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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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