many individuals at the same time, or on account of the +structures in question being more or less variable round a specific +mean. In the latter case--which is the only case that Mr. Allen's +measurements have to do with--the law of averages will of course +determine that half the whole number of variations in any given +structure, in any given generation, will be above the mean line. But, +equally of course, no one has ever denied that where, for either of +these reasons, natural selection is provided with sufficient material, +it is correspondingly capable of improving the specific type without the +assistance of any other form of homogamy; so to speak, they protect +themselves by their very numbers, and their superiority over others +leads to their survival and accumulation. But what is the result? _The +result can only be monotypic evolution._ No matter how great the number, +or how great the range, of variations round an average specific mean, +out of such material natural selection can never produce _polytypic_ +evolution: it may _change_ the type to any extent during successive +generations, and in a single line of change; but it cannot _branch_ the +type, unless some other form of homogamy intervenes. Therefore, when Mr. +Wallace adduces the well-known fact that all structures vary more or +less round a specific mean as proof that natural selection need not be +incommoded by free intercrossing, but can of itself produce all the +known phenomena of specific evolution, he fails to perceive that his +argument refers only to one aspect of such evolution (viz. the +transformation of species in time), and does not apply to the aspect +with which alone my paper on _Physiological Selection_ was concerned +(viz. the multiplication of species in space). + +The same thing may be shown in this way. It is perfectly obvious that +where the improvement of type in a linear series is concerned (monotypic +evolution), free intercrossing, far from being a hindrance to the +process, _is the very means by which the process is accomplished_. +Improvement here ascends by successive steps, in successive generations, +simply _because_ of the general intercrossing of the generally most fit +with the result that the species, _as a whole_, gradually becomes +transformed into another species, _as a whole_. Therefore, it would be +mere fatuity in any one to adduce free intercrossing as a "difficulty" +against natural selection alone being competent to produce evolution of +this kind. But where the kind of evolution is that whereby the species +is _differentiated_--where it is required, for instance, to produce +different structures in different portions of the species, such as the +commencement of a fighting spur on the wing of a duck, or _novel_ +characters of any sort in different groups of the species--free +intercrossing is no longer a condition to, but an absolute preventive +of, the process; and, therefore, unless checked as between each portion +of the species by some form of homogamy other than natural selection, +it must effectually inhibit any _segregation_ of specific types, or +divergence of character. + +Hence it is that, while no Darwinian has ever questioned the power of +unaided selection to cause _improvement of character in successive +generations_, in common now with not a few other Darwinians I have +emphatically denied so much as the abstract possibility of selection +alone causing a _divergence of character in two or more simultaneous +lines of change_. + +And, although these opposite views cannot be reconciled, I am under the +impression that they do admit of being explained. For I take them to +indicate a continued failure to perceive the all-important distinction +between evolution as monotypic and polytypic. Unless one has fully +grasped this distinction, and constantly holds it in mind, he is not in +a position to understand the "difficulty" in question; nor can he avoid +playing fast and loose with natural selection as possibly the sole cause +of evolution, and as necessarily requiring the co-operation of some +other cause. But if he once clearly perceives that "evolution" is a +logical genus, of which the monotypic and the polytypic forms are +species, he will immediately escape from his confusion, and find that +while the monotypic form may be caused by natural selection alone the +polytypic form can never be so caused. + + * * * * * + +The second difficulty which I have to mention as at first sight +attaching to the views of Mr. Gulick and myself on the subject of +Isolation is, that in an isolated section of a species Mr. Francis +Galton's law of regression in the average character of offspring to the +typical character of the group through reversion or atavism (_Natural +Inheritance_, p. 97) must have the effect of neutralizing the +segregative influence of mere apogamy. That such, however, cannot be the +case has been well shown by Mr. Gulick in his paper on _Intensive +Segregation_. Without at all disputing the validity of Mr. Galton's law, +he proves that "it can hold in full force only where there is free +crossing, otherwise no divergent race could ever be formed by any amount +of selection and independent breeding[14]." This is so self-evident that +I need not quote his demonstration of the point. + + [14] _Zool. Journal Lin. Soc._, vol. xxiii. p. 313. + + * * * * * + +In conclusion, then, and having regard to the principle of isolation as +a whole, or in all the many and varied forms in which this principle +obtains, I trust that I have redeemed the promise with which I set +out--viz. to show that in relation to the theory of descent this +principle is of an importance second to no other, not even excepting +heredity, variability, and the struggle for existence. This has now been +fully shown, inasmuch as we have clearly seen that the importance of the +struggle for existence, and consequent survival of the fittest, arises +just because survival of the fittest is a form, and a very stringent +form, of isolation; while, as regards both heredity and variability, we +are now in a position to see that the more fully we recognize their +supreme importance as principles concerned in organic evolution, the +more must we also recognize that any rational theory of such evolution +becomes, in the last resort, a theory of the different modes in which +efficient isolation can be secured. For, in whatever degree the process +of organic evolution has been dependent upon heredity with variability, +in that degree must it also have been dependent upon the means of +securing homogamy, whereby alone the force of heredity can be made to +expend itself in the innumerable directions of progressive change, +instead of continually neutralizing the force of variability by +promiscuous intercrossing. + + + + +CHAPTER III. + +PHYSIOLOGICAL SELECTION. + + +So far we have been concerned with the principle of Isolation in +general. We have now to consider that form of isolation which arises in +consequence of mutual infertility between the members of any group of +organisms and those of all other similarly isolated groups occupying +simultaneously the same area. + + * * * * * + +Against the view that natural selection is a sufficient explanation of +the origin of species, there are two fatal difficulties: one, the +contrast between natural species and domesticated varieties in respect +of cross-sterility; the other, the fact that natural selection cannot +possibly give rise to polytypic as distinguished from monotypic +evolution. Now it is my belief that the theory of physiological +selection fully meets both these difficulties. Indeed I hold this to be +undeniable in a formal or logical sense: the only question is as to the +evidence which can be adduced for the theory in a practical or +biological sense. Therefore in this chapter, where the theory has first +of all to be stated, I shall restrict the exposition as much as possible +to the former, leaving for subsequent consideration the biological +side. + +The following is a brief outline sketch of this theory[15]. + + [15] _See Nineteenth Century_, January, 1887, pp. 61, 62. + +Of all parts of those variable objects which we call organisms, the most +variable is the reproductive system; and the variations may carry with +them functional changes, which may be either in the direction of +increased or of diminished fertility. Consequently variations in the way +of greater or less fertility frequently take place, both in plants and +animals; and probably, if we had adequate means of observing this point, +we should find that there is no one variation more common. But of course +where infertility arises--whether as a result of changed conditions of +life, or, as we say, spontaneously--it immediately becomes extinguished, +seeing that the individuals which it affects are less able (if able at +all) to propagate and to hand on the variation. If, however, the +variant, while showing some degree of infertility with the parent form, +continues to be as fertile as before when mated with similar variants, +under these circumstances there is no reason why such differential +fertility should not be perpetuated. + +Stated in another form this suggestion enables us to regard many, if not +most, species as the records of variations in the reproductive systems +of their ancestors. When variations of a non-useful kind occur in any of +the other systems or parts of organisms, they are, as a rule, +immediately extinguished by intercrossing. But whenever they arise in +the reproductive system in the way here suggested, they tend to be +preserved as new natural varieties, or incipient species. At first the +difference would only be in respect of the reproductive systems; but +eventually, on account of independent variation, other differences would +supervene, and the variety would take rank as a true species. + +Now we must remember that physiological isolation is not like those +other forms of isolation (e.g. geographical) which depend for their +occurrence on accidents of the environment, and which may therefore take +place suddenly in a full degree of completeness throughout a large +section of a species. Physiological isolation depends upon distinctive +characters belonging to organisms themselves; and it would be opposed to +the whole theory of descent with progressive modification to imagine +that absolute sterility usually arises, in a single generation between +two sections of a perfectly fertile species. Therefore evolutionists +must believe that in most, if not in all cases--could we trace the +history, say of any two species, which having sprung from a single +parent stock on a common area, are now absolutely sterile with one +another--we should find that this mutual sterility had been itself a +product of gradual evolution. Starting from complete fertility within +the limits of a single parent species, the infertility between +derivative or divergent species, _at whatever stage in their evolution +this began to occur_, must usually at first have been well-nigh +imperceptible, and thenceforth have proceeded to increase stage by +stage. + +But, if it be true that physiological isolation between genetically +allied groups must usually itself have been the product of a gradual +evolution; and if, when fully evolved, it constitutes a condition of the +first importance to any further differentiation of these groups (by +preventing fusion again into one group, more or less resembling the +original parent form), do we not perceive at least a strong probability +that in the lower stages of its evolution such mutual infertility must +have acted as a segregating influence between the diverging types, in a +degree proportional to its own development? The importance of mutual +sterility as a condition to divergent evolution is not denied, _when +this sterility is already present in an absolute degree_; and we have +just seen that, before it can have attained to this absolute degree _it +must presumably, and as a rule, itself have been the subject of a +gradual development_. Does it not therefore become, on merely antecedent +grounds, in a high degree probable, that from the moment of its +inception this isolating agency must have played the part of a +segregating cause, in a degree proportional to that of its completeness +as a physiological character? + +Whoever answers this question in the affirmative will have gone most of +the way towards accepting, on merely antecedent grounds, the theory of +physiological selection. And therefore it is that I have begun this +statement of the theory by introducing it upon these grounds, thereby +hoping to show how extremely simple--how almost self-evident--is the +theory which it will now be my endeavour to substantiate. I may here add +that the theory was foreshadowed by Mr. Belt in 1874[16], clearly +enunciated in its main features by Mr. Catchpool in 1884[17], and very +fully thought out by Mr. Gulick during a period of about fifteen years, +although he did not publish until a year after the appearance of my own +paper in 1886[18]. + + [16] _Nicaragua_, p. 207. + + [17] _Nature_, vol. xxxi. p. 4. + + [18] _Zool. Journal, Lin. Soc._, vol. xix. pp. 337-411 (1886); and + for Mr. Gulick's papers, _ibid._, vol. xx. pp. 189-274 (1887), vol. + xxiii. pp. 312-380 (1889). Mr. Gulick has recently drawn my + attention, in a private letter, to the fact that as early as 1872 a + paper of his was read at the British Association, bearing the title + _Diversity of Evolution under one set of External Conditions_, and + that here the principle of physiological segregation is stated. + Although it does not appear that Mr. Gulick then appreciated the + great importance of this principle, it entitles him to claim + priority. + +I must next proceed to state some of the leading features of +physiological selection in further detail. + +It has already been shown that Darwin clearly perceived that the very +general occurrence of some degree of infertility between allied species +cannot possibly be attributed to the _direct_ agency of natural +selection. His explanation was that the slight structural modifications +entailed by the transformation of one specific type into another, so +react upon the highly delicate reproductive system of the changing type +as to render it in some degree infertile with its parent type. Now the +theory of physiological selection begins by traversing this view. It +does not, however, deny that in _some_ cases the morphological may be +the prior change; but it strenuously denies that this must be so in +_all_ cases. Indeed, according to my statement in 1886, the theory +inclines to the view that, _as a rule_, the physiological change is +prior. At the same time, the theory, as I have always stated it, +maintains that it is immaterial whether, "in the majority of instances," +the physiological change has been prior to the morphological, or vice +versa; since in either case the physiological change will equally make +for divergence of character. + +To show this clearly the best way will be to consider the two cases +separately, taking first that in which the physiological change has +priority. In this case our theory regards any morphological changes +which afterwards supervene as due to the independent variability which +will sooner or later arise under the physiological isolation thus +secured. But to whatever causes the subsequent morphological changes may +be due, the point to notice is that they are as a general rule, +consequent upon the physiological change. For in whatever _degree_ such +infertility arises between two sections of a species occupying the same +area, in that _degree_ is their interbreeding prevented, and, therefore, +opportunity is given for a subsequent divergence of type, whether by the +influence of independent variability alone, or also by that of natural +selection, as now acting more or less independently on each of the +partially separated groups. In short, all that was said in the foregoing +chapters with respect to isolation in general, here applies to +physiological isolation in particular; and by supposing such isolation +to have been the prior change, we can as well understand the subsequent +appearance of morphological divergence on continuous areas, as in other +forms of isolation we can understand such divergence on discontinuous +areas, seeing that even a moderate degree of cross-infertility may be as +effectual for purposes of isolation as a high mountain-chain, or a +thousand miles of ocean. + +Here, then, are two sharply-defined theories to explain the very general +fact of there being some greater or less degree of cross-infertility +between allied species. The older, and hitherto current theory, +supposes the cross-infertility to be but an _accident_ of specific +divergence, which, therefore, has nothing to do with _causing_ the +divergence. The newer theory, on the other hand, supposes the +cross-infertility to have often been a necessary _condition_ to the +divergence having begun at all. Let us now consider which theory has +most evidence in its favour. + +First of all we have to notice the very general occurrence of the fact +in question. For when we include the infertility of hybrids, as well as +first crosses, the occurrence of some degree of infertility between +allied species is so usual that Mr. Wallace recommends experiments to +ascertain whether careful observation might not prove, even of species +which hybridize, "that such species, when crossed with their near +allies, do always produce offspring which are more or less sterile +_inter se_[19]." This seems going too far, but nevertheless it is the +testimony of a highly competent naturalist to the very general +occurrence of an association between the morphological differentiation +of species and the fact of a physiological isolation. Now I regard it as +little short of self-evident that this general association between +mutual infertility and innumerable secondary, or relatively variable +morphological distinctions, is due to the former having been an original +and a necessary condition to the occurrence of the latter, in cases +where intercrossing has not been otherwise prevented. + + [19] _Darwinism_, p. 169. + +The importance of physiological isolation, _when once fully developed_, +cannot be denied, for it is evident that if such isolation could be +suddenly destroyed between two allied species occupying a common area, +they would sooner or later become fused into a common type--supposing, +of course, no other form of isolation to be present. The necessity then +for this physiological form of isolation in _maintaining_ a specific +differentiation which has been already _attained_ cannot be disputed. +Yet it has been regarded as "Darwinian heresy" to suggest that it can +have been of any important service _during the process of attainment_, +or while the specific differentiation is being advanced, and this +notwithstanding that the physiological change must presumably have +developed _pari passu_ with the morphological, and notwithstanding that +in countless cases the former is associated with every conceivable +variety of the latter. + +Again, why should the physiological change be thus associated with +_every conceivable variety_ of morphological change? Throughout the +length and breadth of both vegetable and animal kingdoms we find this +association, in the great majority of cases, where new species arise. +Therefore, on the supposition that in all such cases the physiological +change has been adventitiously induced by the morphological changes, we +have to face an apparently unanswerable question--Why should the +reproductive mechanism of all organic beings have been thus arranged, as +it were, to change in immediate response to the very slightest +alteration in the complex harmony of "somatic" processes, which now more +than ever is recognized as exercising so comparatively little influence +on the _hereditary_ endowments of this mechanism? Consider the +difference between a worm and the bird that is eating it, an oak tree +and the gall-insect that is piercing it: are we to suppose that in all +cases, no matter how greatly the types differ, they must agree in this, +that when any parts of these complex structures change, ever so +slightly, the reproductive system is almost certain to be adventitiously +affected, yet always thus affected in the same peculiar way? + +If it be answered that the reproductive system is known to be very +sensitive to slight changes in the external conditions of life, the +answer proves too much. For though this is true, yet our opponents must +acknowledge that the reproductive system is not so sensitive, _in this +particular respect_, as their interpretation of the origin of specific +infertility requires. The proof of this point is overwhelming, for there +is the evidence from the entire range of our domesticated productions, +both vegetable and animal. Here the amount of structural change, which +has been slowly accumulated by artificial selection, is often much +greater in amount, and incomparably more rapid, than that which has been +induced between allied species by natural selection; and yet there is +scarcely any indication of the reproductive system having been affected +in the particular way that our opponents' theory requires. There are +many instances of its having been affected in sundry other ways +(chiefly, however, without any accompanying morphological change); but +among all the thousands of our more or less enormously modified +artificial types, there is scarcely one instance of such a peculiar +sexual relation between the modified descendants of a common type as so +usually obtains between allied species in nature. Yet in all other +respects evolutionists are bound to believe that the process of +modification has been in both cases strictly analogous. Why then this +conspicuous difference with respect to the reproductive system? + +The answer is simple. It has never been the object of breeders or of +horticulturists to select variations in the direction of +cross-infertility, for the swamping effects of intercrossing are much +more easily and rapidly prevented by artificial isolation. Consequently, +although they have been able to modify natural types in so many +directions and in such high degrees with regard to _morphology_, there +has been no accompanying physiological modification of the kind +required. But in nature there is no such thing as artificial, i.e. +intentional, isolation. Consequently, on common areas it must usually +happen that those changes of morphology which are associated with +cross-infertility are the only ones which can arise. Hence the very +remarkable contrast between our domesticated varieties and natural +species with regard to cross-infertility is just what the present theory +would expect, or, indeed, require. But on any other theory it has +hitherto remained inexplicable. + +In particular, the contrast in question has constituted one of the main +difficulties with which the theory of natural selection has hitherto had +to contend, not only in the popular mind, but also in the judgement of +naturalists, including the joint-authors of the theory themselves. Thus +Darwin says:-- + + The fertility of varieties is, with reference to my theory, of + equal importance with the sterility of species, for it seems to + make a broad and clear distinction between varieties and + species[20]. + + [20] _Origin of Species_, p. 136. + +And Mr. Wallace says:-- + + One of the greatest, or perhaps we may say the greatest, of all the + difficulties in the way of accepting the theory of natural + selection as a complete explanation of the origin of species, has + been the remarkable difference between varieties and species in + respect of fertility when crossed[21]. + + [21] _Darwinism_, p. 152. + +Now, in view of this conspicuous contrast, Darwin suggested that species +in a state of nature "will have been exposed during long periods of time +to more uniform conditions than have domesticated varieties, and [that] +this may well make a wide difference in the result." Now we have to +remember that species, living and extinct, are numbered by millions, and +represent every variety of type, constitution, and habits; is it +probable, then, that this one peculiarity of the reproductive system +should be due, in so many cases, to some merely incidental effect +produced on that system by uniform conditions of life? Again, _ex +hypothesi_, at the time when a variety is first forming, the influence +exercised by uniform conditions of life (whatever in different cases +this may happen to be) cannot be present as regards that variety: yet +this is just the time when its infertility with the parent (or allied) +form is most likely to have arisen; for it is just then that the nascent +variety would otherwise have been most liable to extinction by free +intercrossing--even supposing that in the presence of such intercrossing +the variety could ever have come into existence at all. + +Mr. Wallace meets the difficulty by arguing that sterility between +allied species may have been brought about by the direct influence of +natural selection. But, as previously remarked, this view is expressly +opposed to that of Darwin, who held that Wallace's contention is +erroneous. + +It will be seen, then, that both Darwin, and Wallace, fully recognize +the necessity of finding some explanation of the infertility of allied +species, over and above the mere reaction of morphological +differentiation on the physiology of the reproductive system, and they +both agree in suggesting additional causes, though they entirely +disagree as to what these causes are. Now, the theory of physiological +selection likewise suggests an additional cause--or, rather, a new +explanation--and one which is surely the most probable. For what is to +be explained? The very general association of a certain physiological +peculiarity with that amount of morphological change which distinguishes +species from species, of whatever kind the change may be, and in +whatever family of the animal or vegetable kingdom it may occur. Well, +the theory of physiological selection explains this very general +association by the simple supposition that, at least in a large number +of cases, it was the physiological peculiarity which first of all led to +the morphological divergence, by interposing the bar of sterility +between two sections of a previously uniform species; and by thus +isolating the two sections one from another, started each upon a +subsequently independent course of divergent evolution. + +Or, to put it in another way, if the occurrence of this physiological +peculiarity has been often the only possible means of isolating two +sections of a species occupying a common area, and thus giving rise to a +divergence of specific type (as obviously _must_ have been the case +wherever there was an absence of any other form of isolation), it is +nothing less than a necessary consequence that many allied species +should now present the physiological peculiarity in question. Thus the +association between the physiological peculiarity and the morphological +divergence is explained by the simple hypothesis, that the former has +acted as a necessary condition to the occurrence of the latter. In the +absence of other forms of isolation, the morphological divergence could +not have taken place at all, had not the physiological peculiarity +arisen; and hence it is that we now meet with so many cases where such +divergence is associated with this peculiarity. + + * * * * * + +So far we have been considering the physiological change as historically +the prior one. Here, at first sight, it may seem that the segregative +power of physiological selection must end; for it may well seem +impossible that the physiological change can ever be necessary for the +divergence of morphological varieties into true species in cases where +it has _not_ been the prior change, but has only set in after +morphological changes have proceeded far enough to have already +constituted definite varieties. A little thought, however, will show +that physiological selection is quite as potent a condition to the +differentiation of species when it occurs after varietal divergence has +begun, as it is when it occurs before the divergence--and hence that it +really makes no difference to the theory of physiological selection +whether, in particular cases, the cross-infertility arises before or +after any structural or other modifications with which it is +associated. + +For the theory does not assert that all varieties have been due to +physiological selection. There are doubtless many other causes of the +origin of varieties besides cross-infertility with parent forms; but, as +a general rule, it does not appear that they are by themselves capable +of carrying divergence beyond a merely varietal stage. In order to carry +divergence to the stage of producing _species_, it appears to be a +general condition that, sooner or later, cross-infertility should +arise--seeing that, when varieties do succeed in becoming species, we +almost invariably find that, as a matter of fact, cross-infertility has +arisen. Hence, if cross-infertility has thus usually been a necessary +condition to a varietal divergence becoming specific, it can make no +material difference when the incipient infertility arose. + +It may be asked, however, whether I suppose that, when the physiological +change is subsequent, it is directly _caused_ by change of structure, +size, colour, &c., or that it arises, so to speak, accidentally, from +other causes which may have affected the sexual system in the required +way. To this question I may briefly reply, that, looking to the absence +of any influence exercised on the reproductive systems of our +domesticated plants and animals by the great and varied changes which so +many of these forms present, it would seem that among natural varieties +such closely analogous changes are presumably not the usual causes of +the physiological change, even where the latter are subsequent to the +former. Nevertheless, I do not deny that in some of these cases changes +of structure, size, colour, &c., may be the causes of the physiological +change by reacting on the sexual system in the required way. But in +such cases free intercrossing will have prevented the perpetuation of +any morphological changes, save those which have the power of so +reacting on the reproductive system as to produce the physiological +change, and thus to protect themselves against the full and adverse +power of free intercrossing. We know that slight or initial changes of +structure, colour, &c., frequently occur as varieties, and yet that on +common areas very few of these varieties become distinct species: free +intercrossing prevents any such further divergence of character. But if +in the course of many such abortive attempts, as it were, to produce a +new species, nature happens to hit upon a structural or a colour +variation which is capable of reacting on the sexual system in the +particular way required, then this variation will be enabled to protect +itself against free intercrossing in proportion to its own development. +Or, in other words, the more it develops as a morphological change, the +more will it increase the physiological change; while the more the +physiological change is thus increased, the more will it in turn promote +the morphological. By such action and reaction the development of each +furthers the development of the other, till from an almost imperceptible +variety, apparently quite fertile with its parent form, there arises a +distinct species absolutely sterile with its parent form. In such cases, +therefore, it is still the physiological conditions which have +_selected_ the particular morphological changes capable of so reacting +on the reproductive system as to produce cross-infertility, and thus to +protect themselves against the destructive power of free intercrossing. +So to speak, free intercrossing is always on the watch to level down +any changes which natural selection, or any other cause of varietal +divergence, may attempt to produce; and therefore, in order to +produce--or to increase--such divergence in the absence of any other +form of isolation, natural selection must hit upon such changes of +structure, form, or colour, as are so correlated with the reproductive +system as to create the physiological isolation that is required. + +To show how the principle of selective fertility may be combined with +what apparently is the most improbable form of isolation for this +purpose--the geographical--I quote the following suggestion made by +Professor Lloyd Morgan in his _Animal Life and Intelligence_:-- + + Suppose two divergent local varieties were to arise in adjacent + areas, and were subsequently (by stress of competition or by + geographical changes) driven together into a single area.... If + their unions be fertile, the isolation will be annulled by + intercrossing--the two varieties will form one mean or average + variety. But if the unions be infertile, the isolation will be + preserved, and the two varieties will continue separate. Suppose + now, and the supposition is by no means an improbable one, that + this has taken place again and again in the evolution of species; + then it is clear that those varietal forms which had continued to + be fertile together would be swamped by intercrossing; while those + varietal forms which had become infertile would remain isolated. + Hence, in the long run, isolated forms occupying a common area + would be infertile, (p. 107.) + +If then cross-sterility may thus arise even in association with +geographical isolation, may it not also arise in its absence? And may it +not thus give rise to the differentiation of varieties on account of +this physiological isolation alone? + +Only two further points need be mentioned to make this statement of +physiological selection as complete as the present _resume_ of its main +principles requires. + +The first is, that, as Mr. Wallace remarks, "every species has come into +existence coincident both in space and time with a pre-existing and +closely allied species." I regard this as important evidence that +physiological selection is one of the natural causes concerned. For the +general fact implied is that every species has come into existence on an +area occupied by its parent type, and therefore under circumstances +which render it imperative that intercrossing with that type should be +prevented. In the case of monotypic evolution by natural selection +alone, intercrossing with the parent type is prevented through the +gradual extinction of that type by successive generations of the +developing type. But in the case of polytypic evolution, intercrossing +with the parent type can only be prevented by some form of isolation +other than natural selection; and here it is evident that +cross-infertility with the parent type must be as efficient to that end +as any other form of isolation that can be imagined. Consequently we +might almost have expected beforehand that in a large proportional +number of cases cross-infertility should have been the means employed. +And the fact that this is actually the case so far corroborates the only +theory which is able to explain it. + +The second point is this. + +It appears to be comparatively rare for any cause of specific divergence +to prove effectual on common areas, unless it sooner or later becomes +associated with some degree of cross-infertility. But through this +association, the segregating influence of both the causes concerned is, +as Mr. Gulick has shown, greatly increased. For instance, if the +segregating influence of some degree of cross-infertility be associated +with that of any other form of isolation, then, not only will the two +segregating influences be added, but multiplied together. And thus, by +their mutual action and reaction, divergent evolution is promoted at a +rapidly increasing rate. + +I will now summarize the main points of the theory of physiological +isolation in a categorical form. + +1. If no other form of isolation be present, specific divergence can +only take place when some degree of cross-infertility has previously +arisen between two or more sections of a species. + +2. When such cross-infertility has arisen it may cause specific +divergence, either (_a_) by allowing independent variability in each of +the physiologically isolated groups; (_b_) by becoming associated with +any other cause of differentiation already operating; or (_c_) by both +these means combined. + +3. As some degree of cross-infertility generally obtains between allied +species, we are justified in concluding that this has been the most +frequent--or, at any rate, the most effective--kind of isolation where +the origin of species is concerned; and therefore the kind with which, +in the case of species-formation, natural selection, or any other cause +of specific divergence, has been most usually associated. + +4. Where varietal divergence has begun in the absence of +cross-infertility, such divergence seems, as a general rule, to have +been incapable of attaining to a specific value. + +5. Therefore, in the vast majority of such cases, it must have been +those varietal changes of structure, size, colour, &c., which happened +to have afterwards been assisted by the reproductive change that were on +this account _selected_ as successful candidates for specific +differentiation. + +6. It follows, that it makes no difference to the general theory of +physiological selection in what proportion of cases the physiological +change has been the initial change; for, whether prior or subsequent to +the varietal changes with which it becomes associated, its presence has +been equally important as a condition to specific divergence. + +7. When physiological isolation becomes associated with natural +selection, or any other form of homogamy, the segregative power of both +is augmented. Moreover, so great is the augmentation that even very +moderate degrees of physiological isolation--themselves capable of +effecting little or nothing--become very powerful when associated with +moderate degrees of any other kind of homogamy, and vice versa. + +8. The theory of physiological selection effectually explains the +divergent evolution of specific types and the cross-infertility of such +types when evolved. + + * * * * * + +To prevent, if possible, the continuance of certain misunderstandings +with regard to my original statement of the new theory, let me here +disclaim some views which have been assigned to me. They are: + +1. That the theory of physiological selection is opposed to the theory +of natural selection. Far from this being so, it is--at all events in my +own opinion--a very important aid to it, in preventing free +intercrossing on a common area, and thus allowing divergent evolution to +occur within that area. + +2. That, in advancing the theory of physiological selection as "an +additional suggestion on the origin of species," I wish to represent it +as being the originating cause of _all_ species. What I hold is, that +all species must have owed their origin to _isolation_, in some form or +other; but that as physiological selection is only one among many other +forms of isolation (including natural selection), and as it can only act +on common areas, a large number of species must have been formed without +its aid. + +3. That I imagine physiological varieties always to arise +"sporadically," or as merely individual "sports" of the reproductive +system. On the contrary, I expressly stated that this is _not_ the way +in which I suppose the "physiological variation" to arise, when giving +origin to a new species; but that it arises, whenever it is effectual, +as a "collective variation" affecting a number of individuals +simultaneously, and therefore characterizing "a whole race, or strain." + +4. That I suppose physiological selection always to act alone. This I +have never supposed. The essential point is, not that the physiological +isolation is unassociated with other forms of isolation, but that unless +associated with some degree of physiological isolation, no one of the +other forms is capable of originating species on common areas with any +approach to frequency. This proposition is the essence of the new +theory, and I take it to be proved, not only by general deductive +reasoning which shows that it _must_ be so, but also by the fact of an +otherwise inexplicable association between specific divergence on common +areas and some more or less considerable degree of mutual infertility. + + + + +CHAPTER IV. + +EVIDENCES OF PHYSIOLOGICAL SELECTION. + + +I will now give an outline sketch of the evidences in favour of the +theory which has been set forth in the preceding chapter, stating first +what is the nature of the verification which it requires. + +The theory is deduced from a highly general association between +distinctive specific characters of _any_ kind and a relatively constant +specific character of a _particular_ kind--namely, sexual exclusiveness. +For it is from this highly general association that the theory infers +that this relatively constant specific character has been at least one +of the needful conditions to the development of the other specific +characters with which it is found associated. Hence the necessary +verification must begin by showing the strength of the theory on these +merely deductive, or antecedent, grounds. It may then proceed to show +how far the facts of organic nature corroborate the theory in other and +independent ways. + +First, let it be carefully observed that here we have to do only with +the _fact_ of selective fertility, and with its _consequences_ as +supposed by the theory: we have nothing to do either with its _causes_ +or its _degrees_. Not with its causes, because in this respect the +theory of physiological selection is in just the same position as that +of natural selection: it is enough for both if the needful variations +are provided, without its being incumbent on either to explain the +causes which produce them. Not with its degrees, because, in the first +place, it can only be those degrees of variation which in particular +cases are supposed adequate to induce specific divergence, that fall +within the scope of the theory; and because, in the second place, +degrees which are adequate only to induce--or to assist in inducing, +_varietal_ divergence, must always tend to increase, or pass into higher +degrees. + + +_Antecedent Standing of the Theory._ + +The antecedent standing or logical basis of the theory has already been +in large measure displayed in the preceding chapter; for it was +impossible to state the theory without thereby showing in how +considerable a degree it is self-evident. A brief recapitulation is +therefore all that is here necessary. + +It has been shown that divergent or polytypic evolution on common areas +is inexplicable by natural selection alone. Hence the question arises: +What form of isolation has, under such circumstances, rendered possible +divergent evolution? In answer to this question the theory of +physiological selection suggests that variations in the reproductive +function occur in such a way as to isolate more or less perfectly from +each other different sections of a species. While cross-fertility +remains unimpaired among the members of each section, there is more or +less cross-infertility when members of either section mate with those of +the other. Thus a physiological barrier is interposed between the two +sections; and any divergences of structure, colouring, or instinct +arising in the members of either section will not in any way be affected +by such divergences as arise among the members of the other. + +In support of this suggestion, it has been shown in the preceding +chapter that the very general association of cross-infertility with +specific differentiation points most strongly to the inference that the +former has usually been an indispensable condition to the occurrence of +the latter. It cannot be denied that in many cases the specific +distinction is now maintained by means of that sexual isolation which +cross-infertility confers: it is therefore probable that such isolation +has been instrumental in securing its initial attainment. + +This probability is strengthened by the observed fact that the general +association in question is conspicuously absent in the case of +domesticated varieties, notwithstanding that their multitudinous and +diverse varietal characters usually equal, and frequently surpass, +specific characters in their degrees of divergence. + +Since, then, it would seem to be impossible for divergent evolution on +common areas to take place in the absence of some mode of isolation; +since cross-infertility appears to be the only possible mode under the +given circumstances; and since among domesticated varieties, where +isolation is otherwise secured by artificial means, cross-infertility is +usually absent, the logical foundations of the theory of physiological +selection would seem to be securely laid. + +We may therefore pass to more special lines of evidence. + + +_Evidence from Geographical Distribution._ + +Darwin has adduced very good evidence to show that large areas, +notwithstanding the disadvantages which (on his theory) must arise from +free intercrossing, are what he terms better manufactories of species +than smaller areas, such as oceanic islands. On the other hand, as a +matter of fact, oceanic islands are comparatively rich in peculiar +species. These two statements, however, are not incompatible. Smaller +areas are, as a rule, rich in peculiar species relatively to the number +of their inhabitants; but it does not follow that they are rich in +species as contrasted with larger areas containing very many more +inhabitants. Therefore, the rules are that large areas turn out an +absolutely greater number of specific types than small areas; although, +relatively to the number of individuals or amount of population, the +small areas turn out a larger number of species than the large areas. + +Now, these two complementary rules admit of being explained as Darwin +explains them. Small and isolated areas are rich in species relatively +to the amount of population, because, as we have before seen, this +population has been permitted to develop an independent history of its +own, shielded from intercrossing with parent forms, and from competition +with exotic forms; while, at the same time, the homogamy thus secured, +combined with change of environment, will give natural selection an +improved chance of finding new points of departure for its operation. On +the other hand, large and continuous areas are favourable to the +production of numerous species, first, because they contain a large +population, thus favouring the occurrence of numerous variations; and, +secondly, because the large area furnishes a diversity of conditions in +its different parts, as to food, climate, attitude, &c., and thus so +many different opportunities for the occurrence of sundry forms of +homogamy. Now, it is obvious that of all these sundry forms of homogamy, +physiological selection must have what may be termed a first-rate +opportunity of assisting in the manufacture of species on large areas. +For not only is it upon large and continuous areas that the antagonistic +effects of intercrossing are most pronounced (and, therefore, that the +influence of physiological selection must be most useful in the work of +species-making); but here also the diversity in the external conditions +of life, which the large area supplies to different parts of the +extensive population, cannot fail to furnish physiological selection +with a greater abundance of that particular variation in the +reproductive system on which its action depends. Again, and of still +more importance, on large areas there are a greater _number_ of species +already differentiated from one another as such; thus a greater number +of already sexually differentiated forms are presented for further +differentiation at the hands of physiological selection. For all these +reasons, therefore, we might have expected, upon the new theory, that +large and continuous areas would be good manufactories of species. + +Again, Darwin has shown that not only large areas, but likewise +"dominant" genera within those areas, are rich in species. By dominant +genera he meant those which are represented by numerous individuals, as +compared with other genera inhabiting the same area. This general rule +he explains by the consideration that the qualities which first led to +the form being dominant must have been useful; that these would be +transmitted to the otherwise varying offspring; and, therefore, that +when these offspring had varied sufficiently to become new species, they +would still enjoy their ancestral advantages in the struggle for +existence. And this, doubtless, is in part a true explanation; but I +also think that the reason why dominant genera are rich in species, is +chiefly because they everywhere present a great number of individuals +exposed to relatively great differences in their conditions of life: or, +in other words, that they furnish the best raw material for the +manufacture of species by physiological selection, as explained in the +last paragraph. For, if the fact of dominant genera being rich in +species is to be explained _only_ by natural selection, it appears to me +that the useful qualities which have already led to the dominance of the +ancestral type ought rather to have proved inimical to its splitting up +into a number of subordinate types. If already so far "in harmony with +its environment" as to have become for this reason dominant, one would +suppose that there is all the more reason for its not undergoing change +by the process of natural selection. Or, at least, I do not see why the +fact of its being in an unusual degree of harmony with its environment +should in itself constitute any unusual reason for its modification by +survival of the fittest. On the other hand, as just observed, I do very +plainly see why such a reason is furnished for the modifying influence +of physiological selection. + +Let us next turn to another of Darwin's general rules with reference to +distribution. He took a great deal of trouble to collect evidence of the +two following facts, namely, (1) that "species of the larger genera in +each country vary more frequently than the species of the smaller +genera"; and (2) that "many of the species included within the larger +genera resemble varieties in being very closely, but unequally, related +to each other, and in having restricted ranges[22]." By larger genera he +means genera containing many species; and he accounts for these general +facts by the principle, "that where many species of a genus have been +formed, on an average many are still forming." But _how_ forming? If we +say by natural selection alone, we should expect to find the +multitudinous species differing from one another in respect of features +presenting well-marked adaptive meanings; yet this is precisely what we +do not find. For Darwin's argument here is that "in large genera the +amount of difference between the species is often exceedingly small, so +that in this respect the species of the larger genera resemble varieties +more than do the species of the smaller genera." Therefore the argument, +while undoubtedly a very forcible one in favour of the fact of +_evolution_, appears to me scarcely consistent with the view of this +evolution being due solely to natural selection. On the other hand, the +argument tells strongly (though unconsciously) in favour of +physiological selection. For the larger a genus, or the greater the +number of its species, the greater must be the opportunity for the +occurrence of that particular kind of variation on which the principle +of physiological selection depends. The species of a genus may be +regarded as so many varieties which have already been separated from one +another physiologically; therefore each of them may now constitute a new +starting-point for a further and similar separation--particularly as, in +virtue of their previous segregation, many are now exposed to different +conditions of life. Thus, it seems to me, we can well understand why it +is that genera already rich in species tend to grow richer; while such +is not the case in so great a degree with genera that are poor in +species. Moreover, we can well understand that, multiplication of +species being as a rule, and in the first instance, determined by +changes in the reproductive system, wherever a large number of new +species are being turned out, the secondary differences between them +should be "often exceedingly small"--a general correlation which, so far +as I can see, we are not able to understand on the theory of natural +selection. + + [22] _Origin of Species_, pp. 44, 45. + +The two subsidiary facts, that very closely allied species have +restricted ranges, and that dominant species are rich in varieties, both +seem to tell more in favour of physiological than of natural selection. +For "very closely allied species" is but another name for species which +scarcely differ from one another at all except in their reproductive +systems; and, therefore, the more restricted their ranges, the more +certainly would they have become fused by intercrossing with one +another, had it not been for the barrier of sterility imposed by the +primary distinction. Or rather, I should say, had it not been for the +original occurrence of this barrier, these now closely-allied species +could never have become species. Again, that dominant species should be +rich in varieties is what might have been expected; for the greater the +number of individuals in a species, the greater is the chance of +variations taking place in all parts of the organic type, and +particularly in the reproductive system, seeing that this system is the +most sensitive to small changes in the conditions of life, and that the +greater the number of individuals composing a specific type, the more +certainty there is of some of them encountering such changes. Hence, the +richness of dominant species in varieties is, I believe, mainly due to +the greater opportunity which such species afford of some degree of +cross-infertility arising between their constituent members. + +Here is another general fact, also first noticed by Darwin, and one +which he experiences some difficulty an explaining on the theory of +natural selection. He says:-- + + In travelling from north to south over a continent, we generally + meet at successive intervals with closely-allied or representative + species, evidently filling the same place in the economy of the + land. These representative species often meet and interlock, and as + one becomes rarer and rarer, the other becomes more and more + frequent, till the one replaces the other. But if we compare these + species where they intermingle, they are generally as absolutely + distinct from each other in every detail of structure as are + specimens taken from the metropolis of each.... In the + intermediate region, having intermediate conditions of life, why do + we not now find closely-linking intermediate varieties? This + difficulty for a long time quite confounded me. But I think it can + in large part be explained[23]. + + [23] _Origin of Species_, ed. 6, pp. 134, 135. + +[Illustration:] + +His explanation is that, "as the neutral territory between two +representative species is generally narrow in comparison with the +territory proper to each, ... and as varieties do not essentially differ +from species, the same rule will probably apply to both; and, therefore, +if we take a varying species inhabiting a very large area, we shall have +to adapt two varieties to two large areas, and a third variety to a +narrow intermediate zone." It is hence argued that this third or +intermediate variety, on account of its existing in lesser numbers, will +probably be soon overrun and exterminated by the larger populations on +either side of it. But how is it possible "to adapt two varieties to two +large areas, and a third [transitional] variety to a narrow intermediate +zone," in the face of free intercrossing on a continuous area? Let _A_, +_B_, and _C_ represent the three areas in question. According to the +argument, variety _A_ passes first into variety _B_, and then into +variety _C_, while variety _B_ eventually becomes exterminated by the +inroads both from _A_ and _C_. But how can all this have taken place +with nothing to prevent intercrossing throughout the entire area _A_, +_B_, _C_? I confess that to me it seems this argument can only hold on +the supposition that the analogy between varieties and species extends +to the reproductive system; or, in a sense more absolute than the +argument has in view, that "varieties do not essentially differ from the +species" which they afterwards form, but from the first show some degree +of infertility towards one another. And, if so, we have of course to do +with the principles of physiological selection. + +That in all such cases of species-distribution these principles have +played an important part in the species-formation, appears to be +rendered further probable from the suddenness of transition on the area +occupied by contiguous species, as well as from the completeness of +it--i. e. the absence of connecting forms. For these facts combine to +testify that the transition was originally due to that particular change +in the reproductive systems of the forms concerned, which still enables +those forms to "interlock" without intercrossing. On the other hand, +neither of these facts appears to me compatible with the theory of +species-formation by natural selection alone. + +But this leads us to another general fact, also mentioned by Darwin, and +well recognized by all naturalists, namely, that closely allied species, +or species differing from one another in trivial details, usually occupy +contiguous areas; or, conversely stated, that contiguity of geographical +position is favourable to the appearance of species closely allied to +one another. Now, the large body of facts to which I here allude, but +need not at present specify, appear to me to constitute one of the +strongest of all my arguments in favour of physiological selection. +Take, for instance, a large continental area, and follow across it a +chain of species, each link of which differs from those on either side +of it by the minute and trivial distinctions of a secondary kind, but +all the links of which differ from one another in respect of the primary +distinction, so that no one member of the series is perfectly fertile +with any other member. Can it be supposed that in every case this +constant primary distinction has been superinduced by the secondary +distinctions, distributed as they are over different parts of all these +kindred organisms, and yet nowhere presenting any but a trifling amount +of morphological change? + +For my own part, I cannot believe--any more than Darwin could +believe--that all these numerous, diverse, and trivial changes have +always had the accidental effect of inducing the same peculiar change in +the reproductive system, and so producing it without any reference to +the process of specific divergence. Nor can I believe, as Darwin +incidentally and provisionally suggested, that prolonged exposure to +uniform conditions of life have so generally induced an equally +meaningless result. I can only believe that all the closely allied +species inhabiting our supposed continent, and differing from one +another in so many and such divers points of small detail, are merely so +many records of the fact that selective fertility has arisen among their +ancestry, and has thus given as many opportunities for the occurrence of +morphological differentiations as it has furnished cases of efficient +isolation. Of course, I do not deny that many, or probably most, of +these trivial morphological differentiations have been produced by +natural selection on account of their utility: I merely deny that they +could have been so produced on this common area, but for the sexual +isolation with which every distinct set of them is now found to be +associated. + + +_Evidence from Topographical Distribution of Species._ + +By topographical distribution I mean the distribution of organisms with +reference to comparatively small areas, as distinguished from larger +regions with reference to which the term geographical distribution is +appropriate. + +It will be at once apparent that a study of the topographical +distribution of organic types is of even more importance for us than a +study of their geographical distribution. For while the former study is +conducted, as it were, with a low power of our observing microscope, the +latter is conducted with a high power. The larger facts of geographical +distribution yield, indeed, all the general characters which we might +expect them to yield, on the theory that divergence of specific types on +common areas has been in chief part determined by physiological +conditions. But for the purpose of testing this theory in a still more +exacting manner, it is of the first importance to consider the more +detailed facts of topographical distribution, since we here come to +closer quarters with the problem of specific differentiation. Therefore, +as we have already considered this problem under the most general points +of view, we will now consider it under more special points of view. + + * * * * * + +It is self-evident, as we have seen in the preceding section, that the +greater the number of individuals of the same species on a given area, +the less must be the power of natural selection to split that species +into two or more allied types; because, the more crowded the population, +the greater must be the uniformitarian effect of free intercrossing. +This obvious fact has been insisted upon by several previous writers on +Darwinism; and the only reason why it has not been recognized by all +naturalists is that so few of them have observed the all-important +distinction between monotypic and polytypic evolution. The denser the +population, and therefore the greater the intercrossing and the severer +the struggle for existence within the species, the better will it be for +_transmutation_ of the species by natural selection; but the worse it +will be for _differentiation_ of the species by this form of homogamy. +On the other hand, if physiological selection be entertained as a form +of homogamy, the denser the population, the better opportunity it will +have of differentiating the species, first, because a greater number of +individuals will be present in which the physiological change may arise, +and, secondly, because, if it does arise, the severity of the struggle +for existence will _then_ give natural selection a better chance of +acting rapidly and effectually on each of the isolated sections. + +Hence, where the question is whether selective fertility has played any +large or general part in the differentiation of specific types, the best +criterion we can apply is to ascertain whether it is a general rule that +closely allied species occur in intimate association, so that their +individual members constitute, as it were, a single population, or, on +the other hand, whether they occur rather on different sides of +physical barriers. If they occur intimately associated, the form of +homogamy to which their differentiation was due must have presumably +been the physiological form; whereas, if they are proved to be +correlated with physical barriers, the form of homogamy which was +concerned in their differentiation must presumably have been the +geographical form. + +Now, at first this consideration was a trouble to me, because Moritz +Wagner had strenuously argued--and supported his argument by a +considerable wealth of illustration--that allied species are always +found correlated with physical barriers or discontinuous areas. +Weismann's answer, indeed, had shown that Wagner's statement was much +too general: nevertheless, I was disappointed to find that so much could +be said in favour of the geographical (or topographical) form of +isolation where closely allied species are concerned. Subsequently, +however, I read the writings of Naegeli on this subject, and in them I +find a very different state of matters represented. + +Seeing as clearly as Wagner that it is impossible under any +circumstances for natural selection to cause specific _differentiation_ +unless assisted by some other forms of homogamy, but committing the same +oversight as Wagner and Weismann in supposing that the only other form +of homogamy in nature is geographical isolation, Naegeli, with great +force of reasoning, and by many examples, founded his argument against +the theory of natural selection on the ground that in the vegetable +kingdom closely allied species are most frequently found in intimate +association with one another, not, that is to say, in any way isolated +by means of physical barriers. This argument is everywhere logically +intact; and, as he sustains it by a large knowledge of topographical +botany, his indictment against natural selection as a cause of specific +_differentiation_ appeared to be insurmountable. And, in point of fact, +it _was_ insurmountable; so that the whole problem of the origin of +species by _differentiation on common areas_ has hitherto been left in +utter obscurity. Nor is there now any escape from this obscurity, unless +we entertain the "supplementary factor" of selective fertility. And, +apparently, the only reason why this has not been universally +recognized, is because Darwinians have hitherto failed to perceive the +greatness of the distinction between the _differentiation_ and the +_transmutation_ of species; and hence have habitually met such +overwhelming difficulties as Naegeli presented by an illogical +confounding of these two totally distinct things. + +But if the idea of selective fertility had ever occurred to Naegeli as a +form of segregation which gives rise to specific differentiation, I can +have no doubt that so astute and logical a thinker would have perceived +that his whole indictment against natural selection was answered. For it +is incredible that he should not have perceived how this physiological +form of homogamy (supposing it to arise _before_ or _during_, and not +_after_ the specific differentiation) would perform exactly the same +function on a continuous area, as he allowed that "isolation" does on a +discontinuous one. + +However, be this as it may, there cannot be any question touching the +immense value of his facts and arguments as evidence in favour of +physiological selection--albeit this evidence was given unconsciously, +or, as it were, prophetically. Therefore I will here quote a few +examples of both, from his paper _Du Developpement des Especes +Sociales_[24]. + + [24] _Archives des Sciences physiques et naturelles_ (Geneve), vol. + liii. (1875), pp. 211-236. + +After stating the theory of natural selection, he says that if the +theory is (of itself) a true explanation of the origin (or divergence) +of specific forms, it ought to follow that + + two closely allied forms, derived the one from the other, would + necessarily occupy two different geographical areas [or + topographical stations], since otherwise they would soon become + blended. Until they had already become sufficiently consolidated as + distinct species to render mutual intercrossing highly improbable, + they could not be intermingled without disadvantage [to + differentiation]. Had Darwin endeavoured to support his hypothesis + by facts, he would, at least in the vegetable kingdom, have found + little to favour his cause. I can cite many hundreds of cases, in + which species in every stage of development have been found closely + mingling with one another, and not in any way isolated. Therefore, + I do not think that one can rightly speak of natural selection in + the Darwinian sense in the vegetable kingdom; and, in my + estimation, there is a great difference between the formation of + species by nature and the production of stock by a breeder.... (p. + 212). + + Of the two kinds of distribution (i. e. growing apart and growing + together), Synoicy (or growing together) is by far the most usual + in nature. I reckon that out of a hundred allied vegetable forms, + at least ninety-five would be found to be synoical (p. 219). + +This is a most important point. That so enormous a proportion of +vegetable species should have originated in intimate association with +their parent or sister types, is clearly unintelligible on the theory of +natural selection alone; there obviously _must_ be some other form of +homogamy which, whether or not in all places _associated_ with natural +selection, is the primary condition to the differentiation. Such I hold +with Naegeli, is a logical necessity; and this whether or not I am right +in believing the other form of homogamy in question to be selective +fertility. But I go further and say, Surely there can be no rational +question that this other form of homogamy must have been, at any rate as +a highly general rule, the one which I have assigned. For how is it that +in these ninety-five per cent. of cases, where vegetable species are +growing intimately associated with their nearest allies, there is no +hybridizing, or blending and relapsing to the original undifferentiated +types? We know well the answer. These are fully differentiated species, +and, as such, are protected from mutual intercrossing by the barrier of +mutual sterility. But now, if this bar is thus necessary for preserving +the specific distinctions when they have been fully developed, much more +must it have been so to admit of their development; or, otherwise +stated, since we know that this barrier is associated with "synoical" +species, and since we clearly perceive that were it withdrawn these +species would soon cease to exist, can we reasonably doubt that their +existence (or origin) is due to the previous erection of this barrier? +If synoical species were comparatively rare, the validity of such +reasoning might be open to question; or, even if we should not doubt it +in such cases, at any rate we might well doubt the importance or extent +of selective fertility as a factor in the origination of species. But +the value of Naegeli's writings on the present subject consists in +showing that synoical species constitute so overwhelming a majority of +the vegetable kingdom, that here, at all events, it appears impossible +to rate too highly the importance of the principle I have called +physiological selection. + + + + +CHAPTER V. + +FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION. + + +_Evidence from Topographical Distribution of Varieties._ + +In the last section we have considered the topographical distribution of +closely allied _species_. I now propose to go still further into matters +of detail, by considering the case of natural _varieties_. And here we +come upon a branch of our inquiry where we may well expect to meet with +the most crucial tests of our theory. For if it should appear that these +nascent species more or less resemble fully developed species in +presenting the feature of cross-infertility, the theory would be +verified in the most direct and conclusive manner possible. These +nascent species may be called embryo species, which are actually in +course of differentiation from their parent-type; and therefore, if they +do not exhibit the feature in relation to that type which the present +theory infers to be necessary for the purposes of differentiation, the +theory must be abandoned. On the other hand, if they do exhibit this +feature, it is just the feature which the theory predicted as one that +would be found highly characteristic of such embryo types. +Contrariwise, the theory of natural selection can have no reason to form +any such anticipation; or rather its anticipation would necessarily +require to be the exact opposite. For, according to this theory, the +cross-infertility of allied species is due, either to correlation with +morphological changes which are being produced by the selection, or +else, as Darwin supposed, to "prolonged exposure to uniform conditions +of life"; and thus, in either case, the sterility variation ought to be, +as a general rule at all events, subsequent to the specific +differentiation, and, according to Darwin's view, _long_ subsequent. +Thus we ought not to find that the physiological change is ever, on any +large or general scale, the initial change; nor ought we to find that it +is, on any such scale, even so much as a contemporary change: there +ought, in fact, to be no constant or habitual association between +divergence of embryo-types and the concurrence of cross-infertility. + +Now, it will be my endeavour to prove that there is an extraordinarily +general association between _varietal_ divergence and cross-infertility, +_wherever common areas are concerned_; and in as far as this can be +proved, I take it that the evidence will make wholly in favour of +physiological selection as the prime condition to specific divergence, +while at the same time they will make no less wholly, _and quite +independently_, against natural selection as the unaided cause of such +divergence. + +I shall begin with some further quotations from Naegeli. + + Species may be synoical at all stages of relationship. We come + across varieties, scarcely distinguishable from one another, + growing in the same locality (as, for example, the _Cirsium + heterophyllum_, with smooth or jagged leaves, the _Hieracium + sylvaticum_, with or without caulinary leaves); again, we meet + other varieties more accentuated (as the _H. hoppeanum_, with under + ligules of white or red, the _Campanula_, with white or lilac + flowers, &c.), other varieties even more marked, which might almost + be elevated to the rank of species (_Hieracium alpinum_, with hairs + and glands, and the new form _H. holadenium_, which has only + glands, _Campanula rotundifolia_ with smooth and hairy leaves), or + forms still more distinct, up to well-defined species. I could + enumerate endless examples at all stages. + + It will be seen that in my definition of synoicy I do not mean to + assert that _all_ allied forms are invariably found together, but + that they are much more often seen in groups than singly. Take, for + instance, nine forms closely related (_A_ to _I_). _A_, _E_, _H_ + will be found side by side at one point, _B_, _D_ at another, _C_, + _F_ at a third, &c. These facts are plainly opposed to the theory + of isolation and amixia, and make, on the contrary, in favour of + the social development of species (_loc. cit._, p. 221). + +Not to multiply quotations to the same general effect, I will supply but +one other, referring to a particular case. + + At one spot (_Rothwand_) much exposed to the sun, and difficult of + access, I remarked two closely allied forms, so nearly related to + _H. villosum_ that this would seem to be an intermediary form + between the two. One of these (_H. villosissimum_) is distinguished + by its tongue and thick pubescence, its tolerably large capitula, + and by the lengthened and separated scales of the involucrum; the + other, on the contrary (_H. elongatum_), is less pubescent, has + smaller capitula, and more compact scales on the involucrum than + _H. villosum_. Both are finally distinguishable from the type by + their longer stalks, which are more decidedly aphyllous, and by + their later flowering. At the spot where I found them the two forms + were closely intermingled, and each was represented by a + considerable number of plants. I did not find them anywhere else on + the mountain, nor could I find at the spot where these were growing + a single specimen of the true _H. villosum_, nor a single hybrid + from these two. + + I concluded that these two new forms had, by joining their forces, + expelled the _H. villosum_ from its primitive abode, but had not + succeeded in displacing one another. As to their origin, they had + evidently developed in two different directions from a common point + of departure, namely _H. villosum_. They had succeeded, not only in + separating themselves from the original form, but also in + preventing any intermediary form from interposing. I thought myself + therefore justified in considering this as a case of varieties + which have come into existence subsequently to the Glacial epoch. + The morphological characteristics of the three forms are + sufficiently distinct for them to be designated as species by a + good many writers. They are better defined than some of MM. Frolich + and Fries' weaker species, and as well defined as some of MM. Koch + and Grisebach's (p. 222). + +Now it is clear, without comment, that all this is exactly as it ought +to be, if allied species have been differentiated on common areas by +selective fertility. For if, as Naegeli elsewhere says, "one meets forms +in nature associated with one another, and severally distinguished by +every possible degree of differentiation," not only as Naegeli adds, does +this general fact lead to the inference that species are (usually) +developed when plants grow intimately associated together; but as +certainly it leads to the further inference that such development must +be due to a prior development of cross-infertility between the diverging +varietal forms, cross-infertility which is therefore afterwards so +characteristic of the allied species, when these are found, in their +fully differentiated condition, still occupying the same area in large +and intimately mingled populations. + +To my mind there could not be any inference more strongly grounded than +this, because, with the one exception of the physiological form, no +other form of homogamy can be conceived which shall account for the +origin and permanence of these synoical varieties, in all degrees of +differentiation up to well-defined synoical species. Least of all, as we +have seen, can natural selection alone have had anything to do with such +a state of matters; while, as we have likewise seen, in all its details +it is exactly the state of matters which the theory of physiological +selection requires. + +Nevertheless, although this inference is so strongly grounded, we ought +to remember that it is only an inference. In order fully to verify the +theory of physiological selection, we ought to prove by experiment the +fact of cross-infertility between these synoical varieties, as we learn +that it afterwards obtains between synoical species. It is to be +regretted that the theory of physiological selection did not occur to +the mind of Naegeli, because he would then, no doubt, have ascertained +this by actual experiment. As it is, the great value of his observations +goes no further than establishing a strong presumption, that it _must_ +be selective fertility which causes the progressive differentiation of +synoical varieties; and also that, if so, this _must_ be the principal +factor in the differentiation of vegetable species, seeing that some +ninety-five per cent. are of synoical origin. + + +_Evidence from Experimental Research._ + +My paper on _Physiological Selection_ pointed out that the whole theory +would have to stand or fall with the experimental proof of the presence +or the absence of cross-infertility between varieties of the same +species growing on common areas. From the facts and considerations which +we have hitherto been dealing with, it did indeed appear to me that +there was the strongest conceivable ground for inferring that +cross-infertility between such varieties would be found by experiment to +be a phenomenon of highly general occurrence--amply sufficient ground to +prove that allied species on common areas for the most part owed their +origin to this character of mutual sterility, and not vice versa as +previously supposed. At that time I was not aware that any experiments +had been made in this direction. Soon after the paper was published, +however, my attention was directed to a laborious research which had +been directed to this very point, and carried on for more than thirty +years, by M. Jordan[25]. This had not attracted the general notice which +it undoubtedly deserved; and I have since ascertained that even Darwin +began to look into it only a few months before his death. + + [25] _Remarques sur le fait de l'existence en societe a l'etat + sauvage des especes vegetales affines et sur d'autres faits relatifs + a la question de l'espece_, par Alexis Jordan; lues au congres de + l'Association Francaise pour l'Avancemeat des Sciences, 2^me + session, Lyon, seance de 28 Aout, 1873. + +Having devoted his life to closely observing in divers stations +multitudes of different species of plants--annuals and perennials, +bulbous and aquatic, trees and shrubs--M. Jordan has been able to +satisfy himself, and the French school of botanists to which this line +of observation has given rise, that in most cases (or "nearly +everywhere"), when a Linnean species is indigenous to a country and is +there of common occurrence, this species within that district is +represented by more or less numerous and perfectly constant varieties. +These varieties are constituted by such minute differences of +morphological character that their very existence eluded the +observation of botanists, until M. Jordan began to search specially for +them as the special objects of his scrutiny. Moreover, these varieties +of a Linnean species occupy common areas, and there grow in intimate +association with one another, or as M. Jordan says, "_pele-mele_." So +far, be it noticed, Jordan was proceeding on exactly the same lines as +Naegeli; only he carried his observations over a still wider range of +species on the one hand, and into a still minuter search for varieties +on the other. But the all-important point for us is, that he further +proceeded to test by experiment the physiological relations between +these morphological varieties; and found, in many hundreds of cases, +that they not only came true to seed (i. e. are hereditary and not +merely climatic), but likewise cross-sterile _inter se_. For these +reasons, M. Jordan, who is opposed to the theory of evolution, regards +all such varieties as separately created species; and the inspiring +motive of his prolonged investigations has been a desire to multiply +these proofs of creative energy. But it clearly makes no difference, so +far as evolutionists are concerned with them, whether all this multitude +of sexually isolated forms be denominated species or varieties. + +The points which are of importance to evolutionists--and of the first +order of importance in the present connexion--may be briefly summarized +as follows:-- + +(1) The research embraces large numbers of species, belonging to very +numerous and very varied orders of plants; (2) in the majority of +cases--although not all--indigenous species which are of common +occurrence present constant varieties; (3) these varieties, +nevertheless, may be morphologically so slight as to be almost +imperceptible; (4) they occupy common areas and grow in intimate +association; (5) although many of them have undergone so small an amount +of morphological change, they have undergone a surprising amount of +physiological change; for (6) not only do very many of these varieties +come true to seed; but, (7) when they do, they are always more or less +cross-infertile _inter se_. + +Now, it is self-evident that every one of these seven points is exactly +what the theory of physiological selection requires, while there is not +one of them which it does not require. For if the theory be sound, we +should expect to find large numbers of species belonging to numerous and +varied orders of plants presenting constant varieties on common areas; +we should expect this to be a highly general, though not a universal, +rule; and we should expect it to apply only to species which are +indigenous. Moreover, we should expect these varieties, although but +slightly differentiated morphologically, to present a great +differentiation physiologically--and this in the special direction of +selective fertility, combined, of course, with heredity. + +On the other hand, as I have said, this catalogue of evidences leaves +nothing to be supplied. It gives us all the facts--and no more than all +the facts--which my paper on _Physiological Selection_ anticipated as +the eventual result of a prolonged experimental research. And if I have +to regret my ignorance of these facts when that paper was published, at +any rate it now furnishes the best proof that my anticipations were not +guided by the results of a verification which had already been supplied. +These anticipations were deduced exclusively from the theory itself, as +representing what _ought_ to be the case if the theory were true; and, I +must confess, if I had then been told that they had already been +realized--that it had actually been found to be a general rule that +endemic species present constant and hereditary varieties, intimately +commingled on common areas, morphologically almost indistinguishable, +but physiologically isolated by selective fertility--I should have felt +that the theory had been verified in advance. For there are only two +alternatives: either these things are due to physiological selection, or +else they are due--as M. Jordan himself believes--to special creation. +Which is equivalent to saying that, for evolutionists, the facts must be +held to verify the former theory in as complete a manner as it is +logically possible for the theory to be verified. + + +_Evidence from Prepotency._ + +We have now to consider the bearing of what is called "prepotency" on +the theory of physiological selection. + +Speaking of the vast number of species of Compositae, Darwin says:-- + + There can be no doubt that if the pollen of all these species could + be simultaneously or successively placed on the stigma of any one + species, this one would elect with unerring certainty its own + pollen. This elective capacity is all the more wonderful, as it + must have been acquired since the many species of this great group + of plants branched off from a common progenitor. + +Darwin is here speaking of elective affinity in its fully developed +form, as absolute cross-sterility between fully differentiated species. +But we meet with all lower degrees of cross-infertility--sometimes +between "incipient species," or permanent varieties, and at other times +between closely allied species. It is then known as "prepotency" of the +pollen belonging to the same variety or species over the pollen of the +other variety or species, when both sets of pollen are applied to the +same stigma. Although in the absence of the prepotent pollen the less +potent will fertilize the seed, yet, such is the appetency for the more +appropriate pollen, that even if this be applied to the stigma some +considerable time after the other, it will outstrip or overcome the +other in fertilizing the ovules, and therefore produce the same result +on the next generation as if it had been applied to the mother plant +without any admixture of the less potent pollen, although in some cases +such incipient degrees of cross-infertility are further shown by the +number or quality of the seeds being fewer or inferior. + +Now, in different varieties and in different allied species, all degrees +of such prepotency have been noticed by many observers, from the +faintest perceptible amount up to complete impotency of the alien +pollen--when, of course, there is absolute sterility between the two +varieties or allied species. The inference is obvious. In this graduated +scale of prepotency--beginning with an experimentally almost +imperceptible amount of sexual differentiation between two varieties, +and ending in an absolute partitioning of two allied species--we have +the only remaining fact that is required to complete the case in favour +of the present theory. We are here brought back to the very earliest +stages of physiological differentiation or to the stages which lie +behind Jordan's "Physiological Species"; and therefore, when taken in +conjunction with his results, the phenomena of prepotency may be said to +give us the complete and final demonstration of one continuous +development, which, beginning in an almost imperceptible amount of +cross-infertility, ends in absolute cross-sterility. The "elective +capacity" to which Darwin alludes as having been "acquired" by all the +species of Compositae since they "branched off from a common +progenitor," is thus seen among innumerable other species actually in +process of acquisition; and so we can perfectly well understand, what is +otherwise unintelligible, that closely allied species of plants occur, +in ninety-five per cent. of cases, intimately associated on common +areas, while exhibiting towards one another the character of mutual +sterility. + +But more than this. The importance of the widespread phenomena of +prepotency to the theory of physiological selection does not consist +merely in thus supplying the last link in the chain of evidence touching +the origin of species by selective fertility, or "elective capacity." +These phenomena are of further importance as showing how in plants, at +all events, physiological selection appears to be frequently capable of +differentiating specific types without the necessary assistance of any +other form of homogamy. In my original statement of the theory, I was +careful to insist upon the great value, as differentiating agents, of +even small degrees of other forms of homogamy when co-operating with +physiological selection. But I also stated my belief that in many cases +selective fertility is presumably of itself capable of splitting a +specific type; and the reason why I still believe this is, that I do not +otherwise understand these phenomena of prepotency. I cannot believe +that in all the innumerable cases where they arise, they have been +super-induced by some prior morphological changes going on in some other +part of the organism, or by "prolonged exposure to uniform conditions of +life," on the part of two well-nigh identical forms which have arisen +intimately commingled in exactly the same environment, and under the +operation of a previously universal intercrossing. Even if such a thing +could be imagined as happening occasionally, I feel it difficult to +imagine that it can happen habitually, and yet this view must be held by +those who would attribute prepotency to natural selection. + +It must never be forgotten that the relatively enormous changes as to +size, structure, habit, &c., which are presented by our domesticated +plants as results of artificial selection, do not entail the +physiological character of cross-sterility in any degree, save possibly +in some small number of cases. Although in wild species any +correspondingly small percentage of cases (where natural selection +happens to hit upon parts of the organism modifications of which produce +the physiological change by way of correlation) would doubtless be the +ones to survive on common areas, still it is surely incredible that such +an accidental association between natural selection and +cross-infertility is so habitually the means of specific differentiation +as the facts of prepotency (together with the observations of Jordan +and Naegeli) would necessarily demand. + +Moreover, this view of the matter is still further corroborated by +certain other facts and considerations. For example, the phenomena of +prepotency (whether as between varieties or between closely allied +species) are found to occur when the two forms occupy a common area, +i.e. are growing intermingled with one another. Therefore, but for this +physiological differentiation, there could be absolutely nothing to +prevent free intercrossing. Yet the fact that hybrids are so +comparatively rare in a state of nature--a fact which Sir Joseph Hooker +has pointed out to me as otherwise inexplicable--proves the efficacy of +even a low degree of such differentiation in preventing the +physiologically-differentiated forms from intercrossing. Even in cases +where there is no difficulty in producing artificial hybrids or mongrels +between species or varieties growing on common areas, it is perfectly +astonishing what an extremely small percentage of the hybrid or mongrel +forms are found to occur in nature. And there can be no question that +this is due to the very efficient manner in which prepotency does its +work--efficient, I mean, from the point of view of the new theory; for +upon any other theory prepotency is a meaningless phenomenon, which, +notwithstanding its frequent occurrence, plays no part whatever in the +process of organic evolution. + +I attach considerable importance to the phenomena of prepotency in view +of the contrast which is presented between plants and animals in the +relation of their species to physical barriers. For animals--and +especially the higher animals--appear to depend for their specific +differentiations upon such barriers much more than in the case with +plants. This is no more than we should expect; for, in accordance with +our theory, selective fertility is not so likely to work alone in the +case of the higher animals which mate together, as in plants which are +fertilized through the agency of wind or insects. In the former case +there is no opportunity given for the first rise of cross-infertility, +in the form of prepotency; and even where selective fertility has gained +a footing in other ways, the chances against the suitable mating of +"physiological complements" must be much greater than it is in the +latter case. Hence, among the higher animals, selective fertility ought +much more frequently to be found in association with other forms of +homogamy than it is among plants. And this is exactly what we find. Thus +it seems to me that this contrast between the comparative absence and +presence of physical barriers, where allied species of plants and of +higher animals are respectively concerned, is entitled to be taken as a +further corroboration of our theory. For while it displays exactly such +a general correlation as this theory would expect, the correlation is +one which cannot possibly be explained on any other theory. It is just +where physiological selection can be seen to have the best opportunity +of acting (viz. in the vegetable kingdom) that we find the most +unequivocal evidence of its action; while, on the other hand, it is just +where it can be seen to have the least opportunity of asserting itself +(viz. among the higher animals) that we find it most associated with, +and therefore assisted by, other forms of homogamy, i. e. not only +geographical isolation, but also by sexual preference in pairing, and +the several other forms of homogamy, which Mr. Gulick has shown to arise +in different places as the result of intelligence. + + +_Evidence from Special Cases._ + +Hitherto I have been considering, from the most general point of view, +the most widespread facts and broadest principles which serve to +substantiate the theory of physiological selection. I now pass to the +consideration of one of those special cases in which the theory appears +to have been successfully applied. + +Professor Le Conte has adduced the fossil snails of Steinheim as serving +to corroborate the theory of physiological selection[26]. + + [26] _Evolution and its Relations to Religious Thought_, &c. pp. + 236-7. + +The facts are these. The snail population of this lake remain for a long +time uniform and unchanged. Then a small percentage of individuals +suddenly began to vary as regards the form of their shells, and this in +two or three directions at the same time, each affected individual, +however, only presenting one of the variations. But after all these +variations had begun to affect a proportionally large number of +individuals, some individuals occur in which two or more of the +variations are blended together, evidently, as Weismann says, by +intercrossing of the varieties so blended. Later still, both the +separate varieties and their blended progeny became more and more +numerous, and eventually a single blended type, comprising in itself all +the initial varieties, supplanted the parent form. Then another long +period of stability ensued until another eruption of new variations took +place; and these variations, after having affected a greater and greater +number of individuals, eventually blended together by intercrossing and +supplanted their parent form. So the process went on, comparatively +short periods of variation alternating with comparatively long periods +of stability, the variations, moreover, always occurring suddenly in +crops, then multiplying, blending together, and in their finally blended +type eventually supplanting their parent form. + +Now, the remarkable fact here is that whenever the variations arose, +they only intercrossed between themselves, they did not intercross with +their parent form; for, if they had, not only could they never have +survived (having been at first so few in number and there having been no +geographical barriers in the small lake), but we should have found +evidence of the fact in the half-bred progeny. Moreover, natural +selection can have had nothing to do with the process, because not only +are the variations in the form of the shells of no imaginable use in +themselves; but it would be preposterous to suppose that at each of +these "variation periods" several different variations should always +have occurred simultaneously, all of which were of some hidden use, +although no one of them ever occurred during any of the prolonged +periods of stability. How, then, are we to explain the fact that the +individuals composing each crop of varieties, while able to breed among +themselves, never crossed with their parent form? These varieties, each +time that they arose, were intimately commingled with their parent +form, and would certainly have been reabsorbed into it had intercrossing +in that direction been possible. With Professor Le Conte, therefore, I +conclude that there is only one conceivable answer to this question. +Each crop of varieties must have been _protected from intercrossing with +their parent form_. + +They must have been the result of a variation, which rendered the +affected individuals sterile with their parent form, whilst leaving them +fertile amongst themselves. The progeny of these individuals would then +have dispersed through the lake, physiologically isolated from the +parent population, and especially prone to develop secondary variations +as a direct result of the primary variation. Thus, as we might expect, +two or three variations arose simultaneously, as expressions of so many +different lines of family descent from the original or physiological +variety; these were everywhere prevented from intercrossing with their +parent form, yet capable of blending whenever they or their +ever-increasing progeny happened to meet. Thus, without going into +further details, we are able by the theory of physiological selection to +give an explanation of all these facts, which otherwise remain +inexplicable. + + * * * * * + +In view of the evidence which has now been presented, I will now ask +five questions which must be suitably answered by critics of the theory +of physiological selection. + +1. Can you doubt that the hitherto insoluble problem of inter-specific +sterility would be solved, supposing cross-infertility were proved to +arise before or during the process of specific differentiation, instead +of after that process had been fully completed? + +2. Can you doubt, after duly considering the circumstances under which +allied species of plants have been differentiated--viz. in ninety-five +per cent. of cases intimately commingled on common areas, and therefore +under identical environments--that cross-infertility _must_ have arisen +before or during the specific differentiation? + +3. Can you doubt, after duly considering the facts of prepotency on the +one hand and those of Jordan's physiological varieties on the other, +that cross-infertility _does_ arise before or during the specific +differentiation? + +4. If you cannot express a doubt upon any of these points, can you +explain why you refuse to accept the theory of the origin of species by +means of physiological selection, together with the explanation which +this theory affords of the continued cross-fertility of domesticated +varieties? + +5. Supposing this theory to be true, can you conceive of any other +classes of facts which, either quantitatively or qualitatively, could +more directly or more effectually prove its truth than those which have +now been adduced? + +On these five heads I entertain no doubt. I am convinced that the theory +of physiological selection is the only one that can explain the facts of +inter-specific sterility on the one hand, and, on the other hand, the +contrast which these facts display to the unimpaired fertility of our +domesticated varieties. + +In conclusion, it seems desirable once more to insist that there is no +antagonism or rivalry between the theories of natural and of +physiological selection. For which purpose I will quote the final +paragraph of my original paper. + + So much, then, for the resemblances and the differences between the + two theories. It only remains to add that the two are + complementary. I have already shown some of the respects in which + the newer theory comes to the assistance of the older, and this in + the places where the older has stood most in need of assistance. In + particular, I have shown that segregation of the fit entirely + relieves survival of the fittest from the difficulty under which it + has hitherto laboured of explaining why it is that sterility is so + constantly found between species, while so rarely found between + varieties which differ from one another even more than many + species; why so many features of specific distinction are useless + to the species presenting them; and why it is that incipient + varieties are not obliterated by intercrossing with parent forms. + Again, we have seen that physiological selection, by preventing + such intercrossing, enables natural selection to promote diversity + of character, and thus to evolve species in ramifying branches + instead of in linear series--a work which I cannot see how natural + selection could possibly perform unless thus aided by physiological + selection. Moreover, we have seen that although natural selection + alone could not induce sterility between allied types, yet when + this sterility is given by physiological selection, the forms which + present it would be favoured in the struggle for existence; and + thus again the two principles are found playing, as it were, into + each other's hands. And here, as elsewhere, I believe that the + co-operation enables the two principles to effect very much more in + the way of species-making than either of them could effect if + working separately. On the one hand, without the assistance of + physiological selection, natural selection would, I believe, be all + but overcome by the adverse influences of free + intercrossing--influences all the more potent under the very + conditions which are required for the multiplication of species by + divergence of character. On the other hand, without natural + selection, physiological selection would be powerless to create any + differences of specific type, other than those of mutual sterility + and trivial details of structure, form, and colour--differences + wholly without meaning from a utilitarian point of view. But in + their combination these two principles appear to me able to + accomplish what neither can accomplish alone--namely, a full and + satisfactory explanation of the origin of species. + + + + +CHAPTER VI. + +A BRIEF HISTORY OF OPINIONS ON ISOLATION AS A FACTOR OF ORGANIC +EVOLUTION. + + +This historical sketch must begin with a consideration of Darwin's +opinions on the subject; but as these were considerably modified from +time to time during a period of thirty years by the publications of +other naturalists, it will be impossible to avoid cross-references as +between his writings and theirs. It may also be observed that the _Life +and Letters of Charles Darwin_ was not published until the year 1887, so +that the various opinions which I shall quote from the letters, and +which show some considerable approximation in his later years to the +views which have been put forward by Mr. Gulick and myself, were not +before us at the time when our papers were read. + +The earliest allusion that I can find to geographical isolation in the +writings of Darwin occurs in a correspondence with Sir Joseph Hooker, as +far back as 1844. He there says:-- + + I cannot give my reasons in detail; but the most general conclusion + which the geographical distribution of all organic beings appears + to me to indicate is, that isolation is the chief concomitant or + cause of the appearance of _new_ forms (I well know there are some + staring exceptions)[27]. + + [27] _Life and Letters_, vol. ii. p. 28. + +And again:-- + + With respect to original creation or production of new forms, I + have said that isolation appears the chief element[28]. + + [28] _Ibid._ + +Next, in the earlier editions of the _Origin of Species_ this view is +abandoned, and in its stead we meet with the opinion that geographical +isolation lends a certain amount of assistance to natural selection, by +preventing free intercrossing. But here we must note two things. First, +the distinction between monotypic and polytypic evolution is not +defined. Secondly, the levelling effect of free intercrossing in nature, +and hence its antagonism to divergence of character by natural +selection, is not sufficiently recognized; while, on the other hand, and +in consequence of this, the importance of isolation as a factor of +evolution is underrated--not only in its geographical, but likewise in +all its other forms. + +Taking these two points separately, the only passages in Darwin's +writings, so far at least as I can find, in which any distinction is +drawn between evolution as monotypic and polytypic, are those in which +he deals with a somewhat analogous distinction between artificial +selection as intentional and unconscious. He says, for example:-- + + In the case of methodical selection, a breeder selects for some + definite object, and if the individuals be allowed freely to + intercross, his work will completely fail. But when many men, + without intending to alter the breed, have a nearly common + standard of perfection, and all try to procure and breed from the + best animals, improvement surely but slowly follows from this + unconscious process of selection, notwithstanding that there is no + separation of selected individuals. Thus it will be under + nature[29]. + + [29] _Origin of Species_, p. 80, 6th ed. (1872). + +Here we have what may perhaps be regarded as a glimmering of the +distinction between monotypic and polytypic evolution. But that it is +only a glimmering is proved by the immediately ensuing sentences, which +apply this analogy of unconscious selection _not_ to the case of +monotypic, _but_ to that of polytypic evolution. So likewise, in the +succeeding discussion on "divergence of character," the analogy is again +resorted to for the purpose of showing how polytypic evolution may occur +in nature. + +Thus far, then, it may be said that we have scarcely so much as a +glimmering of the distinction between monotypic and polytypic evolution; +and as the same discussion (with but a few verbal alterations) runs +through all the editions of the _Origin_, it may well be asked why I +should have alluded to such passages in the present connexion. Well, I +have done so because it is apparent that, during the last years of his +life, the distinction between selection as "methodical" and +"unconscious" enabled Darwin much more clearly to perceive that between +evolution as monotypic and polytypic. Thus in 1868 he wrote to Moritz +Wagner (who, as we shall presently see, entirely failed to distinguish +between monotypic and polytypic evolution), expressing his belief-- + + That in many large areas all the individuals of the same species + have been slowly modified, in the same manner, for instance, as the + English racehorse has been improved, that is, by the continued + selection of the fleetest individuals, without any separation. But + I admit that by this process two or more new species could hardly + be formed within the same limited area[30]. + + [30] _Life and Letters_, vol. iii. p. 158. + +Again, in 1876 he wrote another letter to Wagner, in which the following +passage occurs:-- + + I believe that all the individuals of a species can be slowly + modified within the same district, in nearly the same manner as man + effects by what I have called the process of unconscious selection. + I do not believe that one species will give birth to two or more + new species as long as they are mingled together within the same + district[31]. + + [31] _Ibid._ p. 159. + +Two years later he wrote to Professor Semper:-- + + There are two different classes of cases, it appears to me, viz. + those in which species becomes slowly modified in the same country, + and those cases in which a species splits into two, or three, or + more new species; and, in the latter case, I should think nearly + perfect separation would greatly aid in their "specification," to + coin a new word[32]. + + [32] _Ibid._ p. 160. + +Now, these passages show a very much clearer perception of the +all-important distinction between monotypic and polytypic evolution than +any which occur in the _Origin of Species_; and they likewise show that +he was led to this perception through what he supposed to be a somewhat +analogous distinction between "unconscious" and "methodical" selection +by man. The analogy, I need hardly say, is radically unsound; and it is +a curious result of its unsoundness that, whereas in the _Origin of +Species_ it is adduced to illustrate the process of polytypic evolution, +as previously remarked, in the letters above quoted we find it adduced +to illustrate the process of monotypic evolution. But the fact of this +analogy being unsound does not affect the validity of the distinction +between monotypic and polytypic evolution to which it led Darwin, in his +later years, so clearly to express[33]. + + [33] The analogy is radically unsound because unconscious selection + differs from methodical selection only in the _degree_ of + "separation" which it effects. These two forms of selection do not + necessarily differ from one another in regard to the _number_ of + characters which are being simultaneously diversified; for while it + may be the object of methodical selection to breed for modification + of a single character alone, it may, on the other hand, be the + result of unconscious selection to diversify an originally uniform + stock, as Darwin himself observes with regard to horse-breeding. The + real distinction between monotypic and polytypic evolution is, not + at all with reference to the _degree_ of isolation (i. e. _amount_ + of "separation"), but to the _number of cases_ in which any + efficient degree of it occurs (i. e. whether in but a single case, + or in two or more cases). + +Turning next to the second point which we have to notice, it is easy to +show that in the earlier editions of his works Darwin did not +sufficiently recognize the levelling effects of free intercrossing, and +consequently failed to perceive the importance of isolation (in any of +its forms) as a factor of organic evolution. This may be most briefly +shown by quoting his own more matured opinion upon the subject. Thus, +with reference to the swamping effects of intercrossing, he wrote to Mr. +Wallace in 1867 as follows:-- + + I must have expressed myself atrociously: I meant to say exactly + the reverse of what you have understood. F. Jenkin argued in the + _North British Review_ against single variations being perpetuated, + and has convinced me, though not in quite so broad a manner as here + put. I always thought individual differences more important; but I + was blind, and thought that single variations might be preserved + much oftener than I now see is possible or probable. I mentioned + this in my former note merely because I believed that you had come + to a similar conclusion, and I like much to be in accord with you. + I believe I was mainly deceived by single variations offering such + simple illustrations, as when man selects [i.e. isolates][34]. + + [34] _Life and Letters_, vol. iii. pp. 157-8. + +Again, somewhere about the same time, he wrote to Moritz Wagner:-- + + Although I saw the effects of isolation in the case of islands and + mountain-ranges, and knew of a few instances of rivers, yet the + greater number of your facts were quite unknown to me. I now see + that, from the want of knowledge, I did not make nearly sufficient + use of the views which you advocate[35]. + + [35] _Ibid._ pp. 157-8. + +Now it would be easy to show the justice of these self-criticisms by +quoting longer passages from earlier editions of the _Origin of +Species_; but as this, in view of the above passages, is unnecessary, we +may next pass on to another point. + +The greatest oversight that Wagner made in his otherwise valuable essays +on geographical isolation, was in not perceiving that geographical +isolation is only one among a number of other forms of isolation: and, +therefore, that although it is perfectly true, as he insisted, that +polytypic evolution cannot be effected by natural selection alone, it is +very far from true, as he further insisted, that _geographical_ +isolation is the only means whereby natural selection can be assisted in +this matter. Hence it is that, when Darwin said he had not himself "made +nearly sufficient use" of geographical isolation as a factor of specific +divergence, he quite reasonably added that he could not go so far as +Wagner did in regarding such isolation as a condition, _sine qua non_, +to divergent evolution in all cases. Nevertheless, he adds the +important words, "I almost wish I could believe in its importance to the +same extent with you; for you well show, in a manner which never +occurred to me, that it removes many difficulties and objections." These +words are important, because they show that Darwin had come to feel the +force of the "difficulties and objections" with regard to divergent +evolution being possible by means of natural selection alone, and how +readily they could be removed by assuming the assistance of isolation. +Hence, it is much to be deplored that Wagner presented a single kind of +isolation (geographical) as equivalent to the principle of isolation in +general. For he thus failed to present the complete--and, therefore, the +true--philosophy of the subject to Darwin's mind; and in this, as in +certain other respects which I shall notice later on, served rather to +confuse than to elucidate the matter as a whole. + +To sum up. Although in his later years, as shown by his correspondence, +Darwin came to recognize more fully the swamping effects of free +intercrossing, and the consequent importance of "separation" for the +prevention of these effects, and although in this connexion he likewise +came more clearly to distinguish between the "two cases" of monotypic +and polytypic evolution, it is evident that he never worked out any of +these matters--"thinking it prudent," as he wrote with reference to them +in 1878, "now I am growing old, to work at easier subjects[36]." +Therefore he never clearly saw, on the one hand, that free +intercrossing, far from constituting a "difficulty" to _monotypic_ +evolution by natural selection, is the very means whereby natural +selection is in this case enabled to operate; or, on the other hand, +that, in the case of _polytypic_ evolution, the "difficulty" in question +is so absolute as to render such evolution, by natural selection alone, +absolutely impossible. Hence, although in one sentence of the _Origin of +Species_ he mentions three forms of isolation (besides the geographical +form) as serving in some cases to assist natural selection in causing +"divergence of character" (i. e. polytypic evolution[37]), on account of +not perceiving how great and how sharp is the distinction between the +two kinds or "cases" of evolution, he never realized that, where "two or +more new species" are in course of differentiation, _some_ form of +isolation other than natural selection must _necessarily_ be present, +whether or not natural selection be likewise so. The nearest approach +which he ever made to perceiving this necessity was in one of his +letters to Wagner above quoted, where, after again appealing to the +erroneous analogy between monotypic evolution and "unconscious +selection," he says:--"But I admit that by this process (i. e. +unconscious selection) two or more new species could hardly be formed +within the same limited area: some degree of separation, if not +indispensable, would be highly advantageous; and here your facts and +views will be of great value." But even in this passage the context +shows that by "separation" he is thinking exclusively of _geographical_ +separation, which he rightly enough concludes (as against Wagner) need +certainly not be "indispensable." Had he gone a step further, he must +have seen that separation, _in some form or another, is_ "indispensable" +to polytypic evolution. Instead of taking this further step, however, +two years later he wrote to Semper as follows:-- + + [36] _Life and Letters_, vol. iii. p. 161. + + [37] Page 81. The three forms of isolation mentioned are, "from + haunting different stations, from breeding at slightly different + seasons, or from the individuals of each variety preferring to pair + together." + + I went as far as I could, perhaps too far, in agreement with Wagner + [i. e. in the last edition of the _Origin of Species_]; since that + time I have seen no reason to change my mind; but then I must add + that my attention has been absorbed on other subjects[38]. + + [38] _Life and Letters_, vol. iii. p. 159. + +And he seems to have ended by still failing to perceive that the +explanation which he gives of "divergence of character" in the _Origin +of Species_, can only hold on the unexpressed assumption that free +intercrossing is in some way prevented at the commencement, and +throughout the development, of each diverging type. + +Lastly, we have to consider Darwin's opinion touching the important +principle of "Independent Variability." This, it will be remembered, is +the principle which ensures that when a portion (not too large) of a +species is prevented from interbreeding with the rest of the species, +sooner or later a divergence of type will result, owing to the fact that +the average qualities of the separated portion at the time of its +separation cannot have been exactly the same as the average qualities of +the specific type as a whole. Thus the state of Amixia, being a state of +what Mr. Gulick calls Independent Generation, will of itself--i.e. even +if unassisted by natural selection--induce divergence of type, in a +ratio that has been mathematically calculated by Delboeuf. + +Darwin wrote thus to Professor Weismann in 1872:-- + + I have now read your essay with very great interest. Your view of + the origin of local races through "Amixia" is altogether new to me, + and seems to throw an important light on an obscure question[39]. + + [39] _Life and Letters_, vol. iii. p. 155. + +And in the last edition of the _Variation of Animals and Plants_ he adds +the following paragraph:-- + + This view may throw some light on the fact that the domestic + animals which formerly inhabited the several districts in Great + Britain, and the half-wild cattle lately kept in several British + parks, differed slightly from one another; for these animals were + prevented from wandering over the whole country and intercrossing, + but would have crossed freely within each district or park[40]. + + [40] _Variation_, &c., vol. ii. p. 262. + +Now, although I allow that Darwin never attributed to this principle of +Amixia, or Independent Variability, anything like the degree of +importance to which, in the opinion of Delboeuf, Gulick, Giard, and +myself, it is entitled, the above passage appears to show that, as soon +as the "view" was clearly "suggested" to his mind, he was so far from +being unfavourably disposed towards it, that he added a paragraph to the +last edition of his _Variation_ for the express purpose of countenancing +it. Nevertheless, later on the matter appears to have entirely escaped +his memory; for in 1878 he wrote to Semper, that he did "not see at all +more clearly than I did before, from the numerous cases which he +[Wagner] has brought forward, how and why it is that a long isolated +form should almost always become slightly modified[41]." I think this +shows entire forgetfulness of the principle in question, because, if +the latter is good for explaining the _initial_ divergence of type as +between separated stocks of "domesticated animals," much more must it be +competent to explain the _further_ divergence of type which is "almost +always" observable in the case of "a long isolated form" under nature. +The very essence of the principle being that, when divergence of type +has once begun, this divergence must _ipso facto_ proceed at an +ever-accelerating pace, it is manifestly inconsistent to entertain the +principle as explaining the first commencement of divergence, and then +to ignore it as explaining the further progress of divergence. Hence, I +can only conclude that Darwin had forgotten this principle altogether +when he wrote his letter to Semper in 1878--owing, no doubt, as he says +in the sentence which immediately follows, to his having "not attended +much of late years to such questions." + + [41] _Life and Letters_, vol. iii. p. 161. + + * * * * * + +So much, then, for Darwin's opinions. Next in order of time we must +consider Moritz Wagner's essays on what he called the "Law of +Migration[42]." The merit of these essays was, first, the firm expression +of opinion upon the swamping effects of free intercrossing; and, second, +the production of a large body of facts showing the importance of +geographical isolation in the prevention of these effects, and in the +consequent differentiation of specific types. On the other hand, the +defect of these essays was, first, not distinguishing between evolution +as monotypic and polytypic; and, second, not perceiving that +geographical isolation is only one among a number of other forms of +isolation. From these two radical oversights--which, however, were +shared by all other writers of the time, with the partial exception of +Darwin himself, as previously shown--there arose the following and most +lamentable errors. + + [42] _Die Darwin'sche Theorie und das Migrationsgesetz_ (1868): + _Ueber den Einfluss der geographischen Isolirung_, &c. (1870). + +Over and over again Moritz Wagner insists, as constituting the +fundamental doctrine of his attempted reform of Darwinism, that +evolution by natural selection is impossible, unless natural selection +be assisted by geographical isolation, in order to prevent the swamping +effects of intercrossing[43]. Now, if instead of "evolution" he had said +"divergence of type," and if instead of "geographical isolation" he had +said "prevention of intercrossing," he would have enunciated the general +doctrine which it has been the joint endeavour of Mr. Gulick and myself +to set forth. But by not perceiving that "evolution" is of two radically +different kinds--polytypic and monotypic--he entirely failed to perceive +that, while for one of its kinds the _prevention_ of intercrossing is an +absolute necessity, for the other of its kinds the _permission_ of +intercrossing is a necessity no less absolute. And, again, in missing +the fact that geographical isolation is but one of the many ways +whereby intercrossing may be prevented, he failed to perceive that, even +as regards the case of polytypic evolution, he greatly erred in +representing this one form of isolation as being universally a necessary +condition to the process. The necessary condition to this process is, +indeed, the prevention of intercrossing _by some means or another_; but +his unfortunate insistence on geographical separation as the only +possible means to this end--especially when coupled with his no less +unfortunate disregard of monotypic evolution--caused him to hinder +rather than to advance a generalization which he had only grasped in +part. And this generalization is, as now so repeatedly stated, that +while the form of isolation which we know as natural selection depends +for its action upon the intercrossing of all the individuals which it +isolates (i. e. selects), when acting alone it can produce only +monotypic evolution; but that when it is supplemented by any of the +other numerous forms of isolation, it is furnished with the necessary +condition to producing polytypic evolution--and this in as many lines of +divergent change as there may be cases of this efficient separation. + + [43] For instance, speaking of common, or continuous areas, he + says:--"In this case a constant variety, or new species, cannot be + produced, because the free crossing of a new variety with the old + unaltered stock will always cause it to revert to the original type; + in other words, will destroy the new form. The formation of a real + variety, which Darwin, as we know, regards as the commencement of a + new species, will only succeed when a few individuals, having + crossed the barrier of their habitat, are able to separate + themselves for a long time from the old stock." And the last + sentence, given as a summary of his whole doctrine, is--"The + geographical isolation of the form, a necessary consequence of + migration, is the cause of its typical character." + +Nevertheless, while we must lament these shortcomings on the part of +Wagner, we ought to remember that he rendered important services in the +way of calling attention to the swamping effects of free intercrossing, +and, still more, in that of showing the high importance of geographical +isolation as a factor of organic evolution. Therefore, although in an +elaborate criticism of his views Weismann was easily able to dispose of +his generalizations in the imperfect form that they presented, I do not +think it was just in Weismann to remark, "if Wagner had confined himself +to the statement that geographical isolation materially assists the +process of natural selection, and thus also promotes the origination of +new species, he would have met with little or no opposition; but then, +of course, in saying this much, he would not have been saying anything +new." No doubt, as I have just shown, he _ought_ thus (as well as in +other and still more important respects not perceived by Prof. Weismann) +to have limited his statement; but, had he done so, it does not follow +that he would not have been saying anything new. For, in point of fact, +in as far as he said what was true, he did say a great deal that was +also new. Thus, most of what he said of the _principle of separation_ +(apogamy) was as new as it was true, although, as we have seen, he said +it to very little purpose on account of his identifying this principle +as a whole with that of but one of its forms. Again, notwithstanding +this great error, or oversight, he certainly showed of the particular +form in question--viz. geographical isolation--that it was of +considerably _more_ importance than had previously been acknowledged. +And this was so far a valuable contribution to the general theory of +descent. + + * * * * * + +Prof. Weismann's essay, to which allusion has just been made[44], was, +however, in all respects a great advance upon those of Wagner. It was +not only more comprehensive in its view of the whole subject of +geographical isolation, but likewise much more adequate in its general +treatment thereof. Its principal defects, in my judgement, were, first, +the inordinately speculative character of some of its parts, and, +second, the restriction of its analysis to but one form of isolation--a +defect which it shares with the essays of Wagner, and in quite as high a +degree. Furthermore, although this essay had the great merit of +enunciating the principle of Amixia, it did so in a very inefficient +manner. For not only was this principle adduced with exclusive reference +to _geographical_ isolation, but even in regard to this one kind of +isolation it was presented in a highly inconsistent manner, as I will +now endeavour to show. + + [44] _Ueber den Einfluss der Isolirung auf die Artbildung_ (1872). + +Weismann was led to perceive the principle in question by the +consideration that new specific characters, when they first appear, do +not all appear together in the same individuals: they appear one in one +individual, another in another, a third in a third, &c.; and it is only +in the course of successive generations that they all become blended in +the same individuals by free intercrossing. Hence, the eventually +emerging constant or specific type is the resultant of all the +transitory or varietal types, when these have been fused together by +intercrossing. From which Weismann deduces what he considers a general +law--namely, that "the constancy of a specific type does not arise +suddenly, but gradually; and it is established by the promiscuous +crossing of all individuals[45]." From which again it follows, that this +constancy must cease so soon as the condition which maintains it +ceases--i. e. so soon as free intercrossing is prevented by the +geographical isolation of a portion of the species from its parent +stock. + + [45] _Loc. cit._, p. 43. + +Now, to begin with, this statement of the principle in question is not a +good statement of it. There was no need while stating the doctrine that +separation induces differentiation, to found the doctrine on any such +highly speculative basis. In point of fact, there is no real evidence +that specific types do attain their constancy in the way supposed; nor, +for the purposes of the doctrine in question, is it necessary that there +should be. For this doctrine does not need to show how the constancy has +been _attained_; it only has to show that the constancy is _maintained_ +by free intercrossing, with the result that when free intercrossing is +_by any means_ prevented, divergence of character ensues. In short, the +correct way of stating the principle is that which has been adopted by +Delboeuf and Gulick--namely, the average characters of a separated +portion of a species are not likely to be the same as those of the whole +species; with the result that divergence of type will be set up in the +separated portion by intercrossing within that portion. Or the principle +may be presented as I presented it under the designation of "Independent +Variability"--namely, "a specific type may be regarded as the average +mean of all individual variations, any considerable departure from this +average mean being, however, checked by intercrossing," with the result +that when intercrossing is prevented between a portion of a species and +the rest of the species, "this population is permitted to develop an +independent history of its own, shielded from intercrossing with its +parent form[46]." + + [46] _Physiological Selection_, pp. 348, 389. + +Not only, however, is Weismann's principle of "Amixia" thus very +differently stated from that of my "Independent Variability" (apogamy), +or Gulick's "Independent Generation"; but, apparently owing to this +difference of statement, the principle itself is not the same. In +particular, while Weismann holds with us that when new characters arise +in virtue of the mere prevention of intercrossing with parent forms +these new characters will be of non-utilitarian kind[47], he appears to +think that divergence of character under such circumstances is not +likely to go on to a _specific_ value. Now, it is of importance to +observe why he arrives at this conclusion, which is not only so +different from that of Delboeuf, Gulick, and myself, but apparently so +inconsistent with his own recognition of the diversifying effect of +"Amixia" as regards the formation of _permanent varieties_. For, as we +have already seen while considering Darwin's views on this same +principle of "Amixia," it is highly inconsistent to recognize its +diversifying effect up to the stage of constituting fixed varieties, and +then not to recognize that, so much divergence of character having been +already secured by the isolation alone, much more must further +divergence continue, and continue at an ever accelerating pace--as +Delboeuf and Gulick have so well shown. What, then, is the explanation +of this apparent inconsistency on Weismann's part? The explanation +evidently is that, owing to his erroneous statement of the principle, he +misses the real essence of it. For, in the first place, he does not +perceive that this essence consists in an initial difference of average +characters on the part of the isolated colony as compared with the rest +of their species. On the contrary, he loses himself in a maze +of speculation about all species having had what he calls +"variation-periods," or eruptions of general variability alternating +with periods of repose--both being as unaccountable in respect of their +causation as they are hypothetical in respect of their occurrence. From +these speculations he concludes, that isolation of a portion of a +species will then only lead to divergence of character when the +isolation happens to coincide with a "variation-period" on the part of +the species as a whole, and that the divergence will cease so soon as +the "variation-period" ceases. Again, in the second place as previously +remarked, equally with Wagner whom he is criticizing, he fails to +perceive that _geographical_ isolation is not the only kind of +isolation, or the only possible means to the prevention of free +intercrossing. And the result of this oversight is, that he thinks +amixia can act but comparatively seldom upon sufficiently small +populations to become a factor of much importance in the differentiation +of species. Lastly, in the third place, owing to his favourite +hypothesis that all species pass through a "variation-period," he +eventually concludes that the total amount of divergence of type +producible by isolation alone (even in a small population) can never be +greater than that between the extremes of variation which occur within +the whole species at the date of its partition (p. 75). In other words, +the possibility of change due to amixia alone is taken to be limited by +the range of deviation from the general specific average, as manifested +by different individual variations, before the species was divided. Thus +the doctrine of amixia fails to recognize the law of Delboeuf, or the +_cumulative_ nature of divergence of type when once such divergence +begins in a separated section. Therefore, in this all-important--and, +indeed, essential--respect, amixia differs entirely from the principle +which has been severally stated by Delboeuf, Gulick, and myself. + + [47] _Loc. cit._, p. 54. + +Upon the whole, then, we must say that although Professor Weismann was +the first to recognize the diversifying influence of merely +indiscriminate isolation _per se_ (apogamy), he did so only in part. He +failed to distinguish the true essence of the principle, and by +overlaying it with a mass of hypothetical speculation, concealed even +more of it than he revealed. + + * * * * * + +The general theory of Isolation, as independently worked out by Mr. +Gulick and myself, has already been so fully explained, that it will +here be sufficient merely to enumerate its more distinguishing features. +These are, first, drawing the sharpest possible line between evolution +as monotypic and polytypic; second, showing that while for the former +the peculiar kind of isolation which is presented by natural selection +suffices of itself to _transform_ a specific type, in order to work for +the latter, or to _branch_ a specific type, natural selection must +necessarily be assisted by some other kind of isolation; third, that +even in the absence of natural selection, other kinds of isolation may +be sufficient to effect specific divergence through independent +generation alone; fourth, that, nevertheless, natural selection, where +present, will always accelerate the process of divergence; fifth, that +monotypic evolution by natural selection depends upon the _presence_ of +intercrossing, quite as much as polytypic evolution (whether with or +without natural selection) depends upon the _absence_ of it; sixth, +that, having regard to the process of evolution throughout all taxonomic +divisions of organic nature, we must deem the physiological form of +isolation as the most important, with the exception only of natural +selection. + +The only difference between Mr. Gulick's essays and my own is, that, on +the one hand, he has analyzed much more fully than I have the various +forms of isolation; while, on the other hand, I have considered much +more fully than he has the particular form of physiological isolation +which so frequently obtains between allied _species_. This particular +form of physiological isolation I have called "physiological selection," +and claim for it so large a share in the differentiation of specific +types as to find in it a satisfactory explanation of the contrast +between natural species and artificial varieties in respect of +cross-infertility. + + * * * * * + +Mr. Wallace, in his _Darwinism_, has done good service by enabling all +other naturalists clearly to perceive how natural selection alone +produces monotypic evolution--namely, through the free intercrossing of +all individuals which have not been eliminated by the isolating process +of natural selection itself. For he very lucidly shows how the law of +averages must always ensure that in respect of any given specific +character, half the individuals living at the same time and place will +present the character above, and half below its mean in the population +as a whole. Consequently, if it should ever be of advantage to a +species that this character should undergo either increase or decrease +of its average size, form, colour, &c., there will always be, in each +succeeding generation, a sufficient number of individuals--i. e. half of +the whole--which present variations in the required direction, and which +will therefore furnish natural selection with abundant material for its +action, without the need of any other form of isolation. It is to be +regretted, however, that while thus so clearly presenting the fact that +free intercrossing is the very means whereby natural selection is +enabled to effect monotypic evolution, he fails to perceive that such +intercrossing must always and necessarily render it impossible for +natural selection to effect polytypic evolution. A little thought might +have shown him that the very proof which he gives of the necessity of +intercrossing where the _transmutation_ of species is concerned, +furnishes, measure for measure, as good a proof of the necessity of its +absence where the _multiplication_ of species is concerned. In justice +to him, however, it may be added, that this distinction between +evolution as monotypic and polytypic (with the important consequence +just mentioned) still continues to be ignored also by other well-known +evolutionists of the "ultra-Darwinian" school. Professor Meldola, for +example, has more recently said that in his opinion the "difficulty from +intercrossing" has been in large part--if not altogether--removed by Mr. +Wallace's proof that natural selection alone is capable of effecting +[monotypic] evolution; while he regards the distinction between +monotypic and polytypic evolution as mere "verbiage[48]." + + [48] _Nature_, vol. xliii. p. 410, and vol. xliv. p. 29. + +It is in relation to my presentment of the impossibility of natural +selection alone causing polytypic evolution, that Mr. Wallace has been +at the pains to show how the permission of intercrossing (panmixia) is +necessary for natural selection in its work of causing monotypic +evolution. And not only has he thus failed to perceive that the +"difficulty" which intercrossing raises against the view of natural +selection being of itself capable of causing polytypic evolution in no +way applies to the case of monotypic; but as regards this "difficulty," +where it does apply, he says:-- + + Professor G. J. Romanes has adduced it as one of the difficulties + which can alone be overcome by his theory of physiological + selection[49]. + + [49] _Darwinism_, p. 143. + +This, however, is a misapprehension. I have by no means represented that +the difficulty in question can alone be overcome by this theory. What I +have represented is, that it can be overcome by any of the numerous +forms of isolation which I named, and of which physiological selection +is but one. And although, _where common areas are concerned_, I believe +that the physiological form of isolation is the most important form, +this is a very different thing from entertaining the supposition which +Mr. Wallace here assigns to me. + + * * * * * + +I may take this opportunity of correcting a somewhat similar +misunderstanding which has been more recently published by Professor W. +A. Herdman, of Liverpool; and as the case which he gives is one of +considerable interest in itself, I will quote his remarks in extenso. In +his _Opening Address to the Liverpool Biological Society_, Professor +Herdman said:-- + + Some of you will doubtless remember that in last year's address, + while discussing Dr. Romanes' theory of physiological selection, I + quoted Professor Flemming Jenkin's imaginary case of a white man + wrecked upon an island inhabited by negroes, given as an + illustration of the supposed swamping effect by free intercrossing + of a marked variety with the parent species. I then went on to say + in criticism of the result at which Jenkin arrived, viz. that the + characteristics of the white man would be stamped out by + intercrossing with the black:-- + + "Two influences have, I think, been ignored, viz. atavism, or + reversion to ancestral characters, and the tendency of the members + of a variety to breed with one another. Keeping to the case + described above, I should imagine that the numbers of intelligent + young mulattoes produced in the second, third, fourth, and few + succeeding generations would to a large extent intermarry, the + result of which would be that a more or less white aristocracy + would be formed on the island, including the king and all the chief + people, the most intelligent men and the bravest warriors. Then + atavism might produce every now and then a much whiter + individual--a reversal to the characteristics of the ancestral + European--who, by being highly thought of in the whitish + aristocracy, would have considerable influence on the colour and + other characteristics of the next generation. Now such a white + aristocracy would be in precisely the same circumstances as a + favourable variety competing with its parent species," &c. + + You may imagine then my pleasure when, a few months after writing + the above, I accidentally found, in a letter[50] written by the + celebrated African traveller Dr. David Livingstone to Lord + Granville, and dated "Unyanyembe, July 1st, 1872," the following + passage:-- + + [50] In Appendix to H. M. Stanley's _How I found Livingstone_, 2nd + ed. London, 1872, p. 715. + + "About five generations ago, a white man came to the highlands of + Basango, which are in a line east of the watershed. He had six + attendants, who all died, and eventually their headman, called + Charura, was elected chief by the Basango. In the third generation + he had sixty able-bodied spearmen as lineal descendants. This + implies an equal number of the other sex. They are very light in + colour, and easily known, as no one is allowed to wear coral beads + such as Charura brought except the royal family. A book he brought + was lost only lately. The interest of the case lies in its + connexion with Mr. Darwin's celebrated theory on the 'origin of + species,' for it shows that an improved variety, as we whites + modestly call ourselves, is not so liable to be swamped by numbers + as some have thought." + + Here we have a perfect fulfilment of what I last year, in ignorance + of this observation of Livingstone's, predicted as being likely to + occur in such a case. We have the whitish aristocracy in a dominant + condition, and evidently in a fair way to spread their + characteristics over a larger area and give rise to a marked + variety, and it had clearly struck Livingstone fourteen years + before the theory of physiological selection had been heard of, + just as it must strike us now, as an instance telling strongly + against the "swamping" argument as used by Flemming Jenkin and + Romanes. + +Here we have a curious example of one writer supporting the statements +of another, while appearing to be under the impression that he is +controverting those statements. Both Professor Herdman's imaginary case, +and its realization in Livingstone's account, go to show "the tendency +of the members of a variety to breed with one another." This is what I +have called "psychological selection," and, far from "ignoring" it, I +have always laid stress upon it as an obviously important form of +isolation or _prevention_ of free intercrossing. But it is a form of +isolation which can only occur in the higher animals, and, therefore, +the whole of Professor Herdman's criticism is merely a restatement of my +own views as already published in the paper which he is criticizing. +For all that his argument goes to prove is, first, the necessity for +_some_ form of isolation if the overwhelming effects of intercrossing +are to be obviated; and, secondly, the manifest consequence that where +the psychological form is unavailable (as in many of the lower animals +and in all plants), some other form must be present if divergent +evolution is taking place on a common area. + + * * * * * + +Seeing that so much misunderstanding has been shown with reference to my +views on "the swamping effects of intercrossing," and seeing also that +this misunderstanding extends quite as much to Mr. Gulick's views as to +my own, I will here supply brief extracts from both our original papers, +for the double purpose of showing our complete agreement, and of leaving +it to be judged whether we can fairly be held responsible for the +misunderstanding in question. After having supplied these quotations, I +will conclude this historical sketch by considering what Mr. Wallace has +said in reply to the views therein presented. I will transcribe but a +single passage from our papers, beginning with my own. + + Any theory of the origin of species in the way of descent must be + prepared with an answer to the question, Why have species + _multiplied_? How is it that, in the course of evolution, species + have not simply become transmuted in linear series instead of + ramifying into branches? This question Mr. Darwin seeks to answer + "from the simple circumstance that the more diversified the + descendants from any one species becomes in structure, + constitution, and habits, by so much will they be better enabled to + seize on many and widely diversified places in the economy of + nature, and so be enabled to increase in numbers." And he proceeds + to illustrate this principle by means of a diagram, showing the + hypothetical divergence of character undergone by the descendants + of seven species. Thus, he attributes divergence of character + exclusively to the influence of natural selection. + + Now, this argument appears to me unassailable in all save one + particular; but this is a most important particular: the argument + wholly ignores the fact of intercrossing with parent forms. + Granting to the argument that intercrossing with parent forms is + prohibited, and nothing can be more satisfactory. The argument, + however, sets out with showing that it is in limited areas, or in + areas already overstocked with the specific form in question, that + the advantages to be derived from diversification will be most + pronounced. It is where they "jostle each other most closely" that + natural selection will set a premium upon any members of the + species which may depart from the common type. Now, inasmuch as + this jostling or overcrowding of individuals is a needful condition + to the agency of natural selection in the way of diversifying + character, must we not feel that the general difficulty from + intercrossing previously considered is here presented in a special + and aggravated form? At all events, I know that, after having duly + and impartially considered the matter, to me it does appear that + unless the swamping effects of intercrossing with the parent form + on an overcrowded area is in some way prevented to begin with, + natural selection could never have any material supplied by which + to go on with. Let it be observed that I regard Mr. Darwin's + argument as perfectly sound where it treats of the divergence of + _species_, and of their further divergence into _genera_; for in + these cases the physiological barrier is known to be already + present. But in applying the argument to explain the divergence of + individuals into varieties, it seems to me that here, more than + anywhere else, Mr. Darwin has strangely lost sight of the + formidable difficulty in question; for in this particular case so + formidable does the difficulty seem to me, that I cannot believe + that natural selection alone could produce any divergence of + specific character, so long as all the individuals on an + overcrowded area occupy that area together. Yet, if any of them + quit that area, and so escape from the unifying influence of free + intercrossing, these individuals also escape from the conditions + which Mr. Darwin names as those that are needed by natural + selection in order to produce divergence. Therefore, it appears to + me that, under the circumstances supposed, natural selection alone + could not produce divergence; the most it could do would be to + change the whole specific type in some one direction, and thus + induce transmutation of species in a linear series, each succeeding + member of which might supplant its parent form. But in order to + secure _diversity_, _multiplication_, or _ramification_ of species, + it appears to me obvious that the primary condition required is + that of preventing intercrossing with parent forms at the origin of + each branch, whether the prevention be from the first absolute, or + only partial. + +Now for Mr. Gulick, a portion of whose more lengthy discussion of the +subject, however, is all that I need quote:-- + + Having found that the evolution of the fitted is secured through + the prevention of crossing between the better fitted and the less + fitted, can we believe that the evolution of a special race, + regularly transmitting a special kind of fitness, can be realized + without any prevention of crossing with other races that have no + power to transmit that special kind of fitness? Can we suppose that + any advantage, derived from new powers that prevent severe + competition with kindred, can be permanently transmitted through + succeeding generations to one small section of the species while + there is free crossing equally distributed between all the families + of the species? Is it not apparent that the terms of this + supposition are inconsistent with the fundamental laws of heredity? + Does not inheritance follow the lines of consanguinity; and when + consanguinity is widely diffused, can inheritance be closely + limited? When there is free crossing between the families of one + species, will not any peculiarity that appears in one family either + be neutralized by crosses with families possessing the opposite + quality, or, being preserved by natural selection, while the + opposite quality is gradually excluded, will not the new quality + gradually extend to all the branches of the species; so that, in + this way or in that, increasing divergence of form will be + prevented? + + If the advantage of freedom from competition in any given variation + depends on the possession, in some degree, of new adaptations to + unappropriated resources, there must be some cause that favours the + breeding together of those thus specially endowed, and interferes + in some degree with their crossing with other variations, or, + failing this, the special advantage will in succeeding generations + be lost. As some degree of Independent Generation is necessary for + the continuance of the advantage, it is evident that the same + condition is necessary for the accumulation through Natural + Selection of the powers on which the advantage depends. The + advantage of divergence of character cannot be retained by those + that fail to retain the divergent character; and divergent + character cannot be retained by those that are constantly crossing + with other kinds; and the prevention of free crossing between those + that are equally successful is in no way secured by Natural + Selection. + +So much, then, as expressive of Mr. Gulick's opinion upon this subject. +To exactly the same effect Professor Lloyd Morgan has recently published +his judgement upon it thus:-- + + That perfectly free intercrossing, between any or all of the + individuals of a given group of animals, is, so long as the + characters of the parents are blended in the offspring, fatal to + divergence of character, is undeniable. Through the elimination of + less favourable variations, the swiftness, strength, and cunning of + a race may be gradually improved. But no form of elimination can + possibly differentiate the group into swift, strong, and cunning + varieties, distinct from each other, so long as all three varieties + freely interbreed, and the characters of the parents blend in the + offspring. Elimination may and does give rise to progress in any + given group, _as a group_; it does not and cannot give rise to + differentiation and divergence, so long as interbreeding with + consequent interblending of characters be freely permitted. Whence + it inevitably follows, as a matter of simple logic, that where + divergence has occurred, intercrossing and interbreeding must in + some way have been lessened or prevented. Thus a new factor is + introduced, that of _isolation_ or _segregation_. And there is no + questioning the fact that it is of great importance. Its + importance, indeed, can only be denied by denying the swamping + effects of intercrossing, and such denial implies the tacit + assumption that interbreeding and interblending are held in check + by some form of segregation. The isolation explicitly denied is + implicitly assumed[51]. + + [51] _Animal Life and Intelligence_, pp. 98, 99 (1890-1891). + +Similarly, and still more recently, Professor Le Conte writes:-- + + It is evident, then, as Romanes claims, that natural selection + alone tends to _monotypic_ evolution. Isolation of some sort seems + necessary to _polytypic_ evolution. The tree of evolution under the + influence of natural selection alone grows palm-like from its + terminal bud. Isolation was necessary to the starting of lateral + buds, and thus for the profuse ramification which is its most + conspicuous character[52]. + + [52] _The Factors of Evolution_ (1891). + +In order to complete this historical review, it only remains to consider +Mr. Wallace's utterances upon the subject. + +It is needless to say that he stoutly resists the view of Weismann, +Delboeuf, Gulick, and myself, that specific divergence can ever be +due--or, as I understand him, even so much as assisted--by this +principle of indiscriminate isolation (apogamy). It will be remembered, +however, that Mr. Gulick has adduced certain general principles and +certain special facts of geographical distribution, in order to prove +that apogamy eventually leads to divergence of character, provided that +the isolated section of the species does not contain any very large +number of individuals. Now, Mr. Wallace, without making any reference to +this argument of Mr. Gulick, simply states the reverse--namely, that, as +a matter of fact, indiscriminate isolation is not found to be +associated with divergence of character. For, he says, "there is an +entire absence of change, where, if this were a _vera causa_, we should +expect to find it[53]." But the only case which he gives is that of +Ireland. + + [53] _Darwinism_, p. 151. + +This, he says, furnishes "an excellent test case, for we know that it +[Ireland] has been separated from Britain since the end of the glacial +epoch: ... yet hardly one of its mammals, reptiles, or land molluscs has +undergone the slightest change[54]." Here, however, Mr. Wallace shows +that he has failed to understand "the views of those who, like Mr. +Gulick, believe isolation itself to be a cause of modification of +species"; for it belongs to the very essence of these views that the +efficiency of indiscriminate isolation as a "_vera causa_" of organic +evolution varies inversely with the number of individuals (i. e. the +size of the species-section) exposed to its influence. Therefore, far +from being "an excellent test case," the case of Ireland is +unsatisfactory. If we are in search of excellent test cases, in the +sense intended by Mr. Wallace, we ought not to choose a large island, +which from the time of its isolation must have contained large bulks of +each of the geographically separated species concerned: we ought to +choose cases where as small a number as possible of the representatives +of each species were in the first instance concerned. And, when we do +this, the answer yielded by any really "excellent test case" is +unequivocal. + + [54] _Ibid._ + +No better test case of this kind has ever been furnished than that of +Mr. Gulick's land-shells, which Mr. Wallace is specially considering in +the part of his book where the sentence above quoted occurs. How, then, +does he meet this case? He meets it by assuming that in all the numerous +adjacent valleys of a small island there must be as many differences of +environment, each of which is competent to induce slight varietal +changes on the part of its occupants by way of natural selection, +although in no one case can the utility of these slight changes be +surmised. Now, against this explanation there are three overwhelming +considerations. In the first place, it is purely gratuitous, or offered +merely in order to save the hypothesis that there _can_ be no other +cause of even the most trivial change in species than that which is +furnished by natural selection. In the second place, as Mr. Gulick +writes to me in a private letter, "if the divergence of Sandwich Island +land molluscs is wholly due to exposure to different environments, as +Mr. Wallace argues on pages 147-150, then there must be completely +occult influences in the environment that vary progressively with each +successive mile. This is so violent an assumption that it throws doubt +on any theory that requires such support." In the third place, the +assumption that the changes in question must have been due to natural +selection, is wholly incompatible with the facts of isolation +elsewhere--namely, in those cases where (as in that of Ireland) a large +section of species, instead of a small section, has been +indiscriminately isolated. Mr. Wallace, as we have seen, inadvertently +alludes to these "many other cases of isolation" as evidence against +apogamy being _per se_ a cause of specific change. But although, for +the reason above stated, they are without relevancy in this respect, +they appear to me fatal to the explanation which he gives of specific +changes under apogamy where only small sections of species are +concerned. For example, can it be rationally maintained that there are +more differences of environment between every two of the many contiguous +valleys of a small island, such as Mr. Gulick describes, than there are +in the incomparably larger area of the whole of Ireland? But, if not, +and if natural selection is able to work such "occult" wonders in each +successive mile on the Sandwich Islands, why has it so entirely lost +this magic power in the case of Ireland--or in the "many other cases of +isolation" to which Mr. Wallace refers? On his theory there is no +coherent answer to be given to this question, while on our theory the +answer is given in the very terms of the theory itself. The facts are +plainly just what the theory requires that they should be; and +therefore, if they were not as they are, the theory would be deprived of +that confirmation which it now derives from them. + +Thus, in truth, though in an opposite way, the case of Ireland is, as +Mr. Wallace says, "an excellent test case," when once the theory of +apogamy as a "_vera causa_" of specific change is understood; and the +effect of applying the test is fully to corroborate this theory, while +at the same time it as fully negatives the other. For the consideration +whereby Mr. Wallace seeks to explain the inactivity of natural selection +in the case of Ireland is not "coherent." What he says is, "That changes +have not occurred through natural selection, is perhaps due to the less +severe struggle for existence, owing to the smaller number of competing +species[55]." But even with regard to molluscs alone, there is a greatly +larger number of species in Ireland than occurs in any one valley of the +Sandwich Islands; while if we have regard to all the other classes of +animal life, comparison entirely fails. + + [55] _Loc. cit._, p. 151. + +Much more to the point are certain cases which were adduced long ago by +Weismann in his essay previously considered. Nevertheless, although this +essay was published as far back as 1872, and, although it expressly +deals with the question of divergence of character through the mere +prevention of intercrossing (Amixia), Mr. Wallace nowhere alludes to +these cases _per contra_, which are so much more weighty than his own +"test case" of Ireland. Of such are four species of butterflies, +belonging to three genera[56], which are identical in the polar regions +and in the Alps, notwithstanding that the sparse Alpine populations have +been presumably separated from their parent stocks since the glacial +period; or of certain species of fresh water crustaceans (_Apus_), the +representatives of which are compelled habitually to form small isolated +colonies in widely separated ponds, and nevertheless exhibit no +divergence of character, although apogamy has probably lasted for +centuries. These cases are unquestionably of a very cogent nature, and +appear of themselves to prove that apogamy alone is not invariably +capable of inducing divergence--at any rate, so rapidly as we might +expect. There appears, however, to be another factor, the presence or +absence of which makes a great difference. This as stated in the text, +is the degree in which a specific type is stable or unstable--liable or +not liable to vary. Thus, for example, the Goose is what Darwin calls an +"inflexible" type as compared with most other domesticated birds. +Therefore, if a lot of geese were to be indiscriminately isolated from +the rest of their species, the probability is that in a given time their +descendants would not have diverged from the parent type to such an +extent as would a similar lot of ducks under similar circumstances: the +more stable specific type would require a longer time to change under +the influence of apogamy alone. Now, the butterflies and crustaceans +quoted by Weismann may be of a highly stable type, presenting but a +small range of individual variability; and, if so, they would naturally +require a long time to exhibit any change of type under the influence of +apogamy alone. But, be this as it may, Weismann himself adduces these +cases merely for the sake of showing that there are cases which seem to +tell against the general principle of modification as due to apogamy +alone--i.e. the general principle which, under the name amixia, he is +engaged in defending. And the conclusion at which he himself arrives is, +that while it would be wrong to affirm that apogamy _must_ in all cases +produce divergence, we are amply justified in affirming that in many +cases it _may_ have done so; while there is good evidence to prove that +in not a few cases it _has_ done so, and therefore should be accepted +as one of the factors of organic evolution[57]. + + [56] Namely, _Lycaena denzelii_, _L. pheretes_, _Argynnis pales_, + _Erebia mante_. + + [57] Since the above was written, I have heard of some cases which + seem to present greater difficulties to our theory than those above + quoted. These refer to some of the numerous species of land mollusca + which inhabit the isolated rocks near Madeira (Dezertas). My + informant is Dr. Grabham, who has himself investigated the matter, + and reports as follows:-- + + "It is no uncommon thing to meet with examples of the same species, + sub-fossil, recent, and living upon one spot, and presenting no + variation in the long record of descent." Then, after naming these + examples, he adds, "All seem to vary immediately on attaining new + ground, assuming many aspects in different districts." + + Unquestionably these statements support, in a very absolute manner, + Mr. Wallace's opinion, while making directly against my own. It is + but fair, however, to add that the cases are not numerous (some + half-dozen at the most, and all within the limits of a single + genus), and that, even in the opinion of my informant himself, the + facts have not hitherto been sufficiently investigated for any + decisive judgement to be formed upon them. + +My view from the very first has been that variations in the way of +cross-infertility are of frequent occurrence (how, indeed, can they be +otherwise, looking to the complex conditions that have to be satisfied +in every case of full fertility?); and, therefore, however many of such +variations are destined to die out, whenever one arises, "under suitable +conditions," "it must inevitably tend to be preserved as a new natural +variety, or incipient species." Among the higher animals--which are +"comparatively few in number"--I think it probable that some slight +change of form, colour, habit, &c., must be usually needed either to +"superinduce," or, which is quite a different thing, to _coincide_ with +the physiological change But in the case of plants and the lower +invertebrata. I see no reason for any frequent concomitance of this +kind; and therefore believe the physiological change to be, "as a +general rule," the primordial change. At the same time, I have always +been careful to insist that this opinion had nothing to do with "the +essence of physiological selection"; seeing that "it was of no +consequence" to the theory in what proportional number of cases the +cross-sterility had begun _per se_, had been superinduced by +morphological changes, or only enabled to survive by happening to +coincide with any other form of homogamy. In short, "the essence of +physiological selection" consists in _all_ cases of the diversifying +_effect_ of cross-infertility, whensoever and howsoever it may happen in +particular cases to have been _caused_. + +Thus I emphatically reaffirm that "from the first I have always +maintained that it makes no essential difference to the theory _in what +proportional number of cases_ they [the physiological variations] have +arisen 'alone in an otherwise undifferentiated species'"; therefore, +"even if I am wrong in supposing that physiological selection can _ever_ +act alone, the _principle_ of physiological selection, as I have stated +it, is not thereby affected. And this principle is, as Mr. Wallace has +re-stated it, 'that some amount of infertility characterizes the +distinct varieties which are in process of differentiation into +species'--infertility whose absence, 'to obviate the effects of +intercrossing, may be one of the _usual_ causes of their failure to +become developed into distinct species.'" + +These last sentences are quoted from the correspondence in _Nature_[58], +and to them Mr. Wallace replied by saying, "if this is not an absolute +change of front, words have no meaning"; that "if this is 'the whole +essence of physiological selection,' then physiological selection is but +a re-statement and amplification of Darwin's views"; that such a "change +of front" is incompatible, not only with my term "physiological +selection," but also with my having "acknowledged that Mr. Catchpool had +'very clearly put forward the theory of physiological selection'"; and +much more to the same effect. + + [58] Vol. xliii. p. 127. + +Now, to begin with, it is due to Mr. Catchpool to state that his only +publication upon this subject is much too brief to justify Mr. +Wallace's, inference, that he supposes variations in the way of +cross-infertility always to arise "alone in an otherwise +undifferentiated species." What Mr. Catchpool's opinion on this point +may be, I have no knowledge; but, whatever it is, he was unquestionably +the first writer who "clearly stated the leading principles" of +physiological selection, and this fact I am very glad to have +"acknowledged." In my correspondence with Mr. Wallace, however, I not +only named Mr. Catchpool: I also named--and much more prominently--Mr. +Gulick. For even if I were to grant (which I am far indeed from doing) +that there was any want of clearness in my own paper touching the point +in question, I have now repeatedly shown that it is simply impossible +for any reader of Mr. Gulick's papers to misunderstand _his_ views with +regard to it. Accordingly, I replied to Mr. Wallace in _Nature_ by +saying:-- + + Not only have I thus from the first fully recognized the sundry + other causes of specific change with which the physiological + variations may be associated; but Mr. Gulick has gone into this + side of our common theory much more fully, and elaborately + calculated out the high ratio in which the differentiating agency + of any of these other causes must be increased when assisted by--i. + e. associated with--even a moderate degree of the selective + fertility, and vice versa. Therefore, it is simply impossible for + Mr. Wallace to show that "our theory" differs from his in this + respect. Yet it is the only respect in which his reply alleges any + difference. (Vol. xliii. p. 127.) + +I think it is to be regretted that, in his answer to this, Mr. Wallace +alludes only to Mr. Catchpool, and entirely ignores Mr. Gulick--whose +elaborate calculations above alluded to were communicated to the +Linnaean Society by Mr. Wallace himself in 1887. + +The time has now come to prove, by means of quotations, that I have from +the first represented the "principle," or "essence," of physiological +selection to consist in selective fertility furnishing a needful +condition to specific differentiation, in at least a large proportional +number of allied species which afterwards present the reciprocal +character of cross-sterility; that I have never represented variations +in the way of this selective fertility as necessarily constituting the +initial variations, or as always arising "alone, in an otherwise +undifferentiated species"; and that, although I have uniformly given it +as my opinion that these variations do _in some cases_ thus arise +(especially among plants and lower invertebrata), I have as uniformly +stated "that it makes no difference to the theory in what proportional +number of cases they have done so"--or even if, as Mr. Wallace supposes, +they have never done so in any case at all[59]. These statements (all of +which are contradictory of the only points of difference alleged) have +already been published in my article in the _Monist_ of October, 1890. +And although Mr. Wallace, in his reply to that article, ignores my +references to the "original paper," it is scarcely necessary to quote +the actual words of the paper itself, since the reader who is further +interested in this controversy can readily refer to it in the _Journal +of the Linnaean Society_ (vol. xix. pp. 337-411). + + [59] This refers to what I understand Mr. Wallace to say in the + _Nature_ correspondence is the supposition on which his own theory + of the origin of species by cross-infertility is founded. But in the + original statement of that theory itself, it is everywhere + "supposed" that when species are originated by cross-infertility, + the _initial_ change _is_ the physiological change. In his original + statement of that theory, therefore, he literally went further than + I had gone in my "original paper," with reference to supposing the + physiological change to be the initial change. I do not doubt that + this is due to some oversight of expression; but it is curious that, + having made it, he should still continue his endeavour to fix + exactly the same oversight upon me. + +Having arrived at these results with regard to the theory of Isolation +in general and of Physiological Isolation in particular, I arrive also +at the end of this work. And if, while dealing with the post-Darwinian +period, I have imparted to any general reader the impression that there +is still a great diversity of expert opinion; I must ask him to note +that points with reference to which disagreement still exists are but +very subordinate to those with regard to which complete agreement now +prevails. The noise of wrangling disputations which has so filled the +camp of evolutionists since the death of their captain, is apt to hide +from the outside world the solid unanimity that prevails with regard to +all the larger and more fundamental questions, which were similarly the +subjects of warfare in the past generation. Indeed, if we take a fair +and general view of the whole history of Darwinism, what must strike us +as the really significant fact is the astonishing unanimity which has +been so rapidly attained with regard to matters of such immeasurable +importance. It is now but little more than thirty years since the +publication of the _Origin of Species_; and in that period not only have +all naturalists unequivocally embraced the doctrine of descent +considered as a fact; but, in one degree or another, they have all as +unequivocally embraced the theory of natural selection considered as a +method. The only points with regard to which any difference of opinion +still exist, have reference to the precise causation of that mighty +stream of events which, under the name of organic evolution, we have now +all learnt to accept as scientifically demonstrated. But it belongs to +the very nature of scientific demonstration that, where matters of great +intricacy as well as of high generality are concerned, the process of +demonstration must be gradual, even if it be not always slow. It is only +by the labours of many minds working in many directions that, in such +cases, truth admits of being eventually displayed. Line upon line, +precept upon precept, here a little and there a little--such is the +course of a scientific revelation; and the larger the subject-matter, +the more subtle and the more complex the causes, the greater must be the +room for individual differences in our reading of the book of Nature. +Now, if all this be true, must we not feel that in the matter of organic +evolution the measure of agreement which has been attained is out of all +proportion to the differences which still remain--differences which, +although of importance in themselves, are insignificant when compared +with those which once divided the opinions of not a few still living +men? And if we are bound to feel this, are we not bound further to feel +that the very intensity of our disputations over these residual matters +of comparative detail, is really the best earnest that can be given of +the determination of our quest--determination which, like that of our +fathers, cannot fail to be speedily rewarded by the discovery of truth? + +Nevertheless, so long as this noise of conflict is in the Senate, we +cannot wonder if the people are perplexed. Therefore, in conclusion, I +may ask it to be remembered exactly what are the questions--and the only +questions--which still divide the parties. + +Having unanimously agreed that organic evolution is a fact and that +natural selection is a cause, or a factor in the process, the primary +question in debate is whether natural selection is the only cause, or +whether it has been assisted by the co-operation of other causes. The +school of Weismann maintain that it is the only cause; and therefore +deem it worse than useless to search for further causes. With this +doctrine Wallace in effect agrees, excepting as regards the particular +case of the human mind. The school of Darwin, on the other hand--to +which I myself claim to belong--believe that natural selection has been +to a considerable extent supplemented by other factors; and, therefore, +although we further believe that it has been the "main" factor, we agree +with Darwin himself in strongly reprobating all attempts to bar _a +priori_ the progress of scientific investigation touching what, if any, +these other factors may be. Lastly, there are several more or less +struggling schools, chiefly composed of individual members who agree +with each other only to the extent of holding that the causal agency of +natural selection is not so great as Darwin supposed. The Duke of +Argyll, Mr. Mivart and Mr. Geddes may be named in this connexion; +together with the self-styled neo-Lamarckians, who seek to magnify the +Lamarckian principles at the expense of the distinctively Darwinian. + +This primary difference of opinion leads deductively to certain +secondary differences. For if a man starts with the premiss that natural +selection must necessarily be the "exclusive" cause of organic +evolution, he is likely to draw conclusions which another man would not +draw who starts with the premiss that natural selection is but the +"main" cause. Of these subordinate differences the most important are +those which relate to the possible transmission of acquired characters, +to the necessary (or only general) utility of specific characters, and +to the problem touching the inter-sterility of allied species. But we +may well hope that before another ten years shall have passed, even +these still outstanding questions will have been finally settled; and +thus that within the limits of an ordinary lifetime the theory of +organic evolution will have been founded and completed in all its parts, +to stand for ever in the world of men as at once the greatest +achievement in the history of science, and the most splendid monument of +the nineteenth century. + +In the later chapters of the foregoing treatise I have sought to +indicate certain matters of general principle, which many years of study +specially devoted to this great movement of contemporary thought have +led me to regard as almost certainly sound in themselves, and no less +certainly requisite as complements of the Darwinian theory. I will now +conclude by briefly summarizing these matters of general principle in +the form of twelve sequent propositions. And, in doing so, I may ask it +to be noticed that the system which these propositions serve to express +may now claim, at the least, to be a strictly logical system. For the +fact that, not merely in its main outlines, but likewise in its details, +it has been independently constructed by Mr. Gulick, proves at any rate +this much; seeing that, where matters of such intricacy are concerned, +nothing but accurate reasoning from a common foundation of _data_ could +possibly have yielded so exact an agreement. The only difference between +us is, that Mr. Gulick has gone into much further detail than I have +ever attempted in the way of classifying the many and varied forms of +isolation; while I have laid more special stress upon the physiological +form, and found in it what appears to me a satisfactory solution of "the +greatest of all the difficulties in the way of accepting the theory of +natural selection as a complete explanation of the origin of +species"--namely, "the remarkable difference between varieties and +species when crossed." + + + + +GENERAL CONCLUSIONS. + +1. NATURAL SELECTION IS PRIMARILY A THEORY OF THE CUMULATIVE DEVELOPMENT +OF ADAPTATIONS WHEREVER THESE OCCUR; AND THEREFORE IS ONLY INCIDENTALLY, +OR LIKEWISE, A THEORY OF THE ORIGIN OF SPECIES IN CASES WHERE ALLIED +SPECIES DIFFER FROM ONE ANOTHER IN RESPECT OF PECULIAR CHARACTERS, WHICH +ARE ALSO ADAPTIVE CHARACTERS. + +2. HENCE, IT DOES NOT FOLLOW FROM THE THEORY OF NATURAL SELECTION THAT +ALL SPECIES--MUCH LESS ALL SPECIFIC CHARACTERS--MUST NECESSARILY HAVE +OWED THEIR ORIGIN TO NATURAL SELECTION; SINCE IT CANNOT BE PROVED +DEDUCTIVELY FROM THE THEORY THAT NO "MEANS OF MODIFICATION" OTHER THAN +NATURAL SELECTION IS COMPETENT TO PRODUCE SUCH SLIGHT DEGREES OF +MODIFICATION AS GO TO CONSTITUTE DIAGNOSTIC DISTINCTIONS BETWEEN CLOSELY +ALLIED SPECIES; WHILE, ON THE OTHER HAND, THERE IS AN OVERWHELMING MASS +OF EVIDENCE TO PROVE THE ORIGIN OF "A LARGE PROPORTIONAL NUMBER OF +SPECIFIC CHARACTERS" BY CAUSES OF MODIFICATION OTHER THAN NATURAL +SELECTION. + +3. THEREFORE, AND UPON THE WHOLE, AS DARWIN SO EMPHATICALLY HELD, +"NATURAL SELECTION HAS BEEN THE MAIN, BUT NOT THE EXCLUSIVE MEANS OF +MODIFICATION." + +4. EVEN IF IT WERE TRUE THAT ALL SPECIES AND ALL SPECIFIC CHARACTERS +MUST NECESSARILY OWE THEIR ORIGIN TO NATURAL SELECTION, IT WOULD STILL +REMAIN ILLOGICAL TO DEFINE THE THEORY OF NATURAL SELECTION AS +INDIFFERENTLY A THEORY OF SPECIES OR A THEORY OF ADAPTATIONS; FOR, EVEN +UPON THIS ERRONEOUS SUPPOSITION, SPECIFIC CHARACTERS AND ADAPTIVE +CHARACTERS WOULD REMAIN VERY FAR INDEED FROM BEING CONTERMINOUS--MOST OF +THE MORE IMPORTANT ADAPTATIONS WHICH OCCUR IN ORGANIC NATURE BEING THE +COMMON PROPERTY OF MANY SPECIES. + +5. IN NO CASE CAN NATURAL SELECTION HAVE BEEN THE CAUSE OF MUTUAL +INFERTILITY BETWEEN ALLIED, OR ANY OTHER, SPECIES--_I.E._ OF THE MOST +GENERAL OF ALL "SPECIFIC CHARACTERS." + +6. WITHOUT ISOLATION, OR THE PREVENTION OF FREE INTERCROSSING, ORGANIC +EVOLUTION IS IN NO CASE POSSIBLE. THEREFORE, IT IS ISOLATION THAT _HAS_ +BEEN "THE EXCLUSIVE MEANS OF MODIFICATION," OR, MORE CORRECTLY, THE +UNIVERSAL CONDITION TO IT. THEREFORE, ALSO, HEREDITY AND VARIABILITY +BEING GIVEN, THE WHOLE THEORY OF ORGANIC EVOLUTION BECOMES A THEORY OF +THE CAUSES AND CONDITIONS WHICH LEAD TO ISOLATION. + +7. ISOLATION MAY BE EITHER DISCRIMINATE OR INDISCRIMINATE. WHEN +DISCRIMINATE, IT HAS REFERENCE TO RESEMBLANCES BETWEEN INDIVIDUALS +CONSTITUTING THE ISOLATED COLONY OR GROUP; WHEN INDISCRIMINATE, IT HAS +NO SUCH REFERENCE. IN THE FORMER CASE THERE ARISES HOMOGAMY, AND IN THE +LATTER CASE THERE ARISES APOGAMY. + +8. EXCEPT WHERE VERY LARGE POPULATIONS ARE CONCERNED, INDISCRIMINATE +ISOLATION ALWAYS TENDS TO BECOME INCREASINGLY DISCRIMINATE; AND, IN THE +MEASURE THAT IT DOES SO, APOGAMY PASSES INTO HOMOGAMY, BY VIRTUE OF +INDEPENDENT VARIABILITY. + +9. NATURAL SELECTION IS ONE AMONG MANY OTHER FORMS OF DISCRIMINATE +ISOLATION, AND PRESENTS IN THIS RELATION THE FOLLOWING PECULIARITIES:-- +(_A_) THE ISOLATION IS WITH REFERENCE TO SUPERIORITY OF FITNESS; (_B_) +IS EFFECTED BY DEATH OF THE EXCLUDED INDIVIDUALS; AND (_C_) UNLESS +ASSISTED BY SOME OTHER FORM OF ISOLATION, CAN ONLY EFFECT MONOTYPIC AS +DISTINGUISHED FROM POLYTYPIC EVOLUTION. + +10. IT IS A GENERAL LAW OF ORGANIC EVOLUTION THAT THE NUMBER OF +POSSIBLE DIRECTIONS IN WHICH DIVERGENCE MAY OCCUR CAN NEVER BE MORE THAN +EQUAL TO THE NUMBER OF CASES OF EFFICIENT ISOLATION; BUT, EXCEPTING +NATURAL SELECTION, ANY ONE FORM OF ISOLATION NEED NOT NECESSARILY +REQUIRE THE CO-OPERATION OF ANOTHER FORM IN ORDER TO CREATE AN +ADDITIONAL CASE OF ISOLATION, OR TO CAUSE POLYTYPIC AS DISTINGUISHED +FROM MONOTYPIC EVOLUTION. + +11. WHERE COMMON AREAS AND POLYTYPIC EVOLUTION ARE CONCERNED, THE MOST +GENERAL AND MOST EFFICIENT FORM OF ISOLATION HAS BEEN THE PHYSIOLOGICAL, +AND THIS WHETHER THE MUTUAL INFERTILITY HAS BEEN THE ANTECEDENT OR THE +CONSEQUENT OF MORPHOLOGICAL CHANGES ON THE PART OF THE ORGANISMS +CONCERNED, AND WHETHER OR NOT THESE CHANGES ARE OF AN ADAPTIVE +CHARACTER. + +12. THIS FORM OF ISOLATION--WHICH, IN REGARD TO INCIPIENT SPECIES, I +HAVE CALLED PHYSIOLOGICAL SELECTION--MAY ACT EITHER ALONE OR IN +CONJUNCTION WITH OTHER FORMS OF ISOLATION ON COMMON AREAS: IN THE FORMER +CASE ITS AGENCY IS OF MOST IMPORTANCE AMONG PLANTS AND THE LOWER CLASSES +OF ANIMALS; IN THE LATTER CASE ITS IMPORTANCE CONSISTS IN ITS GREATLY +INTENSIFYING THE SEGREGATIVE POWER OF WHATEVER OTHER FORM OF ISOLATION +IT MAY BE WITH WHICH IT IS ASSOCIATED. + + + + +_APPENDICES_ + + + + + +APPENDIX A. + +MR. GULICK'S CRITICISM OF MR. WALLACE'S VIEWS ON PHYSIOLOGICAL +SELECTION. + + +I have received from Mr. Gulick the results of his consideration of Mr. +Wallace's criticism. As these results closely resemble those which I +have myself reached, and as they were independently worked out on the +other side of the globe, I deem it desirable to publish them here for +the sake of comparison. + +In his covering letter Mr. Gulick writes:-- + + Mr. Wallace has most certainly adopted the fundamental principles + of our theory, and in an arbitrary way attempted to claim the + results produced by these principles as the effects of natural + selection. He takes our principles, which in the previous chapter + he has combated; but he makes such disjointed use of them that I am + not willing to recognize his statement as an intelligible + exposition of our theory.... I have endeavoured to indicate at what + points Mr. Wallace has deserted his own principles, and at what + points he has failed to make the best use of ours. To bring out + these points distinctly has been no easy task; but if you regard + this paper on _The Preservation and Accumulation of + Cross-infertility_ as giving any help in elucidating the true + principles, and in showing Mr. Wallace's position in regard to + them, I shall be satisfied. Please make any use of it that may seem + desirable, and then forward it to Professor Dana. + +The following is a general summary of Mr. Gulick's results:-- + + Mr. Wallace's criticism of the theory of Physiological Selection is + unsatisfactory; (l) because he has accepted the fundamental + principle of that theory on pages 173-9, in that he maintains that + without the cross-infertility the incipient species there + considered would be swamped; (2) because he assumes that + physiological selection pertains simply to the infertility of first + crosses, and has nothing to do with the infertility of mongrels and + hybrids; (3) because he assumes that infertility between first + crosses is of rare occurrence between species of the same genus, + ignoring the fact that in many species of plants the pollen of the + species is pre-potent on the stigma of the same species when it has + to compete with the pollen of other species of the same genus; (4) + because he not only ignores Mr. Romanes' statement that + cross-infertility often affects "a whole race or strain," but he + gratuitously assumes that the theory of Physiological Selection + excludes this "racial incompatibility" (which Mr. Romanes maintains + is the more probable form), and bases his computation on the + assumption that the cross-infertility is not associated with any + other form of segregation; (5) because he claims to show that "all + infertility not correlated with some _useful_ variation has a + constant tendency to effect its own elimination," while his + computation only shows that, if the cross-infertility is not + associated with some form of _positive_ segregation, it will + disappear[60]; and (6) because he does not observe that the positive + segregation may be secured by the very form of the physiological + incompatibility.... Without here entering into any computation, it + is evident that, e.g. the prepotency of pollen of each kind with + its own kind, if only very slight, will prevent cross-fertilization + as effectually as a moderate degree of instinctive preference in + the case of an animal. + + [60] "Positive segregation" is Mr. Gulick's term for forms of + homogamy other than that which is due to selective fertility. Of + these other, or "positive" forms, natural selection is one; but as + it is far from being the _only_ one, the criticism points out that + utility is not the _only_ conserving principle with which selective + fertility may be associated. + +The paper likewise indicates a point which, in studying Mr. Wallace's +theory, I have missed. It will be remembered that the only apparent +difference between his theory and mine has been shown to consist in +this--that while I was satisfied to state, in a general way, that +natural selection is probably able to increase a selective fertility +which has already been begun by other causes, Mr. Wallace has sought to +exhibit more in detail the precise conditions under which it can do so. +Now, Mr. Gulick shows that the particular conditions which Mr. Wallace +describes, even if they do serve to promote an increase of +cross-infertility, are conditions which preclude the possibility of +natural selection coming into play at all. So that if, under these +particular conditions, a further increase of cross-infertility does take +place, it does not take place in virtue of natural selection. To me it +appears that this criticism is sound; and, if so, it disposes of even +the one very subordinate addition to our theory which Mr. Wallace +"claims" as the most "distinctive" part of his. + +The following is the criticism in question:-- + + On pages 173-186 Mr. Wallace maintains that "Natural selection is, + in some probable cases at all events, able to accumulate variations + in infertility between incipient species" (p. 174); but his + reasoning does not seem to me conclusive. Even if we grant that the + increase of this character [cross-infertility] occurs by the steps + which he describes, _it is not a process of accumulation by natural + selection_. In order to be a means of cumulative modification of + varieties, races, or species, selection, whether artificial or + adaptational [i.e. natural], must preserve certain forms of an + intergenerating stock, to the exclusion of other forms of the same + stock. Progressive change in the size of the occupants of a + poultry-yard may be secured by raising only bantams the first, only + common fowls the second, and only Shanghai fowls the third year; + but this is not the form of selection that has produced the + different races of fowls. So in nature, rats may drive out and + supplant mice; but this kind of selection modifies neither rats + nor mice. On the other hand, if certain variations of mice prevail + over others, through their superior success in escaping their + pursuers, then modification begins. Now, turning to page 175, we + find that, in the illustrative case introduced by Mr. Wallace, the + commencement of infertility between the incipient species is in the + relations to each other of two portions of a species that are + locally segregated from the rest of the species, and partially + segregated from each other by different modes of life. These two + local varieties, being by the terms of his supposition better + adapted to the environment than the freely interbreeding forms in + other parts of the general area, increase till they supplant these + original forms. Then, in some limited portion of the general area, + there arise two still more divergent forms, with greater mutual + infertility, and with increased adaptation to the environment, + enabling them to prevail throughout the whole area. The process + here described, if it takes place, is not modification by natural + selection. + +On the other hand, it _is_ modification by physiological selection. For, +among the several other forms of isolation which are called +into requisition, the physiological (i.e. ever accumulating +cross-infertility) is supposed to play an important part. That the +modification is not modification by natural selection may perhaps be +rendered more apparent by observing, that in as far as _any_ other mode +of isolation is involved or supposed, so far is the _possible_ agency of +natural selection eliminated _as between the two or more otherwise +isolated sections of a species_; and yet it is modes of isolation other +than that furnished by natural selection (i.e. perishing of the less +fit), that Mr. Wallace here supposes to have been concerned--including, +as I have before shown, the physiological form, to which, indeed, he +really assigns most importance of all. Or, as Mr. Gulick states the +matter in his independent criticism:-- + + In the supposed case pictured by Mr. Wallace, the principle by + which the two segregating forms are kept from crossing, and so are + eventually preserved as permanently distinct forms, is no other + than that which Mr. Romanes and myself have discussed under the + terms Physiological Selection and Segregate Fecundity. Not only is + Mr. Wallace's exposition of the divergence and the continuance of + the same in accord with these principles which he has elsewhere + rejected, but his whole exposition is at variance with his own + principle, which, in the previous chapter, he vigorously maintains + in opposition to my statement that many varieties and species of + Sandwich Island land molluscs have arisen, while exposed to the + same environment, in the isolated groves of the successive valleys + of the same mountain range. If he adhered to his own theory, "the + greater infertility between the two forms in one portion of the + area" would be attributed to a difference between the _environment_ + presented in that portion and that presented in the other portions; + and the difficulty would be to consistently show how this greater + infertility could continue unabated when the varieties thus + characterized spread beyond the environment on which the character + depends. But, without power to continue, the process which he + describes would not take place. Therefore, in order to solve the + problem of the _origin_ and _increase_ of infertility between + species, he tacitly gives up his own theory, and adopts not only + the theory of Physiological Selection but that of Intensive + Segregation[61] through Isolation, though he still insists on + calling the process natural selection; for on page 183 he says, "No + form of infertility or sterility between the individuals of a + species can be increased by natural selection unless correlated + with some useful variation, while all infertility not so correlated + has a constant tendency to effect its own elimination." Even this + claim he seems to unwittingly abandon when on page 184 he says: + "The moment it [a species] becomes separated either by geographical + or selective isolation, or by diversity of station or of habits, + then, while each portion must be kept fertile _inter se_, there is + nothing to prevent infertility arising between the two separated + portions." + + [61] By Intensive Segregation Mr. Gulick means what I have called + Independent Variability. + +The criticism proceeds to show yet further inconsistencies and +self-contradictions in Mr. Wallace's treatment of this subject; but it +now seems needless to continue. Nor, indeed, should I have quoted this +much but for the sake of so fully justifying my own criticism by showing +the endorsement which it has received from a completely independent +examination. + + + + +APPENDIX B. + +AN EXAMINATION BY MR. FLETCHER MOULTON OF MR. WALLACE'S CALCULATION +TOUCHING THE POSSIBILITY OF PHYSIOLOGICAL SELECTION EVER ACTING ALONE. + + +We have seen that the only important point of difference between Mr. +Wallace's more recent views and my own on the problem of inter-specific +sterility, has reference to the question whether variations in the way +of cross-infertility can _ever_ arise and act "alone, in an otherwise +undifferentiated species," or whether they can _never_ so arise and act. +It is Mr. Wallace's opinion that, even if they ever do arise alone, at +all events they can never act in differentiating a specific type, seeing +that the chances against their suitable mating must be so great: only if +they be from the first associated with some other form of homogamy, +which will have the effect of determining their suitable mating, does he +think that they can act in the way supposed by our theory of "selective +fertility"[62]. On the other hand, as previously and frequently stated, +I have so strong a belief in the segregating power of physiological +selection, or selective fertility, that I do not think it is necessary +for this principle to be _always___ associated with some other form of +homogamy. From the first, indeed, I have laid great stress (as, also, +has Mr. Gulick) on the re-enforcing influence which association with any +other form of homogamy must exercise upon the physiological form, and +vice versa; but I have also said that, in my opinion, the physiological +form may in many cases be able to act entirely alone, or without +assistance derived from any other source. The question here is, as we +have already so fully seen, a question of but secondary importance; +since, whether or not the physiological form of homogamy ever acts +alone, even Mr. Wallace now allows, or rather argues, that it acts in +combination--and this so habitually, as well as with so much effect, +that it constitutes a usual condition to the origination of species. +Nevertheless, although the only relevancy of his numerical computation +of chances--whereby he thinks that he overturns my theory _in toto_--is +such relevancy as it bears to this question of secondary importance, I +have thought it desirable to refer the question, together with Mr. +Wallace's views upon it, to the consideration of a trained +mathematician. + + [62] His sentence, "all fertility not correlated with some _useful_ + variation has a constant tendency to effect its own elimination," + still further restricts the possible action of physiological + selection to cases where at least one of the other forms of homogamy + with which it is associated is natural selection. Or, in other + words, it is represented that physiological selection must always be + associated with natural selection, even if it be likewise associated + with any other form of exclusive breeding. But as this further + limitation appears to me self-evidently unjustifiable (seeing that + utility is not the only possible means of securing effective + isolation) I here neglect it, and take the wider ground marked out + above. It is needless to say that this is giving Mr. Wallace every + possible advantage, by not holding him to his still narrower ground. + +As this "subordinate question" depends entirely on numerical +computations involving the doctrine of chances, I should first of all +like to remark, that in reference to biological problems of the kind now +before us, I do not myself attach much importance to a merely +mathematical analysis. The conditions which such problems involve are so +varied and complex, that it is impossible to be sure about the validity +of the _data_ upon which a mathematical analysis is founded. +Nevertheless, for the sake of meeting these criticisms upon their own +ground, I will endeavour to show that, even as mathematical +calculations, they are quite untrustworthy. And, in order to do this +effectually, I will quote the results of a much more competent, as well +as a much more thorough, inquiry. I applied to Mr. Moulton for this +purpose, not only because he is one of the ablest mathematicians of my +acquaintance; but also because his interest in biology, and his +knowledge of Darwinian literature, render him well fitted to appreciate +exactly, and in all their bearings, the questions which were submitted +to his consideration. I need only add that his examination was +completely independent, and in no way influenced by me. Having +previously read my paper on _Physiological Selection_, Mr. Gulick's +paper on _Divergent Evolution_, and Mr. Wallace's book on _Darwinism_, +he was in possession of all the materials; and I merely requested the +favour of his opinion upon the whole case from a mathematical point of +view. The following is his reply; and I give it _in extenso_, because it +serves to place in another light some of the general considerations +which it has already been my endeavour to present[63]. + + [63] In our _Nature_ correspondence of 1890-1891, Mr. Wallace + remarked: "If Dr. Romanes will carefully work out numerically (as I + have attempted to do) a few cases showing the preservative and + accumulative agency of pure physiological selection within an + otherwise undifferentiated species, he will do more for his theory + than volumes of general disquisition or any number of assertions + that it _does_ possess this power." Several months before this was + written I had already in my hands Mr. Moulton's letter, with its + accompanying calculations. + +After some introductory remarks on Mr. Wallace's "adoption of the theory +of physiological selection pure and simple," and "the pure caricature of +it which he puts forward as" mine, the letter proceeds thus:-- + + The reason why it is so easy to attack your theory is that it is so + easy to confuse the survival of an _individual_ with the survival + of a _peculiarity_ of _type_. No one has ever said that an + _individual_ is _assisted_ by the possession of selective + fertility: that is a matter which cannot affect his chance of + _life_. Nor has any one said that the possession of selective + fertility in an _individual_ will _of itself_ increase the chance + of his having _progeny_ that will survive, and in turn become the + progenitors of others that will survive. Taken by itself, the fact + that an _individual_ is capable of fertility with some only of the + opposite sex lessens the chance of his having progeny. Whether or + not he is more or less favourably situated than his _confreres_ for + the battle of life must be decided by the _total sum_ of his + peculiarities; and the question whether or not this selective + fertility will be a hindrance must be decided by considerations + depending on the other peculiarities associated with it. + + But when we come to consider the survival or permanence of a _type_ + or _peculiarity_, the case is quite different. It then becomes not + only a favourable circumstance, but, in my opinion, almost a + necessary condition, that the peculiarity should be associated with + selective fertility[64]. + + [64] As, for example, in the case of sexuality in general. It is not + to the advantage of such individual male Arthropoda as perish after + the performance of the sexual act that they should perform it; but + its performance is necessary for the perpetuation of their + species.--G. J. R. + + Take the case of the Jews. I don't think that intermarriage with + other nations would lessen their fertility, or diminish the number + of their progeny; nor is there any reason to think that this + progeny would be unequal to the struggle for existence. But no one + doubts that the abandonment of their voluntary isolation (which + operates so far as this is concerned as a selective fertility), + would lead to the disappearance of the familiar Jewish type. All + the world would get some of it; but as a whole it would be + "swamped." + + Now although no doubt Wallace would admit all this, he fails to + give it the weight it ought to have. In discussing the question of + its operation he considers too exclusively the case of the + individual. + + Of course, a type can only be perpetuated through the medium of + individuals, and all that his argument amounts to is, that + selective fertility would be so fatal to individuals that _no_ type + which presents it could be formed or perpetuated--a conclusion + which is not only absurd in itself, but contradicted by his own + subsequent adoption of your theory. Besides, apart from + calculations (with which I will deal when I write next), such + reasoning brings its own refutation. Selective fertility is not in + the same category as some of the other influences to which an + important share has been ascribed in the formation of the existing + types. _It exists as a recognized phenomenon._ Hence all these + numerical proofs that it would lead to extinction, because it is so + disadvantageous to the possessor, prove too much. They would show + that the degree of selective fertility which so frequently + characterizes species is a most onerous gift; and that, were it not + present, there would be a vastly increased chance of fertility, + which would render the races fitter and lead to their increased + survival. Why then has it not been got rid of? + + The two answers which no doubt would be given seem to me to support + rather than to make against your theory. In the first place, + Wallace might say that this infertility is an advantage because it + keeps pure a type which is specially fitted to its surroundings, as + shown by its continued existence. But if this be so, and it is + necessary to protect the _developed_ type, how much more necessary + to protect the _incipient_ type! In the second place, he might say + that this selective fertility is not so disadvantageous when the + species has been formed, because the individual can choose his mate + from his like; whereas, when it is beginning to be formed, he must + mate blindly, or without what you call "psychological selection." + But this seems to me to be wholly inapplicable to at least half the + animal, and to all the vegetable kingdom. Moreover, with regard to + the other half of the animal kingdom, it merely raises the + question,--How soon will such an incipient type recognize itself? + Seeing it is probable that many families [broods] will belong to + the same [incipient] type, I should not be surprised if it were + found that this sexual recognition and preference sets in very + early. + + But this leads me to the question of your letter. I understand you + to want me to examine and criticize the attempted numerical + arguments against or for your theory. Now it seems to me that it + will be best to take, in the first instance, the vegetable kingdom, + and with regard to it I cannot see how there can be any numerical + argument against the theory. For we often have species side by side + with others nearly allied, but much more numerous. The condition of + these is precisely analogous to that of your incipient species. + They are exposed to fertilization from, say, ten times as numerous + individuals of the allied species. They reject this in favour of + that from the relatively few individuals of their own. Yet the two + species are in competition. I could go through the numerical + arguments of your assailant word for word, applying them to such a + case as this, and they would triumphantly show that the specific + fertility of the rarer kind would lead to its certain extinction. + Yet we know that this is not so. + + Indeed, the too triumphant character of the logic used against you + seems to me to be capable of being turned to your use. If + cross-infertility is so intensely disadvantageous to the + individuals presenting it, it cannot have been _that_ which made + these individuals and their progeny survive. It is therefore a + burden which they have carried. But we find that it is more or less + present in all the closely allied types that occur on common areas: + therefore it must be a necessary feature in the formation of such + types; for it cannot be an accident that it is present in so many. + In other words, it must be the price which the individual and his + progeny pay for their formation into a type. And this is your + theory pure and simple. + + The more I consider the matter, the more I feel that it is + impossible to decide as to the sufficiency of selective fertility + to explain the formation of species, if we consider merely the + effect it would have on the number of individuals, as contrasted + with what it would be if no such peculiarity had developed itself. + Indeed, I may say that on pondering over the matter I have come to + the conclusion, that mere fertility is probably a comparatively + unimportant factor in the preservation of the species, after a + certain sufficient degree of fertility is attained. I do not wish + to be misunderstood. To a certain point fertility is not only + advantageous but necessary, in order to secure survival of the + type; but I feel that little reliance can be placed on calculations + based on the numerical co-efficient of fertility (i. e. the ratio + of the number of offspring to the number of parents) in determining + the relative chance of type-survival. + + Take, for instance, the oak tree. It produces thousands of acorns, + almost the whole of which die without producing any progeny. Have + we any reason to believe that if the number of acorns borne by oak + trees were diminished, even so much as to one-tenth, the race of + oaks would perish? It may of course be said that, if all other + things are equal, the probabilities of survival must be increased + by increased fertility of this kind; but I feel convinced that when + numerical fertility has attained to a high point in circumstances + in which actual increase of the race cannot take place to any + substantial extent, the numerical value of this fertility sinks + down into a factor of the second or third order of importance--that + is to say, into the position of a factor whose effects are only to + be considered when we have duly allowed for the full effects of all + the main factors. Until we have done that, we gain little or + nothing in the way of accuracy of conclusion by taking into + consideration the minor factors. It may be very well to neglect the + effect of the attraction of Jupiter in our early researches on the + motion of the Moon; and our doing so will not prevent the results + being approximate and having considerable value, because we are + retaining the two main factors that establish the motion, viz. the + effects of the Earth and the Sun. But if we exclude the effect of + one of these main factors, our results would be worthless; and it + would not be rendered substantially less so by the fact that we had + taken Jupiter into account in arriving at them. + + You must not imagine, however, that I think it wholly profitless to + see whether there would be any substantial effect on numerical + fertility were _selective_ fertility to manifest itself. But if we + want to derive any assistance from calculation, it must be by + applying it with a good deal more precision and definiteness than + anything that Wallace shows. And, in the first place, it is useless + to confuse the vegetable and animal kingdoms. In the former you + have union unaffected by choice; in the latter, so far at all + events as the higher animals are concerned, you have "psychological + selection." In order to give you a specimen of what can safely be + done by calculation if you take a problem of sufficient + definiteness, I have chosen the case of a flowering plant in which + a certain proportion of the race have developed the peculiarity of + being sterile with the remainder, while retaining the normal + fertility of the race in unions among themselves. In order to give + the greatest advantage to your critics, I have assumed that such + flowers as possess the peculiarity are not self-fertilizable; for + it is clear that if we suppose that they are self-fertilizable, the + fertility need be very slightly affected. + + As I have excluded self-fertilization, it is necessary, if we are + to get any trustworthy results, that one should consider the mode + in which fertilization will be produced. I have taken the case of + fertilization by insects, and have assumed that each flower is + visited a certain number of times by insects during the period when + fertilization is possible; and, further, that the insects which + visit it have on the average visited a certain number of flowers of + the same species before they came there. Of course nothing but + observation can fix these latter numbers; but I should not be + surprised at finding that they are of considerable magnitude[65]. In + order to make the results a little more intelligible, I have + grouped them under the numbers which represent the average number + of flowers that an insect visits in a journey. This is a little + more than twice as great as the number which represents the number + of flowers he has on the average visited before coming to the + individual whose fertility we are considering. + + [65] In this anticipation Mr. Moulton is right. The well-known + botanist, Mr. Bennett, read a most interesting paper on the subject + before the British Association in 1881. His results have since been + corroborated by other observers. In particular, Mr. R. M. Christy + has recorded the movements of 76 insects while visiting at least + 2,400 flowers. (_Entomologist_, July 1883, and _Zool. Journal Lin. + Soc._, August 1883.) The following is an analysis of his results. In + the case of butterflies, in twelve observations on nearly as many + species, there are recorded altogether 99 visits to fifteen species + of flowers; and of these 99 visits 94 were constant to the same + species, leaving only 5 visits to any other, or second species. In + the case of the hive-bee, there were 8 individuals observed: these + visited altogether 258 flowers, and all the visits paid by the same + individual were paid to the same species in each of the eight cases. + Lastly, as regards bumble-bees, there were altogether observed 55 + individuals belonging to four species. These paid altogether 1751 + visits to 94 species of flowers. Of these 1751 visits, 1605 were + paid to one species, 131 to two species, 16 to three, 6 to four, and + 1 to five. Adding all these results together, we find that 75 + insects (butterflies and bees) visited 117 species of flowers: of + these visits, 1957 were constant to one species of flower; 136 were + paid also to a second species, 16 also to a third, 6 also to a + fourth, and 1 also to a fifth. Or, otherwise stated, while 1957 were + absolutely constant, from such absolute constancy there were only + 159 deviations. Moreover, if we eliminate three individual humble + bees, which paid nearly an equal number of visits to two species + (and, therefore, would have ministered to the work of physiological + selection almost as well as the others), the 159 deviations become + reduced to 72, or about four per cent. of the whole.--G. J. R. + + I send you the formula and the calculation on which it is based in + an Appendix; but as I know you have a holy horror of algebraical + formulae, I give you here a few numerical results. + + The cases I have worked out are those in which the number of + insects visiting each flower is 5, or 10, or 15; and I have also + taken 5, 10, and 15, to represent the number of flowers which an + insect visits each journey. This makes nine cases in all; and I + have applied these to two instances--viz. one in which one-fifth of + the whole race have developed cross-infertility, and the other in + which one-tenth only have done so. Taking first the instance where + one-fifth have developed the peculiarity, I find that if on the + average five insects visit a flower, and each insect on the average + visits five flowers on a journey, the fertility is diminished by + about one-tenth. If, however, the average number of flowers the + insect visits is ten, the reduction of fertility is less than one + per cent. And it becomes inappreciable if the average number is + fifteen. If on the average ten insects visit each flower, then, if + each insect visits on the average five flowers on a journey, the + reduction of fertility is a little over one per cent.; but if it + visits ten or fifteen the reduction is inappreciable. If fifteen + insects visit the flower on an average, then, if these insects on + the average visit five or more flowers on a journey, the reduction + of fertility is inappreciable. + + By the term inappreciable I mean that it is not substantially + greater than one-tenth of one per cent.--i.e. not more than + one-thousandth. + + Of course, if the proportion of individuals acquiring the + peculiarity is less, the effect on the fertility under the above + hypothesis will be greater; and it will not be counteracted so + fully unless the number of insect visits is larger, or unless the + insects visit more flowers on a journey. Thus if only one-tenth of + the race have developed the peculiarity, then, if each flower is + visited on the average by five insects who visit five flowers on + each trip, the fertility will be reduced about one-third. If, + however, the insects visit on the average ten flowers per trip, it + will be only diminished about one-tenth; and if they visit fifteen + on each trip, it will be only diminished about one-fortieth. If in + the same case we suppose that each flower receives ten insect + visits, then, if the insects visit on an average five flowers per + trip, the fertility will be diminished about one-eighth. If they + visit ten on a trip, it will be diminished about one-hundredth, and + the diminution is inappreciable if they visit fifteen on a trip. + Similarly, if a flower receives fifteen insect visits, the + diminution is about one-twenty-fifth, if insects visit on the + average five flowers on a trip; and is inappreciable if they visit + ten or fifteen. + + These figures will show you that it is exceedingly possible that a + peculiarity like this, the effect of which at first sight would + seem to be so prejudicial to fertility, may in fact have little or + no influence upon it; and if you set against this the overwhelming + importance of such a peculiarity in segregating the type so as to + give it a chance of becoming a fixed species, you will, I think, + feel that your hypothesis has nothing to fear from a numerical + examination. + + I have not examined the case of fertilization by other means; nor + have I examined the case of fertilization in animals, where + psychological selection can come in. To obtain any useful results, + one would have to consider very carefully the circumstances of each + case; and at present, at all events, I do not think it would be + useful to do so. Nor have I attempted to show the converse of the + problem--viz. the effect of swamping where cross-fertilization is + possible. I shall be very glad to examine any one of these cases if + you want me to do so; but I should prefer to leave it until I hear + from you again. + + If you contrast the results that I have given above with those + given on pages 181 to 183 of Wallace's book, you will see the + enormous difference. His calculations can only apply to the animal + kingdom in those cases in which there is only a union between one + individual of each sex; and before you can deal with the question + of such animals, you will have to take into consideration many + elements besides that of mere fertility, if you wish to get any + tolerably accurate result[66]. + + [66] Here follows the Appendix presenting the calculations on which + the above results are founded; but it seems unnecessary to reproduce + it on the present occasion.--G. J. R. + +The above analysis leaves nothing to be added by me. But, in conclusion, +I may once more repeat that the particular point with which it is +concerned is a point of very subordinate importance. For even if Mr. +Wallace's computation of chances had been found by Mr. Moulton to have +been an adequate computation--and, therefore, even if it had been thus +proved that physiological homogamy must always be associated with some +other form of homogamy in order to produce specific divergence--still +the importance of selective fertility as a factor of organic evolution +would not have been at all diminished. For such a result would merely +have shown that, not only "in many cases" (as I originally said), but +actually in all cases, the selective fertility which I hold to have been +so generally concerned in the differentiation of species has required +for this purpose the co-operation of some among the numerous other forms +of homogamy. But inasmuch as, by hypothesis, no one of these other or +co-operating factors would of itself have been capable of effecting +specific divergence in any of the cases where its association with +selective fertility is concerned, the mathematical proof that such an +association is _always_--and not merely _often_--necessary, would not +have materially affected the theory of the origin of species by means of +physiological selection. We have now seen, however, that a competent +mathematical treatment proves the exact opposite; and, therefore, that +Mr. Wallace's criticism fails even as regards the very subordinate point +in question. + + + + +APPENDIX C. + +SOME EXTRACTS FROM THE AUTHOR'S NOTE-BOOKS. + + +_Bearing of Weismannism on Physiological Selection._--If in view of +other considerations I could fully accept Professor Weismann's theory of +heredity, it would appear to me in no small measure to strengthen my own +theory of physiological selection. For Weismann's theory supposes that +all changes of specific type must have their origin in variations of a +continuous germ-plasm. But _the more the origin of species is referred +directly to variations arising in the sexual elements, the greater is +the play given to the principles of physiological selection_[67]; while, +on the other hand, the less standing-ground is furnished to the theory +that cross-infertility between allied species is due to "external +conditions of life," "prolonged exposure to uniform change of +conditions," "structural modifications re-acting on the sexual +functions"; or, in short, that "somatogenetic" changes of any kind can +of themselves induce the "blastogenetic" change of cross-infertility +between progeny of the same parental stock. + + [67] _Doctrine of Descent and Darwinism_, Eng. trans. p. 139. + + +_Cross-infertility and Diversity of Life._--Observe that one great +consequence of duly recognizing the importance of intercrossing is +indefinitely to raise our estimate of the part played by the principle +of cross-infertility in diversifying organic nature. For whenever in any +line of descent the bar of sterility arises, there the condition is +given for a new crop of departures (species of a genus); and when genera +are formed by the occurrence of this bar, there natural selection and +all other equilibrating causes are supplied with new material for +carrying on adaptational changes in new directions. Thus, owing to +cross-infertility, all these causes are enabled to work out numberless +adaptations in many directions (i. e. lines of descent) simultaneously. + + +_Cross-infertility and Stability._--The importance of sterility as a +diagnostic feature is obvious if we consider that more than any other +feature it serves to give _stability_ to the type; and unless a type is +stable or constant, it cannot be ranked as a species. That Darwin +himself attributes the highest importance to this feature as diagnostic, +see _Forms of Flowers_, pp. 58, 64. + + +_Cross-infertility and Specific Differentiation._--In their elaborate +work on the many species of the genus Hieracium, Naegeli and Peter are +led to the general conclusion that the best defined species are always +those which display absolute sterility _inter se_; while the species +which present most difficulty to the systematist are always those which +most easily hybridize. Moreover, they find, as another general rule +applicable to the whole genus, that there is a constant correlation +between inability to hybridize and absence of intermediate varieties, +and, conversely, between ability to hybridize and the presence of such +varieties. + + +_Cross-infertility in Domesticated Cattle._--Mr. J. W. Crompton, who has +had a large experience as a professional cattle-breeder, writes to me +(March 2, 1887)-- + + "That form of barrenness, very common in some districts, which + makes heifers become what are called 'bullers'--that is, + irregularly in 'season,' wild, and failing to conceive--is + certainly produced by excess of iron in their drinking-water, and I + suspect also by a deficiency of potash in the soil." + +He also informs me that pure white beasts of either sex are so well +known by experienced breeders to be comparatively infertile together, +that they are never used for breeding purposes, so that "in some parts +of the country, where a tendency to sterility had become so confirmed in +the white race that they utterly died out," only the coloured breeds are +now to be found. He goes on to say that if "a lot of white heifers were +put to a lot of white bulls, I think you would probably get a fertile +breed of pure white cattle.... I think, in short, that domestication has +produced just what your theory suggests, a new variety inclined to prove +sterile with its parent stock." + +Commenting on the origin of domesticated cattle, Professor Oscar Schmidt +remarks (_Doctrine of Descent_, p. 139)-- + + "Ruetimeyer's minute researches on domestic cattle have shown that, + in Europe at least, three well-defined species of the diluvial + period have contributed to their formation--_Bos primigenius_, + _longifrons_, and _frontosus_. These species once lived + geographically separate, but contemporaneously; and they and their + specific peculiarities have perished, to rise again in our domestic + races. These races breed together with unqualified fertility. In + the form of skull and horns they recall one or other of the extinct + species; but collectively they constitute a new main species. That + from their various breeds, the three or any one of the aboriginal + species would ever emerge in a state of pristine purity, would be + an utterly ludicrous assertion." + +Now, seeing that these "aboriginal species," although living +"contemporaneously," were "geographically separate," we can well +understand that their divergence of type from a common ancestor did not +require, as a condition to their divergence, that any cross-sterility +should have arisen between them. The geographical isolation was enough +to secure immunity from mutual intercrossing, and therefore, as our +present theory would have expected as probable, morphological divergence +occurred without any corresponding physiological divergence, as must +almost certainly have been the case if such polytypic evolution had +occurred on a common area. Indeed, one of the two lines of experimental +verification of our theory consists in selecting cases where nearly +allied species are separated by geographical barriers, and proving that, +in such cases, there is no cross-sterility. + + +_Fertility of Domesticated Varieties._--Some writers have sought to +explain the contrast between domesticated varieties and natural species +in respect of fertility when crossed, by the consideration that it is +only those natural species which have proved themselves so far flexible +as to continue fertile under changed conditions of life that can have +ever allowed themselves to become domesticated. But although this +condition may well serve to explain the unimpaired fertility under +domestication of such species as for this very reason have ever become +domesticated, I fail to see how it explains the further and altogether +different fact, that this fertility continues unimpaired between all the +newly differentiated morphological types which have been derived from +the original specific type. It is one thing that this type should +continue fertile after domestication: it is quite another thing that +fertility should continue as between all its modified descendants, even +although the amount of modification may extend much further than that +which usually obtains between different natural species. + + +_Testing for Cross-infertility_ among varieties growing on the same area +is a much more crucial line of verification than testing for unimpaired +fertility between allied species which occupy different areas, because +while in the former case we are dealing with "incipient species" with a +view to ascertaining whether the divergence which they have already +undergone is accompanied by physiological isolation, in the latter case +we can never be sure that two allied species, which are now widely +disconnected geographically, have always been so disconnected. They may +both have originated on the same area; or one may have diverged from the +other before it migrated from that area; or even if, when it migrated, +it was unchanged, and if in its new home it afterwards split into two +species by physiological selection, the newer species would probably +prove infertile, not only with its parent type, but also with its +grand-parent in any other part of the world. + + +_Seebohm on Isolation._--Seebohm is so strongly influenced by the +difficulty from "the swamping effects of free intercrossing," that he is +driven by it to adopt Asa Gray's hypothesis of variations as +teleological. Indeed, he goes as far as Wagner, for he maintains that in +no case can there be divergence or multiplication of species without +isolation. He makes the important statement that "the more the +geographical distribution of birds is studied, the more doubtful it +seems to be that any species of bird has ever been differentiated +without the aid of geographical isolation" (_Charadriidae_, p. 17). If +this is true, it makes in favour of physiological selection by showing +the paramount importance of the swamping effects of intercrossing, and +consequent importance of isolation. But it makes against physiological +selection by showing that the geographical form of isolation is +sufficient to explain all the cases of specific differentiation in +birds. But I must remember that the latter point rests largely on +negative inference, and that birds, owing to their highly locomotive +habits, are the class of animals where physiological selection is likely +to be most handicapped. + + +_Herbert on Hybridization._--Herbert tells us that when he first +astonished the Horticultural Society by laying before them the results +of his experiments on hybridization, his brother botanists took serious +alarm. For it appeared to them that this "intermixture of species would +confuse the labours of botanists, and force them to work their way +through a wilderness of uncertainty." Therefore he was bluntly told by +several of these gentlemen, "I do not thank you for your mules." Now, +although naturalists have travelled far and learnt much since those +days, it appears to me that a modern evolutionist might still turn to +the horticulturist with the same words. For assuredly he has no reason +to thank the horticulturist for his mules, until he has found a +satisfactory answer to the question why it is that natural species +differ so profoundly as regards their capacity for hybridizing. + + +_Advance on Herbert's Position._--- If it be said that all my work +amounts to showing what Herbert said long ago--viz. that the only true +or natural distinction between organic types is the sexual +distinction--I answer that my work does much more than this. For it +shows that the principle of sterility is the main condition to the +differentiation, not merely of species and genera, but also to the +evolution of adaptations everywhere, in higher as well as in lower +taxonomic divisions. Moreover, even though naturalists were everywhere +to consent to abandon specific designations, and, as Herbert advises, to +"entrench themselves behind genera," there would still remain the facts +of what are now called specific differences (of the secondary or +morphological kind), and by whatever name these are called, they alike +demand explanation at the hands of the evolutionist. + + +_Fritz Mueller on Cross-infertility._--Fritz Mueller writes, "Every plant +requires, for the production of the strongest possible and most prolific +progeny, a certain amount of difference between male and female elements +which unite. Fertility is diminished as well when this degree is too low +(in relatives too closely allied) as when it is too high (in those too +little related)." Then he adds, as a general rule, "Species which are +wholly sterile with pollen of the same stock, and even with pollen of +nearly allied stocks, will generally be fertilized very readily by the +pollen of another species. The self-sterile species of the genus +Abutilon, which are, on the other hand, so much inclined to +hybridization, afford a good example of this theory, which appears to be +confirmed also by Lobelia, Passiflora, and Oncidium" (_American +Naturalist_, vol. viii, pp. 223-4, 1874). + + +_Different groups of plants exhibit remarkable differences in the +capability of their constituent species to hybridize._--In so far as +these differences have reference only to first crosses, they have no +bearing either for or against my theory. Only in so far as the +differences extend to the production of fertile hybrids does any +question arise for me. First of all, therefore, I must ascertain whether +(or how far) there is any correlation between groups whose species +manifest aptitude to form first crosses, and groups where first crosses +manifest aptitude to produce fertile hybrids. Next, whatever the result +of this inquiry should be, if I find that certain natural groups of +plants exhibit comparatively well-marked tendencies to form fertile +hybrids, the question will arise, Are these tendencies correlated with +_paucity_ of species? If they are, the fact would make strongly in +favour of physiological selection. For the fact would mean that in these +natural groups, owing to "the nature of the organisms" included under +them, less opportunity is given to physiological selection in its work +of differentiating specific types than is given by other natural groups +where the nature of the organism renders them more prone to mutual +sterility. But in prosecuting this branch of verification, I must +remember to allow for possibilities of differential degrees of +geographical isolation in the different groups compared. + +On this subject Focke writes me as follows:--"In a natural group +(family, order, genus) showing considerable variability in the structure +of the flower, we may expect to find [or do find] a greater number of +mules than in a group whose species are only distinguished by +differences in the shape of the leaves, or in growth, &c. I do not +know, however, which in this connexion of things is the cause and which +the effect. A useful ancestral structure of the flower may be conserved +by an otherwise varying progeny, on condition that the progress of +diversity be not disturbed by frequent intercrossings. [Therefore, if +this condition be satisfied, the structure of the flower in different +members of the group will continue constant: here the cause of +_constancy_ in the flower (however much variability there may be in the +leaves, &c.) is its original _inability_ to hybridize.] On the other +hand, in species or groups ready to hybridize [or capable of +hybridizing], the fixation of a new specific type will require some +change in the structure of the flower, and a change considerable enough +to alter the conditions of fertilization. [Here the reason of the +_in_constancy of the flower in different members of the group is the +original _aptitude_ of their ancestral forms to hybridize.] Perhaps +there is something in this suggestion, but certainly there are other +efficient physiological relations, which are at present unknown. Your +theory of physiological selection may serve to explain many difficult +facts." + + +_The Importance of Prepotency._--A. Kerner shows by means of his own +observations on sundry species of plants which hybridize in the wild +state, that they do so very much more frequently if both, or even if +only one of the parent forms be rare in the neighbourhood. This fact can +only be explained by supposing that, even in species most prone to +hybridizing under Nature, there is some degree of prepotency of pollen +of the same species over that of the other species; so that where both +species are common, it is correspondingly rare that the foreign pollen +gets a chance. But if there were no prepotency, the two species would +blend; and this Kerner supposes must actually take place wherever two +previously separated species, thus physiologically circumstanced, happen +to be brought together. (Kerner's paper is published in _Oester. Bot. +Zeitschrift_, XXI, 1871, where he alludes to sundry other papers of his +own advocating similar views.) + +The relation of these observations to Jordan's _especes affines_ is +obvious. We have only to suppose that some such slight and constant +difference characterizes the sexual elements of these allied varieties +as demonstrably characterizes their morphology, and we can understand +how pollen-prepotency would keep the forms distinct--such forms, +therefore, being so many records of such prepotency. + +Both from Kerner's work, and still more from that of Jordan and Naegeli, +I conclude that (at all events in plants) prepotency is the way in which +physiological selection chiefly acts. That is to say, _sudden_ and +_extreme_ variations in the way of sexual incompatibility are probably +rare, as compared with some degree of prepotency. According as this +degree is small or great so will be the amount of the corresponding +separation. This view would show that in plants the principle of +physiological selection is one of immensely widespread influence, +causing (on the same areas) more or less permanent varieties much below +specific rank. And when we remember on how delicate a balance of +physiological conditions complete correspondency of pollen to ovules +depends, we may be prepared to expect that the phenomenon of prepotency +is not of uncommon occurrence. + + +_Self-fertilization and Variability._--It occurred to Count Berg Sagnitz +that, if physiological selection is a true principle in nature, +vegetable species in which self-fertilization obtains ought to be more +rich in constant varieties than are species in which cross-fertilization +rules. For, although even in the latter case physiological isolation may +occasionally arise, it cannot be of such habitual or constant occurrence +as it must be in the former case. Acting on this idea, Count Berg +Sagnitz applied himself to ascertain whether there is any general +correlation between the habit of self-fertilization and the fact of +high variability; and he says that in all the cases which he has +hitherto investigated, the correlation in question is unmistakable. + + +_Additional Hypothesis concerning Physiological Selection._--In +reciprocal crosses _A_ x _B_ is often more fertile than _B_ x _A_. If +hybrid _AB_ is more fertile with _A_, and hybrid _BA_ with _B_, than +vice versa, there would be given a good analogy on which to found the +following hypothesis. + +Let _A_ and _B_ be two intergenerating groups in which segregate +fecundity is first beginning. Of the hybrids, _AB_ will be more fertile +with _A_, and _BA_ with _B_, than vice versa. The interbreeding of _AB_ +with _A_ will eventually modify sexual characters of _A_ by assimilating +it to those of _AB_, while the interbreeding of _BA_ with _B_ will +similarly modify sexual characters of _B_ by assimilating it to those of +_BA_. Consequently, _A_ will become more and more infertile with _B_, +while _B_ becomes more and more infertile with _A_. Fewer and fewer +hybrids will thus be produced till mutual sterility is complete. + +To sustain this hypothesis it would be needful to prove experimentally, +(1) that hybrid forms _AB_ are more fertile with _A_ than with _B_, +while hybrid forms _BA_ are more fertile with _B_ than with _A_ [or, it +may be possible that the opposite relations would be found to obtain, +viz. that _AB_ would be more fertile with _B_, and _BA_ with _A_]; (2) +that, if so, effect of intercrossing _AB_ with _A_ is to make progeny +more fertile with _A_ than with _B_, while effect of intercrossing _BA_ +with _B_ is to make progeny more fertile with _B_ than with _A_. + +Such experiments had best be tried with species where there is already +known to be a difference of fertility between reciprocal crosses (e.g. +Matthiola annua and M. glabra, see _Origin of Species_, p. 244). + + + + +INDEX + + +A. + +ALLEN, Mr. J. A., on variation under nature, 34. + +Amixia, 12-28, 110-115, 117-133. + +Apogamy, 5, 6, 10, 18, 28. + + +B. + +BELT, on physiological selection, 44. + +BERG SAGNITZ, Count, on self-fertilization and variability, 177. + +Breeding, separate and segregate, 5. + +Butterflies of polar regions and Alps, 133. + + +C. + +CATCHPOOL, Mr., on physiological selection, 44, 137. + +Cross-infertility, 46; + and varietal divergence, 82; + and diversity of life, 169; + and stability, 170; + and specific differentiation, 170; + in domesticated cattle, 170; + testing for, 172; + Fritz Mueller on, 174. + + +D. + +_Darwin_, Charles, on isolation, 2, 106; + on diversity under nature, 31; + on the fertility of varieties, 50; + on the origin of cross-infertility, 51; + on distribution, 68; + on prepotency, 89; + on geographical isolation, 101, 108; + on methodical selection, 102; + on modification in large areas, 103; + on the swamping effects of intercrossing, 105; + on independent variability, 109; + on domestic animals, 110. + +DELBOEUF, law of independent variability, 13. + +Differentiation under natural selection, 37. + +Diversity of life and cross-infertility, 169. + +Domesticated cattle and cross-infertility, 170, 172. + + +E. + +Evidences of physiological selection, 62. + +Evolution, monotypic and polytypic, 21, 75, 102, 107, 112, 129. + +Experimental research in physiological selection, 85. + + +F. + +Fertility of domesticated varieties, 172. + +FOCKE, Herr, on hybridization, 175. + + +G. + +GALTON, Mr. Francis, law of regression, 39. + +General conclusions, 144. + +Geographical distribution and physiological selection, 65. + +GIARD, M., on apogamy, 14. + +GRABHAM, Dr., on mollusca of Madeira, 135. + +GULICK, Rev. J., on natural Selection as a mode of isolation, 9; + on divergence, 11; + on segregate breeding, 19; + on geographical distribution, 27; + on the prevention of intercrossing, 127; + on Mr. Wallace's criticisms, 151. + + +H. + +HERBERT, on hybridization, 173; + advance on his position, 174. + +HERDMAN, Prof., on physiological isolation, 123. + +Historical sketch of opinions on isolation, 101. + +Homogamy, 5, 6; + forms of, 7, 19, 29. + +Hybridization, HERBERT on, 173; + in plants, 175. + +Hypothesis, additional, concerning physiological selection, 178. + + +I. + +Independent variability, 12-29. + +Isolation, defined, 2; + forms of, 3, 6; + geographical, 3; + discriminate and indiscriminate, 5; + physiological, 9, 41, 58; + its importance, 39; + sketch of opinions on, 101; + general conclusions, 144; + SEEBOHM on, 173. + + +J. + +JORDAN, M., on cross sterile varieties of plants, 86; + his researches summarized, 87. + + +K. + +KERNER, Prof. A., on prepotency, 176. + + +L. + +LANKESTER, Prof. Ray, on divergent evolution, 15. + +LE CONTE, Prof., on fossil snails of steinheim, 95; + on isolation, 129. + +LIVINGSTONE, Dr. David, Quoted, 123. + + +M. + +MELDOLA, Prof., on difficulty from intercrossing, 121. + +Misunderstandings of Physiological selection, 59. + +Monotypic evolution, see Evolution. + +MORGAN, Prof. Lloyd, on sterility, 56; + on isolation, 128. + +MOULTON, Mr. Fletcher, an examination of Mr. Wallace's calculations on +physiological selection, 157. + +MUeLLER, Fritz, on cross-infertility, 174. + + +N. + +NAeGELI, on isolation, 76; + on synoicy, 78, 82. + +Natural selection, a form of discriminate isolation, 9, 10, 23; + leads to monotypic evolution, 24-29; + difficulties of, 41, 51. + + +P. + +Panmixia, 12. + +Physiological selection, 9, 41; + summarized, 58; + misunderstandings of, 59; + evidences of, 81-119; + and Weismannism, 169; + additional hypothesis, 178. + +Polytypic evolution, see Evolution. + +Prepotency, 89; importance of, 176. + + +S. + +SCHMIDT, Prof. Oscar, on domesticated cattle, 171. + +SEEBOHM on isolation, 173. + +Segregation, 28. + +Selection, physiological, see Physiological selection. + +Self-fertilization and variability, 177. + +Snails of Sandwich Islands, 16, 130; + fossil of Steinheim, 95. + +Specific differentiation and cross-infertility, 170. + +Stability and cross-infertility, 170. + +Synoicy, 78. + + +T. + +Topographical distribution and physiological selection, 74; + of varieties, 81. + +Transformation, serial and divergent, 21, 121. + + +V. + +Variability and self-fertilization, 177. + +Variation in birds, 34. + +Varieties, topographical distribution of, 81. + + +W. + +WAGNER, Maritz, 3; + on geographical isolation, 76; + quoted, 103; + law of migration, 111. + +WALLACE, Mr. A. R., 3, 17; + quoted, 34, 47, 51, 57,130-136; + criticized by Gulick, 152. + +WEISMANN, Prof., on geographical isolation, 76, 114-118. + +Weismannism and physiological selection, 169. + + + + +TITLE LIST OF OPEN COURT PUBLICATIONS ARRANGED ALPHABETICALLY BY AUTHORS + + +ANESAKI, M. + +345. BUDDHIST AND CHRISTIAN GOSPELS, Being Gospel Parallels from Pali +Texts. Now first compared from the originals by Albert J. Edmunds. +Edited with parallels and notes from the Chinese Buddhist Triptaka by +_M. Anesaki._ $1.50 net. + + +BAYNE, JULIA TAFT. + +323. HADLEY BALLADS. _Julia Taft Bayne._ 75c net. + + +BERKELEY, GEORGE. + +307. A TREATISE CONCERNING THE PRINCIPLES OF HUMAN KNOWLEDGE. _George +Berkeley._ Cloth, 60c net. (3s. net.) + +308. THREE DIALOGUES BETWEEN HYLAS AND PHILONOUS. _George Berkeley._ +Cloth, 60c net. (3s. net.) + + +BINET, ALFRED. + +201. THE PSYCHIC LIFE OF MICRO-ORGANISMS. _Alfred Binet._ 75c. (3s. 6d.) + +270. 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It is opposed on the one hand +to the dogmatism of groundless a priori assumptions, and on the other +hand, to the scepticism of negation which finds expression in the +agnostic tendencies of to-day. + +Monism Means a Unitary World-Conception + +There may be different aspects and even contrasts, diverse views and +opposite standpoints, but there can never be contradiction in truth. +Monism is not a one-substance theory, be it materialistic or +spiritualistic or agnostic; it means simply and solely CONSISTENCY. 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(22 instances) and "i.e." (14 instances). + +Different hyphenation patterns were left as in the original text: + + prepotent (1 instance) pre-potent (1 instance) + presupposes (1) pre-supposes (1) + reacting(5) re-acting (1) + restatement (1) re-statement (2) + superinduced (2) super-induced (1) + + + + + + + + +End of the Project Gutenberg EBook of Darwin, and After Darwin (Vol 3 of 3), by +George John Romanes + +*** END OF THIS PROJECT GUTENBERG EBOOK DARWIN, AFTER DARWIN (VOL 3 OF 3) *** + +***** This file should be named 37777.txt or 37777.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/3/7/7/7/37777/ + +Produced by Marilynda Fraser-Cunliffe, LN Yaddanapudi and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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