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diff --git a/37777-h/37777-h.htm b/37777-h/37777-h.htm new file mode 100644 index 0000000..eaa83ad --- /dev/null +++ b/37777-h/37777-h.htm @@ -0,0 +1,8223 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> + <head> + <meta http-equiv="Content-Type" content="text/html;charset=iso-8859-1" /> + <meta http-equiv="Content-Style-Type" content="text/css" /> + <title> + The Project Gutenberg eBook of Darwin, And After Darwin III, by George John Romanes + </title> + <style type="text/css"> + +body { + margin-left: 10%; + margin-right: 10%; +} + + h1,h2,h3,h4 { + text-align: center; /* all headings centered */ + clear: both; +} + +p { + margin-top: .75em; + text-align: justify; + margin-bottom: .75em; +} + +hr { + width: 33%; + margin-top: 2em; + margin-bottom: 2em; + margin-left: auto; + margin-right: auto; + clear: both; +} + +hr.tb {width: 45%;} +hr.chap {width: 65%} +hr.full {width: 95%;} + +hr.r5 {width: 5%; margin-top: 1em; margin-bottom: 1em;} +hr.r65 {width: 65%; margin-top: 3em; margin-bottom: 3em;} + +ul.index { list-style-type: none; } +li.ifrst { margin-top: 1em; } +li.indx { margin-top: .5em; } +li.isub1 {text-indent: 1em;} + +table { + margin-left: auto; + margin-right: auto; +} + +span.ralign { position: absolute; right: 15%; top: auto;} + +.pagenum { /* uncomment the next line for invisible page numbers */ + /* visibility: hidden; */ + position: absolute; + left: 92%; + font-size: smaller; + text-align: right; +} /* page numbers */ + +.blockquot { + margin-left: 5%; + margin-right: 10%; +} + +.center {text-align: center;} + +.smcap {font-variant: small-caps;} + +/* Images */ +.figcenter { + margin: auto; + text-align: center; +} + +/* Footnotes */ +.footnote {margin-left: 10%; margin-right: 10%; font-size: 0.9em;} + +.footnote .label {position: absolute; right: 84%; text-align: right;} + +.fnanchor { + vertical-align: super; + font-size: .8em; + text-decoration: + none; +} + +/* Transcriber's notes */ +.transnote {background-color: #E6E6FA; + color: black; + font-size:smaller; + padding:0.5em; + margin-bottom:5em; + font-family:sans-serif, serif; } + </style> + </head> +<body> + + +<pre> + +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 + + + + + + +</pre> + + + + + +<h1>DARWIN, AND AFTER DARWIN</h1> + +<h2>III</h2> + +<h2>POST-DARWINIAN QUESTIONS<br /> +ISOLATION AND PHYSIOLOGICAL SELECTION</h2> + +<hr class="r65" /> + +<h3>BY THE SAME AUTHOR.</h3> +<hr class="r5" /> + +<p>DARWIN, AND AFTER DARWIN. An Exposition of the +Darwinian Theory and a Discussion of Post-Darwinian +Questions.</p> + +<div class="blockquot"><p>1. <span class="smcap">The Darwinian Theory.</span> With portrait of Darwin. +460 pages. 125 illustrations. Cloth, $2.00.</p> + +<p>2. <span class="smcap">Post-Darwinian Questions.</span> Edited by Prof. C. +Lloyd Morgan. With portrait of G. J. Romanes. +338 pages. Cloth, $1.50.</p> + +<p>3. <span class="smcap">Post-Darwinian Questions. Isolation and Physiological +Selection.</span> Edited by Prof. C. Lloyd +Morgan. With portrait of Mr. J. T. Gulick. 181 +pages. Cloth, $1.00.</p> + +<p>All three volumes together, $4.00 net.</p></div> + +<p>AN EXAMINATION OF WEISMANNISM. With portrait of +Weismann. 236 pages. Cloth, $1.00.</p> + +<p>THOUGHTS ON RELIGION. Edited by Charles Gore, M.A., +Canon of Westminster. Third Edition. 184 pages. Cloth, +gilt top, $1.25.</p> + +<hr class="r5" /> + +<p class="center">THE OPEN COURT PUBLISHING COMPANY,<br /> +<span class="smcap">324 Dearborn Street, Chicago.</span></p> + +<hr class="r65" /> + +<div class="figcenter" style="width: 332px;"> +<img src="images/illus-004.jpg" width="332" height="600" alt="John J. Gulick" title="John J. Gulick" /></div> + +<hr class="chap" /> + + + + +<h1>DARWIN, AND AFTER DARWIN</h1> + +<h3><i>AN EXPOSITION OF THE DARWINIAN THEORY<br /> +AND A DISCUSSION OF<br /> +POST-DARWINIAN QUESTIONS</i></h3> + +<h3><span style="font-size:small;">BY THE LATE</span><br /> +GEORGE JOHN ROMANES, M.A., LL.D., F.R.S.<br /> +<span style="font-size:small;"><i>Honorary Fellow of Gonville and Caius College, Cambridge</i></span></h3> + + +<h2>III<br /> +POST-DARWINIAN QUESTIONS<br /> +ISOLATION<br /> +AND PHYSIOLOGICAL SELECTION</h2> + + +<h4>Chicago<br /> +THE OPEN COURT PUBLISHING COMPANY<br /> +1906</h4> + +<hr class="r65" /> + +<p class="center">CHAPTER 1. COPYRIGHTED BY<br /> +THE OPEN COURT PUBLISHING CO.<br /> +1897.</p> + + + +<p class="center">The Lakeside Press<br /> +R. R. DONNELLEY & SONS CO., CHICAGO</p> + +<hr class="chap" /> + +<p class="pagenum"><a name="Page_v" id="Page_v">[Pg v]</a></p> + + + + +<h2>PREFACE</h2> + + +<p>Of the six chapters which constitute this concluding +volume of G. J. Romanes' <i>Darwin, and after +Darwin</i>, three, the first two and the last, were in +type at the time of his death. I have not considered +myself at liberty to make any alterations of moment +in these chapters. For the selection and arrangement +of all that is contained in the other three +chapters I am wholly responsible.</p> + +<p>Two long controversial Appendices have been +omitted. Those marked A and B remain in accordance +with the author's expressed injunctions. In +a third, marked C, a few passages from the author's +note-books or MSS. have been printed.</p> + +<p>The portrait of the Rev. J. Gulick, which forms the +frontispiece, was prepared for this volume before the +author's death. Mr. Gulick's chief contributions to +the theory of physiological selection are to be found +in the Linnean Society's <i>Journal</i> (<i>Zoology</i>, vols. xx<span class="pagenum"><a name="Page_vi" id="Page_vi">[Pg vi]</a></span> +and xxiii), and in four letters to <i>Nature</i> (vol. xli. +p. 536; vol. xlii. pp. 28 and 369; and vol. xliv. +p. 29).</p> + +<p>I have to thank Mr. Francis Galton, D.C.L., F.R.S. +and Mr. F. Howard Collins for valuable assistance +generously rendered for the sake of one whom all +who knew him held dear. For he was, if I may +echo the words of Huxley, "a friend endeared to +me, as to so many others, by his kindly nature, and +justly valued by all his colleagues for his powers +of investigation and his zeal for the advancement of +science."</p> + +<p><span class="smcap" style="margin-left:80%;">C. Lloyd Morgan.</span></p> + +<p style="margin-left:10%;"><span class="smcap">Bristol</span>, <i>May 1897</i>.</p> + +<hr class="chap" /> + +<p class="pagenum"><a name="Page_vii" id="Page_vii">[Pg vii]</a></p> + + + + +<h2>CONTENTS</h2> + + +<p class="center">CHAPTER I.</p> + +<p class="blockquot"><span class="smcap">Isolation</span> <span class="ralign"><a href="#Page_1">1</a></span></p> + +<p class="center">CHAPTER II.</p> + +<p class="blockquot"><span class="smcap">Isolation</span> (<i>continued</i>) <span class="ralign"><a href="#Page_28">28</a></span></p> + +<p class="center">CHAPTER III.</p> + +<p class="blockquot"><span class="smcap">Physiological Selection</span> <span class="ralign"><a href="#Page_41">41</a></span></p> + +<p class="center">CHAPTER IV.</p> + +<p class="blockquot"><span class="smcap">Evidences of Physiological Selection</span> <span class="ralign"><a href="#Page_62">62</a></span></p> + +<p class="center">CHAPTER V.</p> + +<p class="blockquot"><span class="smcap">Further Evidences of Physiological Selection</span> <span class="ralign"><a href="#Page_81">81</a></span></p> + +<p class="center">CHAPTER VI.</p> + +<p><span class="smcap blockquot">A Brief History of Isolation as a Factor in +Organic Evolution</span> <span class="ralign"><a href="#Page_101">101</a></span></p> + +<p class="blockquot"><span class="smcap">General Conclusions</span> <span class="ralign"><a href="#Page_144">144</a></span></p> + +<hr class="r5" /> + +<p class="blockquot"><span class="smcap">Appendix A. Mr. Gulick's Criticism of Mr. Wallace's +<span class="pagenum"><a name="Page_viii" id="Page_viii">[Pg viii]</a></span>Views on Physiological Selection</span> <span class="ralign"><a href="#Page_151">151</a></span></p> + +<p class="blockquot"><span class="smcap">Appendix B. An Examination by Mr. Fletcher +Moulton of Mr. Wallace's Calculation touching +the Possibility of Physiological Selection +ever acting alone</span> <span class="ralign"><a href="#Page_157">157</a></span></p> + +<p class="blockquot"><span class="smcap">Appendix C. Some Extracts from the Author's +Note-books</span> <span class="ralign"><a href="#Page_169">169</a></span></p> +<hr class="full" /> + + + + +<p class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></p> +<h2><i>ISOLATION</i></h2> +<hr class="chap" /> + + + + +<h2>CHAPTER I.<br /> +<span class="smcap">Isolation.</span></h2> + + +<p>This treatise will now draw to a close by considering +what, in my opinion, is one of the most important +principles that are concerned in the process of organic +evolution—namely, Isolation. I say in <i>my</i> opinion +such is the case, because, although the importance of +isolation is more or less recognized by every naturalist, +I know of only one other who has perceived all that +the principle involves. This naturalist is the Rev. J. +Gulick, and to his essays on the subject I attribute +a higher value than to any other work in the field of +Darwinian thought since the date of Darwin's death<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a>. +For it is now my matured conviction that a new point +of departure has here been taken in the philosophy of +Darwinism, and one which opens up new territories +for scientific exploration of an endlessly wide and +varied character. Indeed I believe, with Mr. Gulick,<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span> +that in the principle of Isolation we have a principle +so fundamental and so universal, that even the great +principle of Natural Selection lies less deep, and +pervades a region of smaller extent. Equalled only +in its importance by the two basal principles of +Heredity and Variation, this principle of Isolation +constitutes the third pillar of a tripod on which is +reared the whole superstructure of organic evolution.</p> + +<p>By isolation I mean simply the prevention of intercrossing +between a separated section of a species or +kind and the rest of that species or kind. Whether +such a separation be due to geographical barriers, to +migration, or to any other state of matters leading +to exclusive breeding within the separated group, +I shall indifferently employ the term isolation for the +purpose of designating what in all cases is the same +result—namely, a prevention of intercrossing between +A and B, where A is the separated portion and B the +rest of the species or kind.</p> + +<p>The importance of isolation as against dissimilar +forms has always been fully appreciated by breeders, +fanciers, horticulturists, &c., who are therefore most +careful to prevent their pedigree productions from +intercrossing with any other stock. Isolation is indeed, +as Darwin has observed, "the corner-stone of the +breeder's art." And similarly with plants and animals +in a state of nature: unless intercrossing with allied +(i.e. dissimilar) forms is prevented, the principle of +heredity is bound to work for uniformity, by blending +the dissimilar types in one: only when there is +exclusive breeding of similarly modified forms can the +principle of heredity work in the direction of change—i.e. +of evolution.</p> + +<p><span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span> +Now, the forms of isolation—or the conditions +which may lead to exclusive breeding—are manifold. +One of the most important, as well as the most obvious, +is geographical isolation; and no one questions that +this has been an important factor in the process of +evolution, although opinions still vary greatly as to +the degree of its importance in this respect. At one +end of the series we may place the opinion of Mr. +Wallace, who denies that any of what may be termed +the evolutionary effect of geographical isolation is due +to "influence exerted by isolation <i>per se</i>." This effect, +he says, is to be ascribed exclusively to the fact that +a geographically isolated portion of a species must +always encounter a change of environment, and therefore +a new set of conditions necessitating a new set of +adaptations at the hands of natural selection<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a>. At +the other end of the series we must place the opinion +of Moritz Wagner, who many years ago published +a masterly essay<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a>, the object of which was to prove +that, in the absence of geographical isolation (including +migration), natural selection would be powerless to +effect any change of specific type. For, he argued, +the initial variations on which the action of this +principle depends would otherwise be inevitably +swamped by free intercrossing. Wagner adduced +a large number of interesting facts in support of this +opinion; but although he thus succeeded in enforcing +the truth that geographical isolation is an +important aid to organic evolution, he failed to establish +his conclusion that it is an indispensable condition.<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span> +Nevertheless he may have been right—and, as I shall +presently show, I believe he was right—in his fundamental +premiss, that in the presence of free intercrossing +natural selection would be powerless to effect +divergent evolution. Where he went wrong was in +not perceiving that geographical isolation is not the +only form of isolation. Had it occurred to him that +there may be other forms quite as effectual for the +prevention of free intercrossing, his essay could hardly +have failed to mark an epoch in the history of Darwinism. +But, on account of this oversight, he really +weakened his main contention, namely, that in the +presence of free intercrossing natural selection must +be powerless to effect divergent evolution. This main +contention I am now about to re-argue. At present, +therefore, we have only to observe that Wagner did it +much more harm than good by neglecting to perceive +that free intercrossing may be prevented in many other +ways besides by migration, and by the intervention of +geographical barriers.</p> + +<p>In order that we may set out with clearer views +upon this matter, I will make one or two preliminary +remarks on the more general facts of isolation as these +are found to occur in nature.</p> + +<p>In the first place, it is obvious that isolation +admits of degrees: it may be either total or partial; +and, if partial, may occur in numberless grades of +efficiency. This is so manifest that I need not wait +to give illustrations. But now, in the second place, +there is another general fact appertaining to isolation +which is not so manifest, and a clear appreciation +of which is so essential to any adequate consideration +of the subject, that I believe the reason why<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span> +evolutionists have hitherto failed to perceive the full +importance of isolation, is because they have failed +to perceive the distinction which has now to be pointed +out. The distinction is, that isolation may be either +discriminate or indiscriminate. If it be discriminate, +the isolation has reference to the resemblance of the +separated individuals to one another; if it be indiscriminate, +it has no such reference. For example, if +a shepherd divides a flock of sheep without regard to +their characters, he is isolating one section from the +other indiscriminately; but if he places all the white +sheep in one field, and all the black sheep in another +field, he is isolating one section from the other +discriminately. Or, if geological subsidence divides +a species into two parts, the isolation will be indiscriminate; +but if the separation be due to one of +the sections developing, for example, a change of +instinct determining migration to another area, or +occupation of a different habitat on the same area, +then the isolation will be discriminate, so far as the +resemblance of instinct is concerned.</p> + +<p>With the exception of Mr. Gulick, I cannot find +that any other writer has hitherto stated this +supremely important distinction between isolation as +discriminate and indiscriminate. But he has fully +as well as independently stated it, and shown in +a masterly way its far-reaching consequences. Indiscriminate +isolation he calls Separate Breeding, while +discriminate isolation he calls Segregate Breeding. +For the sake, however, of securing more descriptive +terms, I will coin the words Apogamy and Homogamy. +Apogamy, of course, answers to indiscriminate isolation, +or separate breeding. Homogamy, on the other<span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span> +hand, answers to discriminate isolation, or segregate +breeding: only individuals belonging to the same +variety or kind are allowed to propagate. Isolation, +then, is a genus, of which Apogamy and Homogamy +are species<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a>.</p> + +<p>Now, in order to appreciate the unsurpassed importance +of isolation as one of the three basal +principles of organic evolution, let us begin by +considering the discriminate species of it, or Homogamy.</p> + +<p>To state the case in the most general terms, we +may say that if the other two basal principles are +given in heredity and variability, the whole theory +of organic evolution becomes neither more nor less +than a theory of homogamy—that is, a theory of +the causes which lead to discriminate isolation, or +the breeding of like with like to the exclusion of +unlike. For the more we believe in heredity and +variability as basal principles of organic evolution, +the stronger must become our persuasion that discriminate +breeding leads to divergence of type, while +indiscriminate breeding leads to uniformity. This, +in fact, is securely based on what we know from the +experience supplied by artificial selection, which consists<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span> +in the intentional mating of like with like to the +exclusion of unlike.</p> + +<p>The point, then, which in the first instance must be +firmly fastened in our minds is this:—so long as there +is free intercrossing, heredity cancels variability, and +makes in favour of fixity of type. Only when assisted +by some form of discriminate isolation, which +determines the exclusive breeding of like with like, +can heredity make in favour of change of type, or +lead to what we understand by organic evolution.</p> + +<p>Now the forms of discriminate isolation, or homogamy, +are very numerous. When, for example, any +section of a species adopts somewhat different habits +of life, or occupies a somewhat different station in +the economy of nature, homogamy arises within that +section. There are forms of homogamy on which +Darwin has laid great stress, as we shall presently +find. Again, when for these or any other reasons a +section of a species becomes in any small degree +modified as to form or colour, if the species happens +to be one where any psychological preference in +pairing can be exercised—as is very generally the +case among the higher animals—exclusive breeding +is apt to ensue as a result of such preference; for +there is abundant evidence to show that, both in birds +and mammals, sexual selection is usually opposed to +the intercrossing of dissimilar varieties. Once more, +in the case of plants, intercrossing of dissimilar +varieties may be prevented by any slight difference in +their seasons of flowering, of topographical stations, +or even, in the case of flowers which depend on +insects for their fertilization, by differences in the +instincts and preferences of their visitors.</p> + +<p><span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span> +But, without at present going into detail with +regard to these different forms of discriminate isolation, +there are still two others, both of which are of much +greater importance than any that I have hitherto +named. Indeed, these two forms are of such immeasurable +importance, that were it not for their +virtually ubiquitous operation, the process of organic +evolution could never have begun, nor, having begun, +continued.</p> + +<p>The first of these two forms is sexual incompatibility—either +partial or absolute—between different +taxonomic groups. If all hares and rabbits, for +example, were as fertile with one another as they +are within their own respective species, there can be +no doubt that sooner or later, and on common areas, +the two types would fuse into one. And similarly, +if the bar of sterility could be thrown down as +between all the species of a genus, or all the genera of +a family, <i>not otherwise prevented from intercrossing</i>, +in time all such species, or all such genera, would +become blended into a single type. As a matter +of fact, complete fertility, both of first crosses and +of their resulting hybrids, is rare, even as between +species of the same genus; while as between genera +of the same family complete fertility does not appear +ever to occur; and, of course, the same applies to +all the higher taxonomic divisions. On the other +hand, some degree of infertility is not unusual as +between different varieties of the same species; and, +wherever this is the case, it must clearly aid the further +differentiation of those varieties. It will be my +endeavour to show that in this latter connexion +sexual incompatibility must be held to have taken<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span> +an immensely important part in the differentiation +of varieties into species. But meanwhile we have +only to observe that <i>wherever</i> such incompatibility is +concerned, it is to be regarded as an isolating agency +of the very first importance. And as it is of a +character purely physiological, I have assigned to it +the name Physiological Isolation; while for the particular +case where this general principle is concerned +in the origination of specific types, I have reserved +the name Physiological Selection.</p> + +<p>The other most important form of discriminate +isolation to which I have alluded is Natural Selection. +To some evolutionists it has seemed paradoxical +thus to regard natural selection as a form of isolation; +but a little thought will suffice to show that +such is really the most accurate way of regarding it. +For, as Mr. Gulick says, "Natural selection is the +exclusive breeding of those better adapted to the environment: +... it is a process in which the fittest are +prevented from crossing with the less fitted, by the +exclusion of the less fitted." Therefore it is, strictly +and accurately, a mode of isolation, where the +isolation has reference to adaptation, and is secured +in the most effectual of possible ways—i.e. by the +destruction of all individuals whose intercrossing would +interfere with the isolation. Indeed, the very term +"natural <i>selection</i>" shows that the principle is tacitly +understood to be one of isolation, because this name +was assigned to the principle by Darwin for the +express purpose of marking the analogy that obtains +between it and the intentional isolation which is +practised by breeders, fanciers, and horticulturists. +The only difference between "natural selection" and<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span> +"artificial selection" consists in this—that under the +former process the excluded individuals must necessarily +perish, while under the latter they need not do +so. But clearly this difference is accidental: it is in +no way essential to the process considered as a process +of discriminate isolation. For, as far as homogamous +breeding is concerned, it can matter nothing whether +the exclusion of the dissimilar individuals is effected +by separation or by death.</p> + +<p>Natural selection, then, is thus unquestionably +a form of isolation of the discriminate kind; and +therefore, notwithstanding its unique importance in +certain respects, considered as a principle of organic +evolution it is less fundamental—and also less extensive—than +the principle of isolation in general. In +other words, it is but a part of a much larger whole. +It is but a particular form of a general principle, +which, as just shown, presents many other forms, not +only of the discriminate, but likewise of the indiscriminate +kind. Or, reverting to the terminology of +logic, it is a sub-species of the species Homogamy, +which in its turn is but a constituent part of the +genus Isolation.</p> + +<p>So much then for homogamy, or isolation of the +discriminate order. Passing on now to apogamy, or +isolation of the indiscriminate kind, we may well be +disposed, at first sight, to conclude that this kind of +isolation can count for nothing in the process of evolution. +For if the fundamental importance of isolation +in the production of organic forms be due to its +segregation of like with like, does it not follow +that any form of isolation which is indiscriminate +must fail to supply the very condition on which all<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span> +the forms of discriminate isolation depend for their +efficacy in the causing of organic evolution? Or, to +return to our concrete example, is it not self-evident +that the farmer who separated his stock into two +or more parts indiscriminately, would not effect any +more change in his stock than if he had left them +all to breed together?</p> + +<p>Well, although at first sight this seems self-evident, +it is in fact untrue. For, unless the individuals which +are indiscriminately isolated happen to be a very +large number, sooner or later their progeny will come +to differ from that of the parent type, or unisolated +portion of the previous stock. And, of course, as +soon as this change of type begins, the isolation +ceases to be indiscriminate: the previous apogamy +has been converted into homogamy, with the usual +result of causing a divergence of type. The reason +why progeny of an indiscriminately isolated section +of an originally uniform stock—e.g. of a species—will +eventually deviate from the original type is, to quote +Mr. Gulick, as follows:—"No two portions of a species +possess exactly the same average character, and, +therefore, the initial differences are for ever reacting +on the environment and on each other in such a way +as to ensure increasing divergence as long as the +individuals of the two groups are kept from intergenerating<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a>." +Or, as I stated this principle in my +essay on <i>Physiological Selection</i>, published but a short +time before Mr. Gulick's invaluable contributions to +these topics:—</p> + +<p><span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span></p> +<blockquote><p>As a matter of fact, we find that no one individual "is like +another all in all"; which is another way of saying that a +specific type may be regarded as the average mean of all its individual +variations, any considerable departure from this average +being, however, checked by intercrossing.... Consequently, if +from any cause a section of a species is prevented from intercrossing +with the rest of its species, we might expect that new +varieties should arise within that section, and that in time these +varieties should pass into new species. And this is just what +we do find<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a>.</p></blockquote> + +<p>The name which I gave to this cause of specific +change was Independent Variability, or variability in +the absence of overwhelming intercrossing. But it +now appears to me that this cause is really identical +with that which was previously enunciated by +Delbœuf. Again, in his important essay on <i>The +Influence of Isolation</i>, Weismann concludes, on the +basis of a large accumulation of facts, that the constancy +of any given specific type "does not arise +suddenly, but gradually, and is established by the +promiscuous intercrossing of all individuals." From +which, he says, it follows, that this constancy must +cease so soon as the condition which maintains it +ceases—i. e. so soon as intercrossing (Panmixia) +between all individuals ceases, or so soon as a portion +of a species is isolated from its parent stock. To +this principle he assigns the name of Amixia. But +Weismann's Amixia differs from my Independent +Variability in several important particulars; and +on this account I have designedly abstained from<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span> +adopting his term. Here it is enough to remark +that it answers to the generic term Isolation, without +reference to the <i>kind</i> of isolation as discriminate +or indiscriminate, homogamous or apogamous. On +the other hand, my Independent Variability is merely +a re-statement of the so-called "Law of Delbœuf," +which, in his own words, is as follows:—</p> + +<blockquote><p>One point, however, is definitely attained. It is that the +proposition, which further back we designated paradoxical, +is rigorously true, A constant cause of variation, however +insignificant it may be, changes the uniformity [of type] +little by little, and diversifies it <i>ad infinitum</i>. From the homogeneous, +left to itself, only the homogeneous can proceed; but +if there be a slight disturbance ["léger ferment"] in the +homogeneous, the homogeneity will be invaded at a single +point, differentiation will penetrate the whole, and, after +a time—it may be an infinite time—the differentiation will +have disintegrated it altogether.</p></blockquote> + +<p>In other words, the "Law," which Delbœuf has +formulated on mathematical grounds, and with express +reference to the question of segregate breeding, +proves that, no matter how infinitesimally small the +difference may be between the average qualities of +an isolated section of a species compared with the +average qualities of the rest of that species, if the +isolation continues sufficiently long, differentiation of +specific type is necessarily bound to ensue. But, to +make this mathematical law biologically complete, it +ought to be added that the time required for the +change of type to supervene (supposing apogamy to +be the only agent of change) will be governed by the +range of individual variability which the species in +question presents. A highly stable species (such as +the Goose) might require an immensely long time for<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span> +apogamy alone to produce any change of type in an +isolated portion of the species, while a highly variable +species (such as the Ruff) would rapidly change in +any portion that might be indiscriminately isolated. +It was in order to recognize this additional and very +important factor that I chose the name Independent +<i>Variability</i> whereby to designate the diversifying +influence of merely indiscriminate isolation, or apogamy. +Later on Mr. Gulick published his elaborate +papers upon the divergence of type under all kinds of +isolation; and retained my term Independent, but +changed Variability into Generation. I point this +out merely for the sake of remarking that his Independent +Generation is exactly the same principle +as my Independent Variability, and Delbœuf's Mathematical +Law.</p> + +<p>Now, while I fully agree with Mons. Giard where +he says, in the introductory lecture of his course on +<i>The Factors of Evolution</i><a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a>, that sufficient attention +has not been hitherto given by naturalists to this +important factor of organic evolution (apogamy), +I think I have shown that among those naturalists +who have considered it there is a sufficient amount of +agreement. <i>Per contra</i>, I have to note the opinion +of Mr. Wallace, who steadily maintains the impossibility +of any cause other than natural selection (i.e. +one of the forms of homogamy) having been concerned +in the evolution of species. But at present it is enough +to remark that even Professor Ray Lankester—whose +leanings of late years have been to the side of ultra-Darwinism, +and who is therefore disposed to agree<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span> +with Mr. Wallace wherever this is logically possible—even +Professor Ray Lankester observes:—</p> + +<blockquote><p>Mr. Wallace does not, in my judgement, give sufficient +grounds for rejecting the proposition which he indicates as +the main point of Mr. Gulick's valuable essay on <i>Divergent +Evolution through Cumulative Segregation</i>. Mr. Gulick's +idea is that ... no two portions of a species possess exactly the +same average character, and the initial differences will, if the +individuals of the two groups are kept from intercrossing, +assert themselves continuously by heredity in such a way as +to ensure an increasing divergence of the forms belonging to +the two groups, amounting to what is recognized as specific +distinction. Mr. Gulick's idea is simply the recognition of +a permanence or persistency in heredity, which, <i>caeteris +paribus</i>, gives a twist or direction to the variations of the +descendants of one individual as compared with the descendants +of another<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a>.</p></blockquote> + +<p>Now we have seen that "Mr. Gulick's idea," +although independently conceived by him, had been +several times propounded before; and it is partly +implicated in more than one passage of the <i>Origin +of Species</i>, where free intercrossing, or the <i>absence</i> of +isolation, is alluded to as maintaining the <i>constancy</i> +of a specific type<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a>. Moreover, it is still more fully +recognized in the last edition of the <i>Variation of +Animals and Plants</i>, where a paragraph is added for +the purpose of sanctioning the principle in the +imperfect form that it was stated by Weismann<a name="FNanchor_10_10" id="FNanchor_10_10"></a><a href="#Footnote_10_10" class="fnanchor">[10]</a>. +Nevertheless, to Mr. Gulick belongs the credit, not +only of having been the first to conceive (though the +last to publish) the "idea" in question, and of having +stated it with greater fullness than anybody else; but<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span> +still more of having verified its importance as a factor +of organic evolution.</p> + +<p>For, in point of fact, Mr. Gulick was led to his +recognition of the principle in question, not by any +deductive reasoning from general principles, but by +his own particular and detailed observations of the +land mollusca of the Sandwich Islands. Here there +are an immense number of varieties belonging to +several genera; but every variety is restricted, not +merely to the same island, but actually to the same +valley. Moreover, on tracing this fauna from valley +to valley, it is apparent that a slight variation in the +occupants of valley 2 as compared with those of the +adjacent valley 1, becomes more pronounced in the +next—valley 3, still more so in 4, &c., &c. Thus it +was possible, as Mr. Gulick says, roughly to estimate +the amount of divergence between the occupants of +any two given valleys by measuring the number of +miles between them.</p> + +<p>As already stated, I have myself examined his +wonderful collection of shells, together with a topographical +map of the district; and therefore I am in +a position to testify to the great value of Mr. Gulick's +work in this connexion, as in that of the utility +question previously considered. The variations, which +affect scores of species, and themselves eventually run +into fully specific distinctions, are all more or less +finely graduated as they pass from one isolated +region to the next; and they have reference to +changes of form and colour, which in no one case +presents any appearance of utility. Therefore—and +especially in view of the fact that, as far as he could +ascertain, the environment in the different valleys was<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span> +essentially the same—no one who examines this +collection can wonder that Mr. Gulick attributes the +results which he has observed to the influence of +apogamy alone, without any reference to utility or +natural selection.</p> + +<p>To this solid array of remarkable facts Mr. Wallace +has nothing further to oppose than his customary +appeal to the argument from ignorance, grounded on +the usual assumption that no principle other than +natural selection <i>can</i> be responsible for even the +minutest changes of form or colour. For my own +part, I must confess that I have never been so deeply +impressed by the dominating influence of the <i>a priori</i> +method as I was on reading Mr. Wallace's criticism +of Mr. Gulick's paper, after having seen the material +on which this paper is founded. To argue that every +one of some twenty contiguous valleys in the area of +the same small island must necessarily present such +differences of environment that all the shells in each +are differently modified thereby, while in no one out +of the hundreds of cases of modification in minute +respects of form and colour can any human being +suggest an adaptive reason therefor—to argue thus +is merely to affirm an intrinsically improbable dogma +in the presence of a great and consistent array of +opposing facts.</p> + +<p>I have laid special stress on this particular case of +the Sandwich Islands' mollusca, because the fifteen +years of labour which Mr. Gulick has devoted to their +exhaustive working out have yielded results more +complete and suggestive than any which so far have +been forthcoming with regard to the effects of isolation +in divergent evolution. But, if space permitted, it<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span> +would be easy to present abundance of additional facts +from other sources, all bearing to the same conclusion—namely, +that as a matter of direct observation, no +less than of general reasoning, any unprejudiced mind +will concede to the principle of indiscriminate isolation +an important share in the origination of organic types. +For as indiscriminate isolation is thus seen sooner or +later to become discriminate, and as we have already +seen that discriminate isolation is a necessary condition +to all or any modification, we can only conclude that +isolation in both its kinds takes rank with heredity +and variability as one of the three basal principles +of organic evolution.</p> + +<hr class="tb" /> + +<p>Having got thus far in the way of generalities, we +must next observe sundry further matters of comparative +detail.</p> + +<p>1. In any case of indiscriminate isolation, or +apogamy, the larger the bulk of the isolated section +the more nearly must its average qualities resemble +those of its parent stock; and, therefore, the less +divergence of character will ensue in a given time +from this cause alone. For instance, if one-fourth of +a large species were to be separated from the other +three-fourths (say, by subsidence causing a discontinuity +of area), it would continue the specific characters +unchanged for an indefinitely long time, so far as +the influence of such an indiscriminate isolation is +concerned. But, on the other hand, if only half a +dozen individuals were to be thus separated from +the rest of their species, a comparatively short time +would be needed for their descendants to undergo +some varietal modification at the hands of apogamy.<span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span> +For, in this case, the chances would be infinitely +against the average characters of the original half-dozen +individuals exactly coinciding with those of +all the rest of their species.</p> + +<p>2. In any case of homogamy, however, it is +immaterial what proportional number of individuals +are isolated in the first instance. For the isolation is +here discriminate, or effected by the initial difference +of the average qualities themselves—a difference, +therefore, which presupposes divergence as having +already commenced, and equally bound to proceed +whether the number of intergenerants be large or +small.</p> + +<p>It may here be remarked that, in his essay on +the <i>Influence of Isolation</i>, Professor Weismann fails +to distinguish between the two kinds of isolation. +This essay deals only with one of the many different +forms of isolation—the geographical—and is therefore +throughout concerned with a consideration of diversity +as arising from apogamy alone. But in dealing with +this side of the matter Weismann anticipated both +Gulick and myself in pointing out the law of inverse +proportion, which I have stated in the preceding +paragraph in what appears to me its strictly accurate +form.</p> + +<p>3. Segregate Breeding, or homogamy, which arises +under any of the many forms of discriminate isolation, +must always tend to be <i>cumulative</i>. For, again to +quote Mr. Gulick, who has constituted this fact the +most prominent as it is the most original feature +of his essay, "In the first place, every new form of +Segregation<a name="FNanchor_11_11" id="FNanchor_11_11"></a><a href="#Footnote_11_11" class="fnanchor">[11]</a> that now appears depends on, and is<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span> +superimposed upon, forms of Segregation that have +been previously induced; for when Negative Segregation +arises [i. e. isolation due to mutual sterility], +and the varieties of a species become less and less +fertile with one another, the complete infertility that +has existed between them and some other species +does not disappear, nor does the Positive Segregation +cease [i. e. any other form of isolation previously +existing].... In the second place, whenever +Segregation is directly produced by some quality of +the organism, variations that possess the endowment +in a superior degree will have a larger share in producing +the segregated forms of the next generation, +and accordingly the segregative endowment of the +next generation will be greater than that of the +present generation; and so with each successive +generation the segregation will become increasingly +complete." And to this it may be added, in the +third place, that where the segregation (isolation) is +due to the external conditions of life under which +the organism is placed, or where it is due to natural +selection simultaneously operating in divergent lines +of evolution, the same remarks apply. Hence it +follows that discriminate isolation is, in all its forms, +cumulative.</p> + +<p>4. The next point to be noted is, that the cumulative +divergence of type thus induced can take +place only in as many different lines as there are +different <i>cases</i> of isolation. This is a point which +Mr. Gulick has not expressly noticed; but it is one +that ought to be clearly recognized. Seeing that +isolation secures the breeding of similar forms by +exclusion (immediate or eventual) of those which are<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span> +dissimilar, and that only in as far as it does this +can it be a factor in organic evolution, it follows that +the resulting segregation, even though cumulative, +can only lead to divergence of organic types in as +many directions as there are cases of isolation. For +any one group of intergenerants only <i>serial</i> transformation +is possible, even though the transformation +be cumulative through successive generations in the +single line of change. But there is always a probability +that during the course of such <i>serial transformation +in time</i>, some other case of isolation may supervene, +so as to divide the previously isolated group of intergenerants +into two or more further isolated groups. +Then, of course, opportunity will be furnished for +<i>divergent transformation in space</i>—and this in as +many different lines as there are now different +homogamous groups.</p> + +<p>That this must be so is further evident, if we +reflect that the evolutionary power of isolation +depends, not only on the <i>preventing</i> of intercrossing +between the isolated portion of a species and +the rest of that species, but also upon the <i>permitting</i> +of intercrossing between all individuals of the isolated +portion, whereby the peculiar average of qualities which +they as a whole present may be allowed to assert itself +in their progeny—or, if the isolation has been from the +first discriminate, whereby the resulting homogamy +may thus be allowed to assert itself. Hence any +one case of either species of isolation, discriminate or +indiscriminate, can only give rise to what Mr. Gulick +has aptly called "monotypic evolution," or a chain-like +series of types arising successively in time, as +distinguished from what he has called "polytypic<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span> +evolution," or an arborescent multiplication of types +arising simultaneously in space.</p> + +<p>For example, let us again take the geographical +form of isolation. Where a single small intergenerant +group of individuals is separated from the rest of +its species—say, on an oceanic island—<i>monotypic</i> +evolution may take place through a continuous and +cumulative course of independent variation in a +single line of change: all the <i>individuals</i> composing +any one given generation will closely resemble one +another, although the <i>type</i> may be progressively +altering through a long series of generations. But if +the original species had had two small colonies +separated from itself (one on each of two different +islands, so giving rise to two cases of isolation), then +<i>polytypic</i> evolution would have ensued to the extent +of there having been two different lines of evolution +going on simultaneously (one upon each of +the two islands concerned). Similarly, of course, if +there had been three or four such colonies, there +would have been three or four divergent lines of +evolution, and so on.</p> + +<p>5. In the <i>cases</i> of isolation just supposed there +is only one <i>form</i> of isolation; and it is thus shown +that under one form of isolation there may be as +many lines of divergence as there are separate cases +of such isolation. But now suppose that there are +two or more forms of isolation—for instance, that +on the same oceanic island the original colony has +begun to segregate into secondary groups under +the influence of natural selection, sexual selection, +physiological selection, or any of the other forms +of isolation—then there will be as many lines of<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span> +divergent evolution going on at the same time (and +here on the same area) as there are forms of isolation +affecting the oceanic colony. And this because each +of the <i>forms</i> of isolation has given rise to a different +<i>case</i> of isolation.</p> + +<p>Now, inasmuch as different forms of isolation, when +thus superadded one to another, constitute different +cases of isolation, we may lay down the following +general law as applying to all the forms of isolation—namely, +<i>The number of possible directions in +which divergent evolution can occur, is never greater +than, though it may be equal to, the number of cases +of efficient isolation—or the number of efficiently +separated groups of intergenerants.</i></p> + +<p>6. We have now to consider with some care the +particular and highly important form of isolation +that is presented by natural selection. For while +this form of isolation resembles all the other forms +of the discriminate kind in that it secures homogamy, +there are two points in which it differs from +all of them, and one point in which it differs from +most of them.</p> + +<p>Natural selection differs from <i>all</i> the other known +forms of isolation (whether discriminate or indiscriminate) +in that it has exclusive reference to +<i>adaptations</i> on the one hand, and, on the other hand, +necessitates not only the elimination, but the destruction +of the excluded individuals. Again, natural +selection differs from <i>most</i> of the other forms of +isolation in that, unless assisted by some other +form, it can never lead to polytypic, but only +to monotypic evolution. The first two points of +difference are here immaterial; but the last is one<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span> +of the highest importance, as we shall immediately +perceive.</p> + +<p>In nearly all the other forms of isolation, polytypic +or divergent evolution may arise under the influence +of that form alone, or without the necessary co-operation +of any other form. This we have already +seen, for example, in regard to geographical isolation, +under which there may be as many different lines +of transmutation going on simultaneously as there +are different cases of isolation—say, in so many +different oceanic islands. Again, in regard to physiological +isolation the same remark obviously applies; +for it is evident that even upon the same geographical +area there may be as many different lines of transmutation +going on simultaneously as there are cases +of this form of isolation. The bar of mutual sterility, +whenever and wherever it occurs, must always render +polytypic evolution possible. And so it is with almost +all the other forms of isolation: that is to say, one +<i>form</i> does not necessarily require the assistance of +another <i>form</i> in order to create an additional <i>case</i> +of isolation. But it is a peculiarity of natural selection, +considered as a form of isolation, that it does +necessarily require the assistance of some other form +before it can give rise to an additional case of isolation; +and therefore before it can give rise to any +<i>divergence</i> of character in ramifying lines, as distinguished +from <i>transformation</i> of characters in a single +line. Or, in other words, natural selection, when acting +alone, can never induce polytypic evolution, but +only monotypic.</p> + +<p>That this important conclusion is a necessary +deduction from the theory of natural selection itself,<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span> +a very few words will be enough to show. For, +according to the theory, survival of the fittest is +a form of isolation which acts through utility, by +<i>destroying</i> all the individuals whom it fails to isolate. +Hence it follows that survival of the fittest is a form +of isolation which, if acting alone, cannot <i>possibly</i> +effect divergent evolution. For, in the first place, +there is nothing in this form of isolation to ensure +that the fitter individuals should fail to interbreed +with the less fit which are able to survive; and, in +the second place, in all cases where the less fit are +not sufficiently fit to be suffered to breed, they are +exterminated—i. e. not permitted to form a distinct +variety of their own. If it be said that survival of +the fittest may develop simultaneously two or more +lines of <i>useful</i> change, the answer is that it can +only do this if each of the developing varieties is +isolated from the others by some <i>additional form</i> +of isolation; for, if not, there can be no commencement +of utilitarian <i>divergence</i>, since whatever number +of utilitarian changes may be in course of simultaneous +development, they must in this case be all +blended together in a single line of specific transmutation. +Nay, even if specific divergence has +actually been commenced by natural selection when +associated with some other form of homogamy, if +the latter should afterwards be withdrawn, natural +selection would then be unable to maintain even so +much divergence of character as may already have +been attained: free intercrossing between the two +collateral, and no longer isolated branches, would +ensure their eventual blending into a common stock. +Therefore, I repeat, natural selection, when acting<span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span> +alone, can never induce polytypic evolution, but +only monotypic.</p> + +<p>Now I regret to say that here, for the first and +only time throughout the whole course of the +present treatise, I find myself in seeming opposition +to the views of Darwin. For it was the decidedly +expressed opinion of Darwin that natural selection +<i>is</i> competent to effect polytypic, or divergent, evolution. +Nevertheless, I believe that the opposition +is to a large extent only apparent, or due merely to +the fact that Darwin did not explicitly state certain +considerations which throughout his discussion on +"divergence of character" are seemingly implied. +But, be this as it may, I have not even appeared +to desert his leadership on a matter of such high importance +without having duly considered the question +in all its bearings, and to the utmost limit of my +ability. Moreover, about two years after the publication +of my first paper<a name="FNanchor_12_12" id="FNanchor_12_12"></a><a href="#Footnote_12_12" class="fnanchor">[12]</a> upon the subject, Mr. Gulick +followed, at somewhat greater length, in the same line +of dissent. Like all the rest of his work, this is so +severely logical in statement, as well as profoundly +thought out in substance, that I do not see how it +is possible for any one to read impartially what he +has written, and then continue to hold that natural +selection, if unassisted by any other form of isolation, +can possibly effect divergence of character—or +polytypic as distinguished from monotypic evolution<a name="FNanchor_13_13" id="FNanchor_13_13"></a><a href="#Footnote_13_13" class="fnanchor">[13]</a>.</p> + +<p>I may here quote from Mr. Gulick's paper three +propositions, serving to state three large and general<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span> +bodies of observable fact, which severally and collectively +go to verify, with an overwhelming mass of +evidence, the conclusion previously reached on grounds +of general reasoning.</p> + +<blockquote><p>The facts of geographical distribution seem to me to justify +the following statements:—</p> + +<p>(1) A species exposed to different conditions in the different +parts of the area over which it is distributed, is not represented +by divergent forms when free interbreeding exists +between the inhabitants of the different districts. In other +words, Diversity of Natural Selection without Separation does +not produce divergent evolution.</p> + +<p>(2) We find many cases in which areas, corresponding in +the character of the environment, but separated from each +other by important barriers, are the homes of divergent forms +of the same or allied species.</p> + +<p>(3) In cases where the separation has been long continued, +and the external conditions are the most diverse in points +that involve diversity of adaptation, there we find the most +decided divergences in the organic forms. That is, where +Separation and Divergent Selection have long acted, the +results are found to be the greatest.</p> + +<p>The 1st and 3rd of these propositions will probably be +disputed by few, if by any. The proof of the 2nd is +found wherever a set of closely allied organisms is so +distributed over a territory that each species and variety +occupies its own narrow district, within which it is shut by +barriers that restrain its distribution while each species of +the environing types is distributed over the whole territory. +The distribution of terrestrial molluscs on the Sandwich +Islands presents a great body of facts of this kind.</p></blockquote> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></p> + + + + +<h2>CHAPTER II.<br /> +<span class="smcap">Isolation</span> (<i>continued</i>).</h2> + + +<p>I will now recapitulate the main doctrines which +have been set forth in the foregoing chapter, and then +proceed to consider the objections which have been +advanced against them.</p> + +<p>It must be remembered that by isolation I mean +exactly what Mr. Gulick does by "Segregation," +and approximately what Professor Weismann does +by "Amixia "—i. e. the prevention of intercrossing.</p> + +<p>Isolation occurs in very many forms besides the +geographical, as will be more fully shown at the end +of this chapter; and in all its forms it admits of +degrees.</p> + +<p>It also occurs in two very different species or +kinds—namely, discriminate and indiscriminate. These +I have called respectively Homogamy and Apogamy. +This all-important distinction has been clearly recognized +by Mr. Gulick, as a result of his own thought +and observation, independently of anything that I have +published upon the subject.</p> + +<p>In view of this distinction Isolation takes rank with +Heredity and Variability as one of the most fundamental +principles of organic evolution. For, if these<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span> +other two principles be granted, the whole theory +of descent resolves itself into an inquiry touching the +causes, forms, and degrees of Homogamy.</p> + +<p>Save in cases where very large populations are +concerned, apogamy must sooner or later give rise +<i>per se</i> to homogamy, owing to the Law of Delbœuf. +which is the principle that I have called Independent +Variability, and Gulick has called Independent +Generation. But of course this does not hinder that +under apogamy various other causes of homogamy +are likely to arise—in particular natural selection.</p> + +<p>That natural selection differs from most of the other +forms of isolation in not being capable of causing +<i>divergent</i> or <i>polytypic</i> evolution must at once become +evident, if we remember that the only way in which +isolation of any form can cause such evolution is by +partitioning a given group of intergenerants into two +or more groups, each of which is able to survive as +thus separated from the other, and so to carry on the +evolution in divergent lines. But the distinguishing +peculiarity of natural selection, considered as a form +of isolation, is that it effects the isolation <i>by killing +off all the individuals which it fails to isolate</i>: consequently, +this form of isolation differs from other +forms in prohibiting the possibility of any ramification +of a single group of intergenerants into two or more +groups, for the purpose of carrying on the evolution +in divergent lines. Therefore, under this form of +isolation alone, evolution must proceed, palm-like, in +a single line of growth. So to speak, the successive +generations continuously ascend to higher things on +the steps supplied by their own "dead selves"; but +in doing so they must climb a single ladder, no<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span> +rung of which can be allowed to bifurcate in the +presence of the uniformity secured <i>for that generation</i> +by the free intercrossing of the most fit. Even +though beneficial variations may arise in two or more +directions simultaneously, and all be simultaneously +selected by survival of the fittest, the effect of free +intercrossing (in the absence of any other form of +isolation) will be to fuse all these beneficial variations +into one common type, and so to end in <i>monotypic</i> +evolution as before. In order to secure <i>polytypic</i> +evolution, intercrossing between the different beneficial +variants which may arise must be prevented; +and there is nothing to prevent such intercrossing in +the process of natural selection <i>per se</i>. In order that +the original group of intergenerants should be divided +and sub-divided into two or more groups of intergenerants, +some additional form of isolation must +necessarily supervene—when, of course, polytypic +evolution will result. And, as Mr. Gulick has shown, +the conclusion thus established by deductive reasoning +is verified inductively by the facts of geographical +distribution.</p> + +<p>How, then, are we to account for the fact that +Darwin attributed to natural selection the power to +cause divergence of character? The answer is sufficiently +simple. <i>He does so by tacitly invoking the aid +of some other form of homogamy in every case.</i> If we +carefully read pp. 86-97 of the <i>Origin of Species</i>, where +this subject is under consideration, we shall find that +in every one of the arguments and illustrations which +are adduced to prove the power of natural selection to +effect "divergence of character," he either pre-supposes +or actually names some other form of homogamy as<span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span> +the originating cause of the diversity that is afterwards +presented to natural selection for further intensification. +To give only one example. At the starting-point of +the whole discussion the priority of such other forms +of homogamy is assumed in the following words:—</p> + +<blockquote><p>But how, it may be asked, can any analogous principle +[to that of diversity caused by artificial selection] apply in +nature? I believe it can and does apply most efficiently +(though it was a long time before I saw how), from the +simple circumstance that the more diversified the descendants +from any one species become in structure, constitution, and +habits, by so much will they be better enabled to seize on +many and widely diversified places in the polity of nature, +and so be enabled to increase in numbers.</p></blockquote> + +<p>Now, without question, so soon as segregate +breeding in two or more lines of homogamy has been +in any sufficient degree determined by some "change +of structure, constitution, or habits," natural selection +will forthwith proceed to increase the divergence in +as many different lines as there are thus yielded discriminately +isolated sections of the species. And this +fact it must have been that Darwin really had before +his mind when he argued that diversification of character +is caused by natural selection, through the benefit +gained by the diversified forms being thus "enabled +to increase in number." Nevertheless he does not expressly +state the essential point, that although diversification +of character, <i>when once begun</i>, is thus <i>promoted</i> +by natural selection, which forthwith proceeds to cultivate +each of the resulting branches, yet diversification +of character can never be <i>originated</i> by natural +selection. The change of "structure," of "constitution," +of "habits," of "station," of geographical area, of reciprocal<span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span> +fertility, and so on—this change, <i>whatever</i> it +may have been, must clearly have been antecedent to +any operation of natural selection through the benefit +which arose from the change. Therefore the change +must in all cases have been due, in the first instance, +to some other form of isolation than the superadded +form which afterwards arose from superior fitness +in the possession of superior benefit—although, so +long as the prior form of isolation endured, or continued +to furnish the necessary condition to the co-operation +of survival of the fittest, survival of the +fittest would have continued to increase the divergence +of character in as many ramifying lines as there were +thus given to its action separate cases of isolation +by other means.</p> + +<p>In short, as divergence of character must in all cases +be due to a prevention of intercrossing, and as in the +process of natural selection there is, <i>ex hypothesi</i>, +nothing to prevent the intercrossing until the divergence +has already arisen, to suppose that natural +selection alone can have caused the divergence, is to +suppose that natural selection can have caused the +conditions of its own activity, which is absurd.</p> + +<p>Seeing, then, that even in cases where any "benefit" +arises from divergence of character, such benefit can +arise only after the divergence has already commenced, +and seeing that on this as on other accounts previously +mentioned it is plainly impossible to attribute the +origin of such divergence to natural selection, we find +that natural selection must be in all cases assisted +by some other form of isolation, if it is to be concerned +in polytypic as distinguished from monotypic +evolution. But this does not hinder that, when it<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span> +is so assisted, natural selection may become—and, +I believe, does become—the most efficient of all +the forms of isolation in promoting divergence of +character. For, in the first place, of all the forms +of isolation natural selection is probably the most +energetic in promoting monotypic evolution; so that +under the influence of such isolation monotypic +evolution probably advances more rapidly than +it does under any other form of isolation. In the +second place, when polytypic evolution has been +begun by any of these other forms of isolation, and +natural selection then sets to work on each of the +resulting branches, although natural selection is thus +engaged in as many different acts of monotypic evolution +as there are thus separate cases supplied to it by +these other forms of isolation, the joint result of all +these different acts is to hurry on the polytypic +evolution which was originally started by the other +forms of isolation. So to speak, natural selection is +the forcing heat, acting simultaneously on each of the +separate branches which has been induced to sprout +by other means; and in thus rapidly advancing the +growth of all the branches, it is still entitled to be +regarded as the most important <i>single</i> cause of diversification +in organic nature, although we must henceforth +cease to regard it as in any instance the +<i>originating</i> cause—or even so much as the <i>sustaining</i> +cause.</p> + +<p>So much by way of summary and recapitulation. +I will now briefly consider the only objections +which, so far as I can see, admit of being brought +against the foregoing doctrine of Isolation as held +by Mr. Gulick and myself. These possible objections<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span> +are but two in number—although but one of them +has been hitherto adduced. This, therefore, I will +take first.</p> + +<p>Mr. Wallace, with his customary desire to show +that natural selection is everywhere of itself capable +of causing organic evolution, seeks to minimize the +swamping effects of free intercrossing, and the consequent +importance of other forms of isolation. His +argument is as follows.</p> + +<p>Alluding to the researches of Mr. J. A. Allen, +and others, on the amount of variation presented +by individuals of a species in a state of nature, +Mr. Wallace shows that, as regards any given part of +the animal under consideration, there is always to +be found a considerable range of individual variation +round the average mean which goes to constitute the +specific character of the type. Thus, for example, +Mr. Allen says of American birds, "that a variation +of from fifteen to twenty per cent. in general size, +and an equal degree of variation in the relative size +of different parts, may be ordinarily expected among +specimens from the same species and sex, taken at +the same locality, while in some cases the variation +is even greater than this." Now, Mr. Wallace is under +the impression that these facts obviate the difficulty +which arises from the presence of free intercrossing—the +difficulty, that is, against the theory of natural +selection when natural selection is supposed to have +been the exclusive means of modification. For, as +he says, "if less size of body would be beneficial, +then, as half the variations in size are above and +half below the mean or existing standard of the +species, there would be ample beneficial variations";<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span> +and similarly with regard to longer or shorter legs, +wings, tails, &c., darker or lighter colour, and so on +through all the parts of any given organism.</p> + +<p>Well, although I have no wish at all to disparage +the biological value of these actual measurements +of the range of individual variation, I must point +out that they are without any value at all in the +connexion which Mr. Wallace adduces them. We +did not require these measurements to tell us the +broad and patent fact that "no being on this earthly +ball is like another all in all"—or, in less Tennysonian +words, that as regards every specific structure +there is a certain amount of individual variability +round an average mean. Indeed, in my own paper +on <i>Physiological Selection</i>—against which Mr. Wallace +is here specially arguing—I expressly said, as +previously remarked, "that a specific type may +be regarded as <i>the average mean of all individual +variations</i>." The fact of such individual variability +round a specific mean has always been well known +to anatomists; it constitutes one of the basal pillars +of the whole Darwinian theory; and is besides a +matter of universal recognition as regards human +stature, features, and so forth. The value of Mr. +Allen's work consists in accurately measuring the +<i>amount</i> or <i>range</i> of individual variation; but the +question of its amount or range is without relevancy +in the present connexion. For the desirability of +isolation as an aid to natural selection even where +monotypic evolution is concerned, does not arise +with any reference to the amount or range of variation: +it arises with reference to the <i>number</i> of variations +which are—or are not—<i>similar</i> and <i>simultaneous</i>. If<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span> +there be a sufficient number which are both similar +and simultaneous, the desirability of any co-operating +form of isolation is correspondingly removed, because +natural selection may then have sufficient material +wherewith to overcome the adverse influence of free +intercrossing, and so of itself to produce monotypic +evolution. Now, variations may be numerous, similar, +and simultaneous, either on account of some common +cause acting on many individuals at the same time, +or on account of the structures in question being +more or less variable round a specific mean. In +the latter case—which is the only case that Mr. +Allen's measurements have to do with—the law of +averages will of course determine that half the whole +number of variations in any given structure, in any +given generation, will be above the mean line. But, +equally of course, no one has ever denied that where, +for either of these reasons, natural selection is provided +with sufficient material, it is correspondingly +capable of improving the specific type without +the assistance of any other form of homogamy; +so to speak, they protect themselves by their very +numbers, and their superiority over others leads to +their survival and accumulation. But what is the +result? <i>The result can only be monotypic evolution.</i> +No matter how great the number, or how great the +range, of variations round an average specific mean, +out of such material natural selection can never +produce <i>polytypic</i> evolution: it may <i>change</i> the type +to any extent during successive generations, and +in a single line of change; but it cannot <i>branch</i> +the type, unless some other form of homogamy +intervenes. Therefore, when Mr. Wallace adduces<span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span> +the well-known fact that all structures vary more +or less round a specific mean as proof that natural +selection need not be incommoded by free intercrossing, +but can of itself produce all the known +phenomena of specific evolution, he fails to perceive +that his argument refers only to one aspect of such +evolution (viz. the transformation of species in time), +and does not apply to the aspect with which alone +my paper on <i>Physiological Selection</i> was concerned +(viz. the multiplication of species in space).</p> + +<p>The same thing may be shown in this way. It is +perfectly obvious that where the improvement of type +in a linear series is concerned (monotypic evolution), +free intercrossing, far from being a hindrance to the +process, <i>is the very means by which the process is +accomplished</i>. Improvement here ascends by successive +steps, in successive generations, simply <i>because</i> +of the general intercrossing of the generally most fit +with the result that the species, <i>as a whole</i>, gradually +becomes transformed into another species, <i>as a whole</i>. +Therefore, it would be mere fatuity in any one to +adduce free intercrossing as a "difficulty" against +natural selection alone being competent to produce +evolution of this kind. But where the kind of +evolution is that whereby the species is <i>differentiated</i>—where +it is required, for instance, to produce different +structures in different portions of the species, such as +the commencement of a fighting spur on the wing of +a duck, or <i>novel</i> characters of any sort in different +groups of the species—free intercrossing is no longer +a condition to, but an absolute preventive of, the +process; and, therefore, unless checked as between +each portion of the species by some form of homogamy<span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span> +other than natural selection, it must effectually +inhibit any <i>segregation</i> of specific types, or divergence +of character.</p> + +<p>Hence it is that, while no Darwinian has ever +questioned the power of unaided selection to cause +<i>improvement of character in successive generations</i>, in +common now with not a few other Darwinians I have +emphatically denied so much as the abstract possibility +of selection alone causing a <i>divergence of character +in two or more simultaneous lines of change</i>.</p> + +<p>And, although these opposite views cannot be +reconciled, I am under the impression that they do +admit of being explained. For I take them to +indicate a continued failure to perceive the all-important +distinction between evolution as monotypic +and polytypic. Unless one has fully grasped this +distinction, and constantly holds it in mind, he is +not in a position to understand the "difficulty" in +question; nor can he avoid playing fast and loose +with natural selection as possibly the sole cause of +evolution, and as necessarily requiring the co-operation +of some other cause. But if he once clearly perceives +that "evolution" is a logical genus, of which the monotypic +and the polytypic forms are species, he will +immediately escape from his confusion, and find that +while the monotypic form may be caused by natural +selection alone the polytypic form can never be +so caused.</p> + +<hr class="tb" /> + +<p>The second difficulty which I have to mention as at +first sight attaching to the views of Mr. Gulick and +myself on the subject of Isolation is, that in an isolated +section of a species Mr. Francis Galton's law of<span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span> +regression in the average character of offspring to +the typical character of the group through reversion +or atavism (<i>Natural Inheritance</i>, p. 97) must have +the effect of neutralizing the segregative influence of +mere apogamy. That such, however, cannot be the +case has been well shown by Mr. Gulick in his paper +on <i>Intensive Segregation</i>. Without at all disputing +the validity of Mr. Galton's law, he proves that "it can +hold in full force only where there is free crossing, +otherwise no divergent race could ever be formed by +any amount of selection and independent breeding<a name="FNanchor_14_14" id="FNanchor_14_14"></a><a href="#Footnote_14_14" class="fnanchor">[14]</a>." +This is so self-evident that I need not quote his demonstration +of the point.</p> + +<hr class="tb" /> + +<p>In conclusion, then, and having regard to the +principle of isolation as a whole, or in all the many and +varied forms in which this principle obtains, I trust that +I have redeemed the promise with which I set out—viz. +to show that in relation to the theory of descent +this principle is of an importance second to no other, +not even excepting heredity, variability, and the +struggle for existence. This has now been fully +shown, inasmuch as we have clearly seen that the importance +of the struggle for existence, and consequent +survival of the fittest, arises just because survival +of the fittest is a form, and a very stringent form, of +isolation; while, as regards both heredity and variability, +we are now in a position to see that the more +fully we recognize their supreme importance as +principles concerned in organic evolution, the more +must we also recognize that any rational theory of +such evolution becomes, in the last resort, a theory<span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span> +of the different modes in which efficient isolation can +be secured. For, in whatever degree the process of +organic evolution has been dependent upon heredity +with variability, in that degree must it also have been +dependent upon the means of securing homogamy, +whereby alone the force of heredity can be made to +expend itself in the innumerable directions of progressive +change, instead of continually neutralizing the +force of variability by promiscuous intercrossing.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></p> + + + + +<h2>CHAPTER III.<br /> +<span class="smcap">Physiological Selection.</span></h2> + + +<p>So far we have been concerned with the principle +of Isolation in general. We have now to consider +that form of isolation which arises in consequence of +mutual infertility between the members of any group +of organisms and those of all other similarly isolated +groups occupying simultaneously the same area.</p> + +<hr class="tb" /> + +<p>Against the view that natural selection is a sufficient +explanation of the origin of species, there are two +fatal difficulties: one, the contrast between natural +species and domesticated varieties in respect of cross-sterility; +the other, the fact that natural selection +cannot possibly give rise to polytypic as distinguished +from monotypic evolution. Now it is my belief that +the theory of physiological selection fully meets both +these difficulties. Indeed I hold this to be undeniable +in a formal or logical sense: the only question is as +to the evidence which can be adduced for the theory +in a practical or biological sense. Therefore in this +chapter, where the theory has first of all to be stated, +I shall restrict the exposition as much as possible +to the former, leaving for subsequent consideration the +biological side.</p> + +<p><span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span> +The following is a brief outline sketch of this +theory<a name="FNanchor_15_15" id="FNanchor_15_15"></a><a href="#Footnote_15_15" class="fnanchor">[15]</a>.</p> + +<p>Of all parts of those variable objects which we +call organisms, the most variable is the reproductive +system; and the variations may carry with them functional +changes, which may be either in the direction +of increased or of diminished fertility. Consequently +variations in the way of greater or less fertility frequently +take place, both in plants and animals; and +probably, if we had adequate means of observing this +point, we should find that there is no one variation +more common. But of course where infertility arises—whether +as a result of changed conditions of life, or, +as we say, spontaneously—it immediately becomes +extinguished, seeing that the individuals which it +affects are less able (if able at all) to propagate and +to hand on the variation. If, however, the variant, +while showing some degree of infertility with the parent +form, continues to be as fertile as before when mated +with similar variants, under these circumstances there +is no reason why such differential fertility should not +be perpetuated.</p> + +<p>Stated in another form this suggestion enables us +to regard many, if not most, species as the records of +variations in the reproductive systems of their ancestors. +When variations of a non-useful kind occur in any +of the other systems or parts of organisms, they are, +as a rule, immediately extinguished by intercrossing. +But whenever they arise in the reproductive system +in the way here suggested, they tend to be preserved +as new natural varieties, or incipient species. At +first the difference would only be in respect of the<span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span> +reproductive systems; but eventually, on account of +independent variation, other differences would supervene, +and the variety would take rank as a true +species.</p> + +<p>Now we must remember that physiological isolation +is not like those other forms of isolation (e.g. +geographical) which depend for their occurrence on +accidents of the environment, and which may therefore +take place suddenly in a full degree of completeness +throughout a large section of a species. Physiological +isolation depends upon distinctive characters +belonging to organisms themselves; and it would +be opposed to the whole theory of descent with +progressive modification to imagine that absolute +sterility usually arises, in a single generation between +two sections of a perfectly fertile species. Therefore +evolutionists must believe that in most, if not in all +cases—could we trace the history, say of any two +species, which having sprung from a single parent +stock on a common area, are now absolutely sterile +with one another—we should find that this mutual +sterility had been itself a product of gradual evolution. +Starting from complete fertility within the limits of a +single parent species, the infertility between derivative +or divergent species, <i>at whatever stage in their evolution +this began to occur</i>, must usually at first have been well-nigh +imperceptible, and thenceforth have proceeded +to increase stage by stage.</p> + +<p>But, if it be true that physiological isolation between +genetically allied groups must usually itself have been +the product of a gradual evolution; and if, when +fully evolved, it constitutes a condition of the first +importance to any further differentiation of these<span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span> +groups (by preventing fusion again into one group, +more or less resembling the original parent form), do +we not perceive at least a strong probability that +in the lower stages of its evolution such mutual infertility +must have acted as a segregating influence +between the diverging types, in a degree proportional +to its own development? The importance of mutual +sterility as a condition to divergent evolution is not +denied, <i>when this sterility is already present in an +absolute degree</i>; and we have just seen that, before +it can have attained to this absolute degree <i>it must +presumably, and as a rule, itself have been the subject +of a gradual development</i>. Does it not therefore +become, on merely antecedent grounds, in a high +degree probable, that from the moment of its inception +this isolating agency must have played the +part of a segregating cause, in a degree proportional +to that of its completeness as a physiological +character?</p> + +<p>Whoever answers this question in the affirmative +will have gone most of the way towards accepting, on +merely antecedent grounds, the theory of physiological +selection. And therefore it is that I have begun this +statement of the theory by introducing it upon these +grounds, thereby hoping to show how extremely simple—how +almost self-evident—is the theory which it will +now be my endeavour to substantiate. I may here +add that the theory was foreshadowed by Mr. Belt +in 1874<a name="FNanchor_16_16" id="FNanchor_16_16"></a><a href="#Footnote_16_16" class="fnanchor">[16]</a>, clearly enunciated in its main features by +Mr. Catchpool in 1884<a name="FNanchor_17_17" id="FNanchor_17_17"></a><a href="#Footnote_17_17" class="fnanchor">[17]</a>, and very fully thought out +by Mr. Gulick during a period of about fifteen years,<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span> +although he did not publish until a year after the +appearance of my own paper in 1886<a name="FNanchor_18_18" id="FNanchor_18_18"></a><a href="#Footnote_18_18" class="fnanchor">[18]</a>.</p> + +<p>I must next proceed to state some of the leading +features of physiological selection in further detail.</p> + +<p>It has already been shown that Darwin clearly +perceived that the very general occurrence of some +degree of infertility between allied species cannot +possibly be attributed to the <i>direct</i> agency of natural +selection. His explanation was that the slight structural +modifications entailed by the transformation of +one specific type into another, so react upon the +highly delicate reproductive system of the changing +type as to render it in some degree infertile with +its parent type. Now the theory of physiological +selection begins by traversing this view. It does +not, however, deny that in <i>some</i> cases the morphological +may be the prior change; but it strenuously +denies that this must be so in <i>all</i> cases. Indeed, +according to my statement in 1886, the theory inclines +to the view that, <i>as a rule</i>, the physiological change +is prior. At the same time, the theory, as I have +always stated it, maintains that it is immaterial whether, +"in the majority of instances," the physiological change +has been prior to the morphological, or vice versa; +since in either case the physiological change will +equally make for divergence of character.</p> + +<p><span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span> +To show this clearly the best way will be to consider +the two cases separately, taking first that in which +the physiological change has priority. In this case +our theory regards any morphological changes which +afterwards supervene as due to the independent variability +which will sooner or later arise under the +physiological isolation thus secured. But to whatever +causes the subsequent morphological changes +may be due, the point to notice is that they are as +a general rule, consequent upon the physiological +change. For in whatever <i>degree</i> such infertility arises +between two sections of a species occupying the same +area, in that <i>degree</i> is their interbreeding prevented, +and, therefore, opportunity is given for a subsequent +divergence of type, whether by the influence of independent +variability alone, or also by that of natural +selection, as now acting more or less independently +on each of the partially separated groups. In short, +all that was said in the foregoing chapters with respect +to isolation in general, here applies to physiological +isolation in particular; and by supposing such isolation +to have been the prior change, we can as well understand +the subsequent appearance of morphological +divergence on continuous areas, as in other forms +of isolation we can understand such divergence on +discontinuous areas, seeing that even a moderate +degree of cross-infertility may be as effectual for +purposes of isolation as a high mountain-chain, or +a thousand miles of ocean.</p> + +<p>Here, then, are two sharply-defined theories to +explain the very general fact of there being some +greater or less degree of cross-infertility between allied +species. The older, and hitherto current theory,<span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span> +supposes the cross-infertility to be but an <i>accident</i> +of specific divergence, which, therefore, has nothing +to do with <i>causing</i> the divergence. The newer theory, +on the other hand, supposes the cross-infertility to +have often been a necessary <i>condition</i> to the divergence +having begun at all. Let us now consider which +theory has most evidence in its favour.</p> + +<p>First of all we have to notice the very general +occurrence of the fact in question. For when we +include the infertility of hybrids, as well as first +crosses, the occurrence of some degree of infertility +between allied species is so usual that Mr. Wallace +recommends experiments to ascertain whether careful +observation might not prove, even of species which +hybridize, "that such species, when crossed with their +near allies, do always produce offspring which are +more or less sterile <i>inter se</i><a name="FNanchor_19_19" id="FNanchor_19_19"></a><a href="#Footnote_19_19" class="fnanchor">[19]</a>." This seems going too +far, but nevertheless it is the testimony of a highly +competent naturalist to the very general occurrence of +an association between the morphological differentiation +of species and the fact of a physiological isolation. +Now I regard it as little short of self-evident that this +general association between mutual infertility and +innumerable secondary, or relatively variable morphological +distinctions, is due to the former having +been an original and a necessary condition to the +occurrence of the latter, in cases where intercrossing +has not been otherwise prevented.</p> + +<p>The importance of physiological isolation, <i>when +once fully developed</i>, cannot be denied, for it is evident +that if such isolation could be suddenly destroyed +between two allied species occupying a common area,<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span> +they would sooner or later become fused into a +common type—supposing, of course, no other form +of isolation to be present. The necessity then for +this physiological form of isolation in <i>maintaining</i> +a specific differentiation which has been already <i>attained</i> +cannot be disputed. Yet it has been regarded +as "Darwinian heresy" to suggest that it can have +been of any important service <i>during the process of +attainment</i>, or while the specific differentiation is +being advanced, and this notwithstanding that the +physiological change must presumably have developed +<i>pari passu</i> with the morphological, and notwithstanding +that in countless cases the former is associated +with every conceivable variety of the latter.</p> + +<p>Again, why should the physiological change be +thus associated with <i>every conceivable variety</i> of +morphological change? Throughout the length and +breadth of both vegetable and animal kingdoms we +find this association, in the great majority of cases, +where new species arise. Therefore, on the supposition +that in all such cases the physiological change +has been adventitiously induced by the morphological +changes, we have to face an apparently unanswerable +question—Why should the reproductive +mechanism of all organic beings have been thus +arranged, as it were, to change in immediate response +to the very slightest alteration in the complex harmony +of "somatic" processes, which now more than +ever is recognized as exercising so comparatively +little influence on the <i>hereditary</i> endowments of this +mechanism? Consider the difference between a worm +and the bird that is eating it, an oak tree and the +gall-insect that is piercing it: are we to suppose that<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span> +in all cases, no matter how greatly the types differ, +they must agree in this, that when any parts of +these complex structures change, ever so slightly, +the reproductive system is almost certain to be +adventitiously affected, yet always thus affected in +the same peculiar way?</p> + +<p>If it be answered that the reproductive system is +known to be very sensitive to slight changes in the +external conditions of life, the answer proves too +much. For though this is true, yet our opponents +must acknowledge that the reproductive system is +not so sensitive, <i>in this particular respect</i>, as their +interpretation of the origin of specific infertility +requires. The proof of this point is overwhelming, +for there is the evidence from the entire range of our +domesticated productions, both vegetable and animal. +Here the amount of structural change, which has been +slowly accumulated by artificial selection, is often +much greater in amount, and incomparably more +rapid, than that which has been induced between +allied species by natural selection; and yet there is +scarcely any indication of the reproductive system +having been affected in the particular way that our +opponents' theory requires. There are many instances +of its having been affected in sundry other +ways (chiefly, however, without any accompanying +morphological change); but among all the thousands +of our more or less enormously modified artificial +types, there is scarcely one instance of such a peculiar +sexual relation between the modified descendants of +a common type as so usually obtains between allied +species in nature. Yet in all other respects evolutionists +are bound to believe that the process of<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span> +modification has been in both cases strictly analogous. +Why then this conspicuous difference with respect to +the reproductive system?</p> + +<p>The answer is simple. It has never been the object +of breeders or of horticulturists to select variations +in the direction of cross-infertility, for the swamping +effects of intercrossing are much more easily and +rapidly prevented by artificial isolation. Consequently, +although they have been able to modify natural types +in so many directions and in such high degrees with +regard to <i>morphology</i>, there has been no accompanying +physiological modification of the kind required. But +in nature there is no such thing as artificial, i.e. intentional, +isolation. Consequently, on common areas +it must usually happen that those changes of morphology +which are associated with cross-infertility +are the only ones which can arise. Hence the very +remarkable contrast between our domesticated varieties +and natural species with regard to cross-infertility +is just what the present theory would expect, or, +indeed, require. But on any other theory it has +hitherto remained inexplicable.</p> + +<p>In particular, the contrast in question has constituted +one of the main difficulties with which the theory +of natural selection has hitherto had to contend, not +only in the popular mind, but also in the judgement +of naturalists, including the joint-authors of the theory +themselves. Thus Darwin says:—</p> + +<blockquote><p>The fertility of varieties is, with reference to my theory, +of equal importance with the sterility of species, for it seems +to make a broad and clear distinction between varieties and +species<a name="FNanchor_20_20" id="FNanchor_20_20"></a><a href="#Footnote_20_20" class="fnanchor">[20]</a>.</p></blockquote> +<p><span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span></p> +<p>And Mr. Wallace says:—</p> + +<blockquote><p>One of the greatest, or perhaps we may say the greatest, of +all the difficulties in the way of accepting the theory of natural +selection as a complete explanation of the origin of species, has +been the remarkable difference between varieties and species in +respect of fertility when crossed<a name="FNanchor_21_21" id="FNanchor_21_21"></a><a href="#Footnote_21_21" class="fnanchor">[21]</a>.</p></blockquote> + +<p>Now, in view of this conspicuous contrast, Darwin +suggested that species in a state of nature "will have +been exposed during long periods of time to more +uniform conditions than have domesticated varieties, +and [that] this may well make a wide difference in the +result." Now we have to remember that species, living +and extinct, are numbered by millions, and represent +every variety of type, constitution, and habits; is +it probable, then, that this one peculiarity of the +reproductive system should be due, in so many cases, +to some merely incidental effect produced on that +system by uniform conditions of life? Again, <i>ex +hypothesi</i>, at the time when a variety is first forming, +the influence exercised by uniform conditions of life +(whatever in different cases this may happen to be) +cannot be present as regards that variety: yet this is +just the time when its infertility with the parent (or +allied) form is most likely to have arisen; for it is +just then that the nascent variety would otherwise +have been most liable to extinction by free intercrossing—even +supposing that in the presence of such +intercrossing the variety could ever have come into +existence at all.</p> + +<p>Mr. Wallace meets the difficulty by arguing that +sterility between allied species may have been brought +about by the direct influence of natural selection.<span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span> +But, as previously remarked, this view is expressly +opposed to that of Darwin, who held that Wallace's +contention is erroneous.</p> + +<p>It will be seen, then, that both Darwin, and Wallace, +fully recognize the necessity of finding some explanation +of the infertility of allied species, over and above +the mere reaction of morphological differentiation on +the physiology of the reproductive system, and they +both agree in suggesting additional causes, though +they entirely disagree as to what these causes are. +Now, the theory of physiological selection likewise +suggests an additional cause—or, rather, a new explanation—and +one which is surely the most probable. +For what is to be explained? The very general +association of a certain physiological peculiarity with +that amount of morphological change which distinguishes +species from species, of whatever kind the +change may be, and in whatever family of the animal +or vegetable kingdom it may occur. Well, the theory +of physiological selection explains this very general +association by the simple supposition that, at least +in a large number of cases, it was the physiological +peculiarity which first of all led to the morphological +divergence, by interposing the bar of sterility between +two sections of a previously uniform species; and by +thus isolating the two sections one from another, +started each upon a subsequently independent course +of divergent evolution.</p> + +<p>Or, to put it in another way, if the occurrence of +this physiological peculiarity has been often the only +possible means of isolating two sections of a species +occupying a common area, and thus giving rise to +a divergence of specific type (as obviously <i>must</i> have<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span> +been the case wherever there was an absence of any +other form of isolation), it is nothing less than a +necessary consequence that many allied species should +now present the physiological peculiarity in question. +Thus the association between the physiological peculiarity +and the morphological divergence is explained +by the simple hypothesis, that the former has acted +as a necessary condition to the occurrence of the +latter. In the absence of other forms of isolation, +the morphological divergence could not have taken +place at all, had not the physiological peculiarity +arisen; and hence it is that we now meet with so +many cases where such divergence is associated with +this peculiarity.</p> + +<hr class="tb" /> + +<p>So far we have been considering the physiological +change as historically the prior one. Here, at first +sight, it may seem that the segregative power of +physiological selection must end; for it may well +seem impossible that the physiological change can +ever be necessary for the divergence of morphological +varieties into true species in cases where it has <i>not</i> +been the prior change, but has only set in after morphological +changes have proceeded far enough to have +already constituted definite varieties. A little thought, +however, will show that physiological selection is quite +as potent a condition to the differentiation of species +when it occurs after varietal divergence has begun, as +it is when it occurs before the divergence—and hence +that it really makes no difference to the theory of +physiological selection whether, in particular cases, the +cross-infertility arises before or after any structural or +other modifications with which it is associated.</p><p><span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span></p> + +<p>For the theory does not assert that all varieties +have been due to physiological selection. There are +doubtless many other causes of the origin of varieties +besides cross-infertility with parent forms; but, as +a general rule, it does not appear that they are by +themselves capable of carrying divergence beyond +a merely varietal stage. In order to carry divergence +to the stage of producing <i>species</i>, it appears to be +a general condition that, sooner or later, cross-infertility +should arise—seeing that, when varieties do succeed +in becoming species, we almost invariably find that, +as a matter of fact, cross-infertility has arisen. Hence, +if cross-infertility has thus usually been a necessary +condition to a varietal divergence becoming specific, +it can make no material difference when the incipient +infertility arose.</p> + +<p>It may be asked, however, whether I suppose that, +when the physiological change is subsequent, it is +directly <i>caused</i> by change of structure, size, colour, &c., +or that it arises, so to speak, accidentally, from other +causes which may have affected the sexual system in +the required way. To this question I may briefly +reply, that, looking to the absence of any influence +exercised on the reproductive systems of our domesticated +plants and animals by the great and varied +changes which so many of these forms present, it +would seem that among natural varieties such closely +analogous changes are presumably not the usual causes +of the physiological change, even where the latter are +subsequent to the former. Nevertheless, I do not +deny that in some of these cases changes of structure, +size, colour, &c., may be the causes of the physiological +change by reacting on the sexual system in the required<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span> +way. But in such cases free intercrossing will +have prevented the perpetuation of any morphological +changes, save those which have the power of so reacting +on the reproductive system as to produce the +physiological change, and thus to protect themselves +against the full and adverse power of free intercrossing. +We know that slight or initial changes of structure, +colour, &c., frequently occur as varieties, and yet that +on common areas very few of these varieties become +distinct species: free intercrossing prevents any such +further divergence of character. But if in the course +of many such abortive attempts, as it were, to produce +a new species, nature happens to hit upon a structural +or a colour variation which is capable of reacting on +the sexual system in the particular way required, then +this variation will be enabled to protect itself against +free intercrossing in proportion to its own development. +Or, in other words, the more it develops as a morphological +change, the more will it increase the physiological +change; while the more the physiological +change is thus increased, the more will it in turn +promote the morphological. By such action and +reaction the development of each furthers the development +of the other, till from an almost imperceptible +variety, apparently quite fertile with its parent form, +there arises a distinct species absolutely sterile with +its parent form. In such cases, therefore, it is still +the physiological conditions which have <i>selected</i> the +particular morphological changes capable of so reacting +on the reproductive system as to produce cross-infertility, +and thus to protect themselves against the +destructive power of free intercrossing. So to speak, +free intercrossing is always on the watch to level<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span> +down any changes which natural selection, or any +other cause of varietal divergence, may attempt to +produce; and therefore, in order to produce—or to +increase—such divergence in the absence of any other +form of isolation, natural selection must hit upon such +changes of structure, form, or colour, as are so correlated +with the reproductive system as to create the +physiological isolation that is required.</p> + +<p>To show how the principle of selective fertility +may be combined with what apparently is the most +improbable form of isolation for this purpose—the +geographical—I quote the following suggestion made +by Professor Lloyd Morgan in his <i>Animal Life and +Intelligence</i>:—</p> + +<blockquote><p>Suppose two divergent local varieties were to arise in +adjacent areas, and were subsequently (by stress of competition +or by geographical changes) driven together into a single +area.... If their unions be fertile, the isolation will be annulled +by intercrossing—the two varieties will form one mean or +average variety. But if the unions be infertile, the isolation +will be preserved, and the two varieties will continue separate. +Suppose now, and the supposition is by no means an improbable +one, that this has taken place again and again in the +evolution of species; then it is clear that those varietal forms +which had continued to be fertile together would be swamped +by intercrossing; while those varietal forms which had become +infertile would remain isolated. Hence, in the long run, isolated +forms occupying a common area would be infertile, +(p. 107.)</p></blockquote> + +<p>If then cross-sterility may thus arise even in association +with geographical isolation, may it not also +arise in its absence? And may it not thus give rise +to the differentiation of varieties on account of this +physiological isolation alone?</p><p><span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span></p> + +<p>Only two further points need be mentioned to +make this statement of physiological selection as +complete as the present <i>résumé</i> of its main principles +requires.</p> + +<p>The first is, that, as Mr. Wallace remarks, "every +species has come into existence coincident both in +space and time with a pre-existing and closely allied +species." I regard this as important evidence that +physiological selection is one of the natural causes +concerned. For the general fact implied is that every +species has come into existence on an area occupied +by its parent type, and therefore under circumstances +which render it imperative that intercrossing with that +type should be prevented. In the case of monotypic +evolution by natural selection alone, intercrossing +with the parent type is prevented through the gradual +extinction of that type by successive generations of +the developing type. But in the case of polytypic +evolution, intercrossing with the parent type can +only be prevented by some form of isolation other +than natural selection; and here it is evident that +cross-infertility with the parent type must be as +efficient to that end as any other form of isolation +that can be imagined. Consequently we might +almost have expected beforehand that in a large +proportional number of cases cross-infertility should +have been the means employed. And the fact that +this is actually the case so far corroborates the only +theory which is able to explain it.</p> + +<p>The second point is this.</p> + +<p>It appears to be comparatively rare for any cause +of specific divergence to prove effectual on common +areas, unless it sooner or later becomes associated with<span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span> +some degree of cross-infertility. But through this +association, the segregating influence of both the +causes concerned is, as Mr. Gulick has shown, greatly +increased. For instance, if the segregating influence +of some degree of cross-infertility be associated with +that of any other form of isolation, then, not only +will the two segregating influences be added, but +multiplied together. And thus, by their mutual +action and reaction, divergent evolution is promoted +at a rapidly increasing rate.</p> + +<p>I will now summarize the main points of the theory +of physiological isolation in a categorical form.</p> + +<p>1. If no other form of isolation be present, specific +divergence can only take place when some degree of +cross-infertility has previously arisen between two or +more sections of a species.</p> + +<p>2. When such cross-infertility has arisen it may +cause specific divergence, either (<i>a</i>) by allowing independent +variability in each of the physiologically +isolated groups; (<i>b</i>) by becoming associated with any +other cause of differentiation already operating; or +(<i>c</i>) by both these means combined.</p> + +<p>3. As some degree of cross-infertility generally +obtains between allied species, we are justified in +concluding that this has been the most frequent—or, +at any rate, the most effective—kind of isolation +where the origin of species is concerned; and +therefore the kind with which, in the case of +species-formation, natural selection, or any other +cause of specific divergence, has been most usually +associated.</p> + +<p>4. Where varietal divergence has begun in the<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span> +absence of cross-infertility, such divergence seems, as +a general rule, to have been incapable of attaining to a +specific value.</p> + +<p>5. Therefore, in the vast majority of such cases, it +must have been those varietal changes of structure, +size, colour, &c., which happened to have afterwards +been assisted by the reproductive change that were +on this account <i>selected</i> as successful candidates for +specific differentiation.</p> + +<p>6. It follows, that it makes no difference to the +general theory of physiological selection in what proportion +of cases the physiological change has been +the initial change; for, whether prior or subsequent +to the varietal changes with which it becomes associated, +its presence has been equally important as a +condition to specific divergence.</p> + +<p>7. When physiological isolation becomes associated +with natural selection, or any other form of homogamy, +the segregative power of both is augmented. Moreover, +so great is the augmentation that even very +moderate degrees of physiological isolation—themselves +capable of effecting little or nothing—become +very powerful when associated with moderate degrees +of any other kind of homogamy, and vice versa.</p> + +<p>8. The theory of physiological selection effectually +explains the divergent evolution of specific +types and the cross-infertility of such types when +evolved.</p> + +<hr class="tb" /> + +<p>To prevent, if possible, the continuance of certain +misunderstandings with regard to my original statement +of the new theory, let me here disclaim some +views which have been assigned to me. They are:</p> + +<p><span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span> +1. That the theory of physiological selection is +opposed to the theory of natural selection. Far from +this being so, it is—at all events in my own opinion—a +very important aid to it, in preventing free intercrossing +on a common area, and thus allowing divergent +evolution to occur within that area.</p> + +<p>2. That, in advancing the theory of physiological +selection as "an additional suggestion on the origin +of species," I wish to represent it as being the +originating cause of <i>all</i> species. What I hold is, that +all species must have owed their origin to <i>isolation</i>, in +some form or other; but that as physiological selection +is only one among many other forms of isolation (including +natural selection), and as it can only act on +common areas, a large number of species must have +been formed without its aid.</p> + +<p>3. That I imagine physiological varieties always +to arise "sporadically," or as merely individual +"sports" of the reproductive system. On the contrary, +I expressly stated that this is <i>not</i> the way in +which I suppose the "physiological variation" to +arise, when giving origin to a new species; but that +it arises, whenever it is effectual, as a "collective +variation" affecting a number of individuals simultaneously, +and therefore characterizing "a whole race, +or strain."</p> + +<p>4. That I suppose physiological selection always to +act alone. This I have never supposed. The essential +point is, not that the physiological isolation is unassociated +with other forms of isolation, but that +unless associated with some degree of physiological +isolation, no one of the other forms is capable of +originating species on common areas with any approach<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span> +to frequency. This proposition is the essence of +the new theory, and I take it to be proved, not only +by general deductive reasoning which shows that +it <i>must</i> be so, but also by the fact of an otherwise +inexplicable association between specific divergence +on common areas and some more or less considerable +degree of mutual infertility.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></p> + + + + +<h2>CHAPTER IV.<br /> +<span class="smcap">Evidences of Physiological Selection.</span></h2> + + +<p>I will now give an outline sketch of the evidences +in favour of the theory which has been set forth in +the preceding chapter, stating first what is the nature +of the verification which it requires.</p> + +<p>The theory is deduced from a highly general +association between distinctive specific characters +of <i>any</i> kind and a relatively constant specific +character of a <i>particular</i> kind—namely, sexual +exclusiveness. For it is from this highly general +association that the theory infers that this relatively +constant specific character has been at least one of +the needful conditions to the development of the +other specific characters with which it is found +associated. Hence the necessary verification must +begin by showing the strength of the theory on these +merely deductive, or antecedent, grounds. It may +then proceed to show how far the facts of organic +nature corroborate the theory in other and independent +ways.</p> + +<p>First, let it be carefully observed that here we have +to do only with the <i>fact</i> of selective fertility, and with +its <i>consequences</i> as supposed by the theory: we have<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span> +nothing to do either with its <i>causes</i> or its <i>degrees</i>. +Not with its causes, because in this respect the +theory of physiological selection is in just the same +position as that of natural selection: it is enough for +both if the needful variations are provided, without +its being incumbent on either to explain the causes +which produce them. Not with its degrees, because, +in the first place, it can only be those degrees of +variation which in particular cases are supposed +adequate to induce specific divergence, that fall +within the scope of the theory; and because, in the +second place, degrees which are adequate only to +induce—or to assist in inducing, <i>varietal</i> divergence, +must always tend to increase, or pass into higher +degrees.</p> + + +<h3><i>Antecedent Standing of the Theory.</i></h3> + +<p>The antecedent standing or logical basis of the +theory has already been in large measure displayed +in the preceding chapter; for it was impossible to +state the theory without thereby showing in how +considerable a degree it is self-evident. A brief +recapitulation is therefore all that is here necessary.</p> + +<p>It has been shown that divergent or polytypic +evolution on common areas is inexplicable by natural +selection alone. Hence the question arises: What +form of isolation has, under such circumstances, +rendered possible divergent evolution? In answer +to this question the theory of physiological selection +suggests that variations in the reproductive function +occur in such a way as to isolate more or less +perfectly from each other different sections of a +species. While cross-fertility remains unimpaired<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span> +among the members of each section, there is more or +less cross-infertility when members of either section +mate with those of the other. Thus a physiological +barrier is interposed between the two sections; and +any divergences of structure, colouring, or instinct +arising in the members of either section will not in +any way be affected by such divergences as arise +among the members of the other.</p> + +<p>In support of this suggestion, it has been shown in +the preceding chapter that the very general association +of cross-infertility with specific differentiation points +most strongly to the inference that the former has +usually been an indispensable condition to the +occurrence of the latter. It cannot be denied that +in many cases the specific distinction is now maintained +by means of that sexual isolation which cross-infertility +confers: it is therefore probable that such +isolation has been instrumental in securing its initial +attainment.</p> + +<p>This probability is strengthened by the observed +fact that the general association in question is +conspicuously absent in the case of domesticated +varieties, notwithstanding that their multitudinous +and diverse varietal characters usually equal, and +frequently surpass, specific characters in their degrees +of divergence.</p> + +<p>Since, then, it would seem to be impossible for +divergent evolution on common areas to take place +in the absence of some mode of isolation; since +cross-infertility appears to be the only possible mode +under the given circumstances; and since among +domesticated varieties, where isolation is otherwise +secured by artificial means, cross-infertility is usually<span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span> +absent, the logical foundations of the theory of +physiological selection would seem to be securely laid.</p> + +<p>We may therefore pass to more special lines of +evidence.</p> + + +<h3><i>Evidence from Geographical Distribution.</i></h3> + +<p>Darwin has adduced very good evidence to show +that large areas, notwithstanding the disadvantages +which (on his theory) must arise from free intercrossing, +are what he terms better manufactories of +species than smaller areas, such as oceanic islands. +On the other hand, as a matter of fact, oceanic +islands are comparatively rich in peculiar species. +These two statements, however, are not incompatible. +Smaller areas are, as a rule, rich in peculiar species +relatively to the number of their inhabitants; but +it does not follow that they are rich in species as +contrasted with larger areas containing very many +more inhabitants. Therefore, the rules are that +large areas turn out an absolutely greater number +of specific types than small areas; although, relatively +to the number of individuals or amount of population, +the small areas turn out a larger number of species +than the large areas.</p> + +<p>Now, these two complementary rules admit of +being explained as Darwin explains them. Small +and isolated areas are rich in species relatively to +the amount of population, because, as we have before +seen, this population has been permitted to develop +an independent history of its own, shielded from +intercrossing with parent forms, and from competition +with exotic forms; while, at the same time, the +homogamy thus secured, combined with change of<span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span> +environment, will give natural selection an improved +chance of finding new points of departure for its +operation. On the other hand, large and continuous +areas are favourable to the production of numerous +species, first, because they contain a large population, +thus favouring the occurrence of numerous variations; +and, secondly, because the large area furnishes +a diversity of conditions in its different parts, as to +food, climate, attitude, &c., and thus so many +different opportunities for the occurrence of sundry +forms of homogamy. Now, it is obvious that of all +these sundry forms of homogamy, physiological +selection must have what may be termed a first-rate +opportunity of assisting in the manufacture of species +on large areas. For not only is it upon large and +continuous areas that the antagonistic effects of +intercrossing are most pronounced (and, therefore, +that the influence of physiological selection must be +most useful in the work of species-making); but here +also the diversity in the external conditions of life, +which the large area supplies to different parts of +the extensive population, cannot fail to furnish physiological +selection with a greater abundance of that +particular variation in the reproductive system on +which its action depends. Again, and of still more +importance, on large areas there are a greater <i>number</i> +of species already differentiated from one another +as such; thus a greater number of already sexually +differentiated forms are presented for further differentiation +at the hands of physiological selection. For +all these reasons, therefore, we might have expected, +upon the new theory, that large and continuous areas +would be good manufactories of species.</p> + +<p><span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span> +Again, Darwin has shown that not only large +areas, but likewise "dominant" genera within those +areas, are rich in species. By dominant genera he +meant those which are represented by numerous +individuals, as compared with other genera inhabiting +the same area. This general rule he explains by the +consideration that the qualities which first led to the +form being dominant must have been useful; that +these would be transmitted to the otherwise varying +offspring; and, therefore, that when these offspring +had varied sufficiently to become new species, they +would still enjoy their ancestral advantages in the +struggle for existence. And this, doubtless, is in part +a true explanation; but I also think that the reason +why dominant genera are rich in species, is chiefly +because they everywhere present a great number of +individuals exposed to relatively great differences in +their conditions of life: or, in other words, that they +furnish the best raw material for the manufacture of +species by physiological selection, as explained in +the last paragraph. For, if the fact of dominant +genera being rich in species is to be explained <i>only</i> +by natural selection, it appears to me that the useful +qualities which have already led to the dominance +of the ancestral type ought rather to have proved +inimical to its splitting up into a number of subordinate +types. If already so far "in harmony with +its environment" as to have become for this reason +dominant, one would suppose that there is all the +more reason for its not undergoing change by the +process of natural selection. Or, at least, I do not +see why the fact of its being in an unusual degree +of harmony with its environment should in itself<span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span> +constitute any unusual reason for its modification by +survival of the fittest. On the other hand, as just +observed, I do very plainly see why such a reason +is furnished for the modifying influence of physiological +selection.</p> + +<p>Let us next turn to another of Darwin's general +rules with reference to distribution. He took a great +deal of trouble to collect evidence of the two following +facts, namely, (1) that "species of the larger genera +in each country vary more frequently than the species +of the smaller genera"; and (2) that "many of the +species included within the larger genera resemble +varieties in being very closely, but unequally, related +to each other, and in having restricted ranges<a name="FNanchor_22_22" id="FNanchor_22_22"></a><a href="#Footnote_22_22" class="fnanchor">[22]</a>." +By larger genera he means genera containing many +species; and he accounts for these general facts by +the principle, "that where many species of a genus +have been formed, on an average many are still +forming." But <i>how</i> forming? If we say by natural +selection alone, we should expect to find the multitudinous +species differing from one another in respect +of features presenting well-marked adaptive meanings; +yet this is precisely what we do not find. For +Darwin's argument here is that "in large genera the +amount of difference between the species is often +exceedingly small, so that in this respect the species +of the larger genera resemble varieties more than do +the species of the smaller genera." Therefore the +argument, while undoubtedly a very forcible one in +favour of the fact of <i>evolution</i>, appears to me scarcely +consistent with the view of this evolution being due +solely to natural selection. On the other hand, the<span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span> +argument tells strongly (though unconsciously) in +favour of physiological selection. For the larger a +genus, or the greater the number of its species, the +greater must be the opportunity for the occurrence +of that particular kind of variation on which the +principle of physiological selection depends. The +species of a genus may be regarded as so many +varieties which have already been separated from one +another physiologically; therefore each of them may +now constitute a new starting-point for a further and +similar separation—particularly as, in virtue of their +previous segregation, many are now exposed to +different conditions of life. Thus, it seems to me, +we can well understand why it is that genera already +rich in species tend to grow richer; while such is not +the case in so great a degree with genera that are +poor in species. Moreover, we can well understand +that, multiplication of species being as a rule, and in +the first instance, determined by changes in the reproductive +system, wherever a large number of new +species are being turned out, the secondary differences +between them should be "often exceedingly small"—a +general correlation which, so far as I can see, we +are not able to understand on the theory of natural +selection.</p> + +<p>The two subsidiary facts, that very closely allied +species have restricted ranges, and that dominant +species are rich in varieties, both seem to tell more +in favour of physiological than of natural selection. +For "very closely allied species" is but another name +for species which scarcely differ from one another +at all except in their reproductive systems; and, +therefore, the more restricted their ranges, the more<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span> +certainly would they have become fused by intercrossing +with one another, had it not been for the +barrier of sterility imposed by the primary distinction. +Or rather, I should say, had it not been +for the original occurrence of this barrier, these now +closely-allied species could never have become species. +Again, that dominant species should be rich in varieties +is what might have been expected; for the +greater the number of individuals in a species, the +greater is the chance of variations taking place in +all parts of the organic type, and particularly in the +reproductive system, seeing that this system is the +most sensitive to small changes in the conditions +of life, and that the greater the number of individuals +composing a specific type, the more certainty +there is of some of them encountering such +changes. Hence, the richness of dominant species +in varieties is, I believe, mainly due to the greater +opportunity which such species afford of some degree +of cross-infertility arising between their constituent +members.</p> + +<p>Here is another general fact, also first noticed by +Darwin, and one which he experiences some difficulty +an explaining on the theory of natural selection. He +says:—</p> + +<blockquote><p>In travelling from north to south over a continent, we generally +meet at successive intervals with closely-allied or representative +species, evidently filling the same place in the economy of the +land. These representative species often meet and interlock, +and as one becomes rarer and rarer, the other becomes more and +more frequent, till the one replaces the other. But if we compare +these species where they intermingle, they are generally as +absolutely distinct from each other in every detail of structure as +are specimens taken from the metropolis of each.... In the<span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span> +intermediate region, having intermediate conditions of life, why +do we not now find closely-linking intermediate varieties? This +difficulty for a long time quite confounded me. But I think it +can in large part be explained<a name="FNanchor_23_23" id="FNanchor_23_23"></a><a href="#Footnote_23_23" class="fnanchor">[23]</a>.</p></blockquote> + +<div class="figcenter" style="width: 600px;"> +<img src="images/illus-083.png" width="600" height="74" alt="" /> +</div> + +<p>His explanation is that, "as the neutral territory +between two representative species is generally narrow +in comparison with the territory proper to each, +... and as varieties do not essentially differ from +species, the same rule will probably apply to both; and, +therefore, if we take a varying species inhabiting +a very large area, we shall have to adapt two varieties +to two large areas, and a third variety to a narrow +intermediate zone." It is hence argued that this +third or intermediate variety, on account of its existing +in lesser numbers, will probably be soon overrun and +exterminated by the larger populations on either side +of it. But how is it possible "to adapt two varieties +to two large areas, and a third [transitional] variety +to a narrow intermediate zone," in the face of free +intercrossing on a continuous area? Let <i>A</i>, <i>B</i>, and +<i>C</i> represent the three areas in question. According to +the argument, variety <i>A</i> passes first into variety <i>B</i>, +and then into variety <i>C</i>, while variety <i>B</i> eventually +becomes exterminated by the inroads both from +<i>A</i> and <i>C</i>. But how can all this have taken place +with nothing to prevent intercrossing throughout the +entire area <i>A</i>, <i>B</i>, <i>C</i>? I confess that to me it seems this +argument can only hold on the supposition that the +analogy between varieties and species extends to the<span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span> +reproductive system; or, in a sense more absolute +than the argument has in view, that "varieties do +not essentially differ from the species" which they +afterwards form, but from the first show some +degree of infertility towards one another. And, if so, +we have of course to do with the principles of physiological +selection.</p> + +<p>That in all such cases of species-distribution these +principles have played an important part in the +species-formation, appears to be rendered further +probable from the suddenness of transition on the +area occupied by contiguous species, as well as from +the completeness of it—i. e. the absence of connecting +forms. For these facts combine to testify that the +transition was originally due to that particular change +in the reproductive systems of the forms concerned, +which still enables those forms to "interlock" without +intercrossing. On the other hand, neither of these +facts appears to me compatible with the theory of +species-formation by natural selection alone.</p> + +<p>But this leads us to another general fact, also +mentioned by Darwin, and well recognized by all +naturalists, namely, that closely allied species, or +species differing from one another in trivial details, +usually occupy contiguous areas; or, conversely stated, +that contiguity of geographical position is favourable +to the appearance of species closely allied to one +another. Now, the large body of facts to which +I here allude, but need not at present specify, appear +to me to constitute one of the strongest of all my +arguments in favour of physiological selection. Take, +for instance, a large continental area, and follow across +it a chain of species, each link of which differs from<span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span> +those on either side of it by the minute and trivial +distinctions of a secondary kind, but all the links +of which differ from one another in respect of the +primary distinction, so that no one member of the +series is perfectly fertile with any other member. Can +it be supposed that in every case this constant +primary distinction has been superinduced by the +secondary distinctions, distributed as they are over +different parts of all these kindred organisms, and +yet nowhere presenting any but a trifling amount of +morphological change?</p> + +<p>For my own part, I cannot believe—any more +than Darwin could believe—that all these numerous, +diverse, and trivial changes have always had the +accidental effect of inducing the same peculiar change +in the reproductive system, and so producing it without +any reference to the process of specific divergence. +Nor can I believe, as Darwin incidentally and provisionally +suggested, that prolonged exposure to +uniform conditions of life have so generally induced +an equally meaningless result. I can only believe +that all the closely allied species inhabiting our +supposed continent, and differing from one another +in so many and such divers points of small detail, are +merely so many records of the fact that selective +fertility has arisen among their ancestry, and has +thus given as many opportunities for the occurrence +of morphological differentiations as it has furnished +cases of efficient isolation. Of course, I do not deny +that many, or probably most, of these trivial morphological +differentiations have been produced by natural +selection on account of their utility: I merely deny +that they could have been so produced on this<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span> +common area, but for the sexual isolation with which +every distinct set of them is now found to be associated.</p> + + +<h3><i>Evidence from Topographical Distribution of +Species.</i></h3> + +<p>By topographical distribution I mean the distribution +of organisms with reference to comparatively +small areas, as distinguished from larger regions with +reference to which the term geographical distribution +is appropriate.</p> + +<p>It will be at once apparent that a study of the +topographical distribution of organic types is of even +more importance for us than a study of their geographical +distribution. For while the former study is +conducted, as it were, with a low power of our +observing microscope, the latter is conducted with +a high power. The larger facts of geographical +distribution yield, indeed, all the general characters +which we might expect them to yield, on the theory +that divergence of specific types on common areas +has been in chief part determined by physiological +conditions. But for the purpose of testing this +theory in a still more exacting manner, it is of the +first importance to consider the more detailed facts +of topographical distribution, since we here come to +closer quarters with the problem of specific differentiation. +Therefore, as we have already considered +this problem under the most general points of view, +we will now consider it under more special points +of view.</p> + +<hr class="tb" /> + +<p>It is self-evident, as we have seen in the preceding<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span> +section, that the greater the number of individuals +of the same species on a given area, the less must +be the power of natural selection to split that species +into two or more allied types; because, the more +crowded the population, the greater must be the +uniformitarian effect of free intercrossing. This obvious +fact has been insisted upon by several previous +writers on Darwinism; and the only reason why it +has not been recognized by all naturalists is that so +few of them have observed the all-important distinction +between monotypic and polytypic evolution. +The denser the population, and therefore the greater +the intercrossing and the severer the struggle for +existence within the species, the better will it be +for <i>transmutation</i> of the species by natural selection; +but the worse it will be for <i>differentiation</i> of the +species by this form of homogamy. On the other +hand, if physiological selection be entertained as +a form of homogamy, the denser the population, the +better opportunity it will have of differentiating the +species, first, because a greater number of individuals +will be present in which the physiological change +may arise, and, secondly, because, if it does arise, the +severity of the struggle for existence will <i>then</i> give +natural selection a better chance of acting rapidly +and effectually on each of the isolated sections.</p> + +<p>Hence, where the question is whether selective +fertility has played any large or general part in the +differentiation of specific types, the best criterion we +can apply is to ascertain whether it is a general +rule that closely allied species occur in intimate +association, so that their individual members constitute, +as it were, a single population, or, on the<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span> +other hand, whether they occur rather on different +sides of physical barriers. If they occur intimately +associated, the form of homogamy to which their +differentiation was due must have presumably been +the physiological form; whereas, if they are proved +to be correlated with physical barriers, the form of +homogamy which was concerned in their differentiation +must presumably have been the geographical +form.</p> + +<p>Now, at first this consideration was a trouble to +me, because Moritz Wagner had strenuously argued—and +supported his argument by a considerable +wealth of illustration—that allied species are always +found correlated with physical barriers or discontinuous +areas. Weismann's answer, indeed, had +shown that Wagner's statement was much too general: +nevertheless, I was disappointed to find that so +much could be said in favour of the geographical +(or topographical) form of isolation where closely +allied species are concerned. Subsequently, however, +I read the writings of Nägeli on this subject, and +in them I find a very different state of matters +represented.</p> + +<p>Seeing as clearly as Wagner that it is impossible +under any circumstances for natural selection to +cause specific <i>differentiation</i> unless assisted by some +other forms of homogamy, but committing the same +oversight as Wagner and Weismann in supposing +that the only other form of homogamy in nature is +geographical isolation, Nägeli, with great force of +reasoning, and by many examples, founded his argument +against the theory of natural selection on the +ground that in the vegetable kingdom closely allied<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span> +species are most frequently found in intimate association +with one another, not, that is to say, in any +way isolated by means of physical barriers. This +argument is everywhere logically intact; and, as he +sustains it by a large knowledge of topographical +botany, his indictment against natural selection as +a cause of specific <i>differentiation</i> appeared to be +insurmountable. And, in point of fact, it <i>was</i> insurmountable; +so that the whole problem of the +origin of species by <i>differentiation on common areas</i> +has hitherto been left in utter obscurity. Nor is there +now any escape from this obscurity, unless we entertain +the "supplementary factor" of selective fertility. +And, apparently, the only reason why this has not +been universally recognized, is because Darwinians +have hitherto failed to perceive the greatness of the +distinction between the <i>differentiation</i> and the <i>transmutation</i> +of species; and hence have habitually met +such overwhelming difficulties as Nägeli presented by +an illogical confounding of these two totally distinct +things.</p> + +<p>But if the idea of selective fertility had ever +occurred to Nägeli as a form of segregation which +gives rise to specific differentiation, I can have no +doubt that so astute and logical a thinker would +have perceived that his whole indictment against +natural selection was answered. For it is incredible +that he should not have perceived how this physiological +form of homogamy (supposing it to arise <i>before</i> +or <i>during</i>, and not <i>after</i> the specific differentiation) +would perform exactly the same function on a continuous +area, as he allowed that "isolation" does on +a discontinuous one.</p><p><span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span></p> + +<p>However, be this as it may, there cannot be any +question touching the immense value of his facts and +arguments as evidence in favour of physiological +selection—albeit this evidence was given unconsciously, +or, as it were, prophetically. Therefore +I will here quote a few examples of both, from his +paper <i>Du Développement des Espèces Sociales</i><a name="FNanchor_24_24" id="FNanchor_24_24"></a><a href="#Footnote_24_24" class="fnanchor">[24]</a>.</p> + +<p>After stating the theory of natural selection, he +says that if the theory is (of itself) a true explanation +of the origin (or divergence) of specific forms, it +ought to follow that</p> + +<blockquote><p>two closely allied forms, derived the one from the other, +would necessarily occupy two different geographical areas [or +topographical stations], since otherwise they would soon become +blended. Until they had already become sufficiently consolidated +as distinct species to render mutual intercrossing highly improbable, +they could not be intermingled without disadvantage +[to differentiation]. Had Darwin endeavoured to support his +hypothesis by facts, he would, at least in the vegetable kingdom, +have found little to favour his cause. I can cite many hundreds +of cases, in which species in every stage of development have +been found closely mingling with one another, and not in any +way isolated. Therefore, I do not think that one can rightly +speak of natural selection in the Darwinian sense in the +vegetable kingdom; and, in my estimation, there is a great +difference between the formation of species by nature and the +production of stock by a breeder.... (p. 212).</p> + +<p>Of the two kinds of distribution (i. e. growing apart and +growing together), Synoicy (or growing together) is by far +the most usual in nature. I reckon that out of a hundred +allied vegetable forms, at least ninety-five would be found to be +synoical (p. 219).</p></blockquote> + +<p>This is a most important point. That so enormous<span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span> +a proportion of vegetable species should have originated +in intimate association with their parent or +sister types, is clearly unintelligible on the theory of +natural selection alone; there obviously <i>must</i> be some +other form of homogamy which, whether or not in +all places <i>associated</i> with natural selection, is the +primary condition to the differentiation. Such +I hold with Nägeli, is a logical necessity; and this +whether or not I am right in believing the other +form of homogamy in question to be selective fertility. +But I go further and say, Surely there can be no +rational question that this other form of homogamy +must have been, at any rate as a highly general rule, +the one which I have assigned. For how is it that +in these ninety-five per cent. of cases, where vegetable +species are growing intimately associated with their +nearest allies, there is no hybridizing, or blending +and relapsing to the original undifferentiated types? +We know well the answer. These are fully differentiated +species, and, as such, are protected from mutual +intercrossing by the barrier of mutual sterility. But +now, if this bar is thus necessary for preserving the +specific distinctions when they have been fully +developed, much more must it have been so to admit +of their development; or, otherwise stated, since we +know that this barrier is associated with "synoical" +species, and since we clearly perceive that were it +withdrawn these species would soon cease to exist, +can we reasonably doubt that their existence (or +origin) is due to the previous erection of this +barrier? If synoical species were comparatively +rare, the validity of such reasoning might be open +to question; or, even if we should not doubt it in<span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span> +such cases, at any rate we might well doubt the +importance or extent of selective fertility as a factor +in the origination of species. But the value of +Nägeli's writings on the present subject consists in +showing that synoical species constitute so overwhelming +a majority of the vegetable kingdom, that +here, at all events, it appears impossible to rate too +highly the importance of the principle I have called +physiological selection.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></p> + + + + +<h2>CHAPTER V.<br /> +<span class="smcap">Further Evidences of Physiological +Selection.</span></h2> + + +<h3><i>Evidence from Topographical Distribution of +Varieties.</i></h3> + +<p>In the last section we have considered the topographical +distribution of closely allied <i>species</i>. I now +propose to go still further into matters of detail, by +considering the case of natural <i>varieties</i>. And here +we come upon a branch of our inquiry where we may +well expect to meet with the most crucial tests of +our theory. For if it should appear that these nascent +species more or less resemble fully developed species +in presenting the feature of cross-infertility, the theory +would be verified in the most direct and conclusive +manner possible. These nascent species may be +called embryo species, which are actually in course +of differentiation from their parent-type; and therefore, +if they do not exhibit the feature in relation +to that type which the present theory infers to be +necessary for the purposes of differentiation, the +theory must be abandoned. On the other hand, if +they do exhibit this feature, it is just the feature +which the theory predicted as one that would be +found highly characteristic of such embryo types.<span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span> +Contrariwise, the theory of natural selection can have +no reason to form any such anticipation; or rather +its anticipation would necessarily require to be the +exact opposite. For, according to this theory, the +cross-infertility of allied species is due, either to +correlation with morphological changes which are +being produced by the selection, or else, as Darwin +supposed, to "prolonged exposure to uniform conditions +of life"; and thus, in either case, the sterility +variation ought to be, as a general rule at all events, +subsequent to the specific differentiation, and, according +to Darwin's view, <i>long</i> subsequent. Thus +we ought not to find that the physiological change +is ever, on any large or general scale, the initial +change; nor ought we to find that it is, on any +such scale, even so much as a contemporary change: +there ought, in fact, to be no constant or habitual +association between divergence of embryo-types and +the concurrence of cross-infertility.</p> + +<p>Now, it will be my endeavour to prove that +there is an extraordinarily general association between +<i>varietal</i> divergence and cross-infertility, <i>wherever +common areas are concerned</i>; and in as far as this +can be proved, I take it that the evidence will make +wholly in favour of physiological selection as the +prime condition to specific divergence, while at the +same time they will make no less wholly, <i>and quite +independently</i>, against natural selection as the unaided +cause of such divergence.</p> + +<p>I shall begin with some further quotations from +Nägeli.</p> + +<blockquote><p>Species may be synoical at all stages of relationship. We +come across varieties, scarcely distinguishable from one another,<span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span> +growing in the same locality (as, for example, the <i>Cirsium +heterophyllum</i>, with smooth or jagged leaves, the <i>Hieracium +sylvaticum</i>, with or without caulinary leaves); again, we meet +other varieties more accentuated (as the <i>H. hoppeanum</i>, with +under ligules of white or red, the <i>Campanula</i>, with white or lilac +flowers, &c.), other varieties even more marked, which might +almost be elevated to the rank of species (<i>Hieracium alpinum</i>, +with hairs and glands, and the new form <i>H. holadenium</i>, which +has only glands, <i>Campanula rotundifolia</i> with smooth and hairy +leaves), or forms still more distinct, up to well-defined species. +I could enumerate endless examples at all stages.</p> + +<p>It will be seen that in my definition of synoicy I do not mean +to assert that <i>all</i> allied forms are invariably found together, but +that they are much more often seen in groups than singly. +Take, for instance, nine forms closely related (<i>A</i> to <i>I</i>). <i>A</i>, <i>E</i>, <i>H</i> +will be found side by side at one point, <i>B</i>, <i>D</i> at another, <i>C</i>, <i>F</i> +at a third, &c. These facts are plainly opposed to the theory of +isolation and amixia, and make, on the contrary, in favour of the +social development of species (<i>loc. cit.</i>, p. 221).</p></blockquote> + +<p>Not to multiply quotations to the same general effect, +I will supply but one other, referring to a particular +case.</p> + +<blockquote><p>At one spot (<i>Rothwand</i>) much exposed to the sun, and +difficult of access, I remarked two closely allied forms, so nearly +related to <i>H. villosum</i> that this would seem to be an intermediary +form between the two. One of these (<i>H. villosissimum</i>) +is distinguished by its tongue and thick pubescence, its tolerably +large capitula, and by the lengthened and separated scales of +the involucrum; the other, on the contrary (<i>H. elongatum</i>), is +less pubescent, has smaller capitula, and more compact scales +on the involucrum than <i>H. villosum</i>. Both are finally distinguishable +from the type by their longer stalks, which are more +decidedly aphyllous, and by their later flowering. At the spot +where I found them the two forms were closely intermingled, +and each was represented by a considerable number of plants. +I did not find them anywhere else on the mountain, nor could +I find at the spot where these were growing a single specimen +of the true <i>H. villosum</i>, nor a single hybrid from these two.</p><p><span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span></p> + +<p>I concluded that these two new forms had, by joining their +forces, expelled the <i>H. villosum</i> from its primitive abode, but +had not succeeded in displacing one another. As to their origin, +they had evidently developed in two different directions from +a common point of departure, namely <i>H. villosum</i>. They had +succeeded, not only in separating themselves from the original +form, but also in preventing any intermediary form from interposing. +I thought myself therefore justified in considering this +as a case of varieties which have come into existence subsequently +to the Glacial epoch. The morphological characteristics of the +three forms are sufficiently distinct for them to be designated as +species by a good many writers. They are better defined than +some of MM. Frolich and Fries' weaker species, and as well +defined as some of MM. Koch and Grisebach's (p. 222).</p></blockquote> + +<p>Now it is clear, without comment, that all this is +exactly as it ought to be, if allied species have been +differentiated on common areas by selective fertility. +For if, as Nägeli elsewhere says, "one meets forms +in nature associated with one another, and severally +distinguished by every possible degree of differentiation," +not only as Nägeli adds, does this general +fact lead to the inference that species are (usually) +developed when plants grow intimately associated +together; but as certainly it leads to the further +inference that such development must be due to +a prior development of cross-infertility between the +diverging varietal forms, cross-infertility which is +therefore afterwards so characteristic of the allied +species, when these are found, in their fully differentiated +condition, still occupying the same area +in large and intimately mingled populations.</p> + +<p>To my mind there could not be any inference more +strongly grounded than this, because, with the one +exception of the physiological form, no other form +of homogamy can be conceived which shall account<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span> +for the origin and permanence of these synoical +varieties, in all degrees of differentiation up to well-defined +synoical species. Least of all, as we have +seen, can natural selection alone have had anything +to do with such a state of matters; while, as we have +likewise seen, in all its details it is exactly the state +of matters which the theory of physiological selection +requires.</p> + +<p>Nevertheless, although this inference is so strongly +grounded, we ought to remember that it is only an +inference. In order fully to verify the theory of +physiological selection, we ought to prove by experiment +the fact of cross-infertility between these synoical +varieties, as we learn that it afterwards obtains between +synoical species. It is to be regretted that the theory +of physiological selection did not occur to the mind +of Nägeli, because he would then, no doubt, have +ascertained this by actual experiment. As it is, the +great value of his observations goes no further than +establishing a strong presumption, that it <i>must</i> be +selective fertility which causes the progressive differentiation +of synoical varieties; and also that, if +so, this <i>must</i> be the principal factor in the differentiation +of vegetable species, seeing that some ninety-five +per cent. are of synoical origin.</p> + + +<h3><i>Evidence from Experimental Research.</i></h3> + +<p>My paper on <i>Physiological Selection</i> pointed out that +the whole theory would have to stand or fall with the +experimental proof of the presence or the absence of +cross-infertility between varieties of the same species +growing on common areas. From the facts and +considerations which we have hitherto been dealing<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span> +with, it did indeed appear to me that there was the +strongest conceivable ground for inferring that cross-infertility +between such varieties would be found by +experiment to be a phenomenon of highly general +occurrence—amply sufficient ground to prove +that allied species on common areas for the most part +owed their origin to this character of mutual sterility, +and not vice versa as previously supposed. At that +time I was not aware that any experiments had been +made in this direction. Soon after the paper was +published, however, my attention was directed to a +laborious research which had been directed to this +very point, and carried on for more than thirty years, +by M. Jordan<a name="FNanchor_25_25" id="FNanchor_25_25"></a><a href="#Footnote_25_25" class="fnanchor">[25]</a>. This had not attracted the general +notice which it undoubtedly deserved; and I have +since ascertained that even Darwin began to look +into it only a few months before his death.</p> + +<p>Having devoted his life to closely observing in +divers stations multitudes of different species of plants—annuals +and perennials, bulbous and aquatic, trees +and shrubs—M. Jordan has been able to satisfy himself, +and the French school of botanists to which this +line of observation has given rise, that in most cases +(or "nearly everywhere"), when a Linnean species +is indigenous to a country and is there of common +occurrence, this species within that district is represented +by more or less numerous and perfectly constant +varieties. These varieties are constituted by such +minute differences of morphological character that<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span> +their very existence eluded the observation of botanists, +until M. Jordan began to search specially for them as +the special objects of his scrutiny. Moreover, these +varieties of a Linnean species occupy common areas, +and there grow in intimate association with one +another, or as M. Jordan says, "<i>pêle-mêle</i>." So far, +be it noticed, Jordan was proceeding on exactly the +same lines as Nägeli; only he carried his observations +over a still wider range of species on the one +hand, and into a still minuter search for varieties +on the other. But the all-important point for us is, +that he further proceeded to test by experiment the +physiological relations between these morphological +varieties; and found, in many hundreds of cases, +that they not only came true to seed (i. e. are hereditary +and not merely climatic), but likewise cross-sterile +<i>inter se</i>. For these reasons, M. Jordan, who is +opposed to the theory of evolution, regards all such +varieties as separately created species; and the +inspiring motive of his prolonged investigations has +been a desire to multiply these proofs of creative +energy. But it clearly makes no difference, so far +as evolutionists are concerned with them, whether +all this multitude of sexually isolated forms be denominated +species or varieties.</p> + +<p>The points which are of importance to evolutionists—and +of the first order of importance in the +present connexion—may be briefly summarized as +follows:—</p> + +<p>(1) The research embraces large numbers of species, +belonging to very numerous and very varied orders +of plants; (2) in the majority of cases—although not +all—indigenous species which are of common occurrence<span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span> +present constant varieties; (3) these varieties, +nevertheless, may be morphologically so slight as to +be almost imperceptible; (4) they occupy common +areas and grow in intimate association; (5) although +many of them have undergone so small an amount +of morphological change, they have undergone a surprising +amount of physiological change; for (6) not +only do very many of these varieties come true to +seed; but, (7) when they do, they are always more or +less cross-infertile <i>inter se</i>.</p> + +<p>Now, it is self-evident that every one of these seven +points is exactly what the theory of physiological +selection requires, while there is not one of them +which it does not require. For if the theory be +sound, we should expect to find large numbers of +species belonging to numerous and varied orders +of plants presenting constant varieties on common +areas; we should expect this to be a highly general, +though not a universal, rule; and we should expect +it to apply only to species which are indigenous. Moreover, +we should expect these varieties, although but +slightly differentiated morphologically, to present a +great differentiation physiologically—and this in the +special direction of selective fertility, combined, of +course, with heredity.</p> + +<p>On the other hand, as I have said, this catalogue +of evidences leaves nothing to be supplied. It gives +us all the facts—and no more than all the facts—which +my paper on <i>Physiological Selection</i> anticipated +as the eventual result of a prolonged experimental +research. And if I have to regret my ignorance of +these facts when that paper was published, at any +rate it now furnishes the best proof that my anticipations<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span> +were not guided by the results of a verification +which had already been supplied. These anticipations +were deduced exclusively from the theory itself, as +representing what <i>ought</i> to be the case if the theory +were true; and, I must confess, if I had then been +told that they had already been realized—that it +had actually been found to be a general rule that +endemic species present constant and hereditary +varieties, intimately commingled on common areas, +morphologically almost indistinguishable, but physiologically +isolated by selective fertility—I should +have felt that the theory had been verified in +advance. For there are only two alternatives: +either these things are due to physiological selection, +or else they are due—as M. Jordan himself believes—to +special creation. Which is equivalent to saying +that, for evolutionists, the facts must be held +to verify the former theory in as complete a manner +as it is logically possible for the theory to be +verified.</p> + + +<h3><i>Evidence from Prepotency.</i></h3> + +<p>We have now to consider the bearing of what is +called "prepotency" on the theory of physiological +selection.</p> + +<p>Speaking of the vast number of species of Compositae, +Darwin says:—</p> + +<blockquote><p>There can be no doubt that if the pollen of all these species +could be simultaneously or successively placed on the stigma of +any one species, this one would elect with unerring certainty its +own pollen. This elective capacity is all the more wonderful, as +it must have been acquired since the many species of this great +group of plants branched off from a common progenitor.</p></blockquote><p><span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span></p> + +<p>Darwin is here speaking of elective affinity in +its fully developed form, as absolute cross-sterility +between fully differentiated species. But we meet +with all lower degrees of cross-infertility—sometimes +between "incipient species," or permanent varieties, +and at other times between closely allied species. +It is then known as "prepotency" of the pollen +belonging to the same variety or species over the +pollen of the other variety or species, when both sets +of pollen are applied to the same stigma. Although +in the absence of the prepotent pollen the less potent +will fertilize the seed, yet, such is the appetency for +the more appropriate pollen, that even if this be +applied to the stigma some considerable time after +the other, it will outstrip or overcome the other in +fertilizing the ovules, and therefore produce the +same result on the next generation as if it had been +applied to the mother plant without any admixture +of the less potent pollen, although in some cases such +incipient degrees of cross-infertility are further shown +by the number or quality of the seeds being fewer +or inferior.</p> + +<p>Now, in different varieties and in different allied +species, all degrees of such prepotency have been +noticed by many observers, from the faintest perceptible +amount up to complete impotency of the +alien pollen—when, of course, there is absolute +sterility between the two varieties or allied species. +The inference is obvious. In this graduated scale +of prepotency—beginning with an experimentally +almost imperceptible amount of sexual differentiation +between two varieties, and ending in an absolute +partitioning of two allied species—we have the only<span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span> +remaining fact that is required to complete the case +in favour of the present theory. We are here brought +back to the very earliest stages of physiological differentiation +or to the stages which lie behind Jordan's +"Physiological Species"; and therefore, when taken +in conjunction with his results, the phenomena of +prepotency may be said to give us the complete and +final demonstration of one continuous development, +which, beginning in an almost imperceptible amount +of cross-infertility, ends in absolute cross-sterility. +The "elective capacity" to which Darwin alludes as +having been "acquired" by all the species of Compositae +since they "branched off from a common +progenitor," is thus seen among innumerable other +species actually in process of acquisition; and so +we can perfectly well understand, what is otherwise +unintelligible, that closely allied species of plants +occur, in ninety-five per cent. of cases, intimately associated +on common areas, while exhibiting towards one +another the character of mutual sterility.</p> + +<p>But more than this. The importance of the widespread +phenomena of prepotency to the theory of +physiological selection does not consist merely in +thus supplying the last link in the chain of evidence +touching the origin of species by selective fertility, +or "elective capacity." These phenomena are of +further importance as showing how in plants, at all +events, physiological selection appears to be frequently +capable of differentiating specific types without the +necessary assistance of any other form of homogamy. +In my original statement of the theory, I was careful +to insist upon the great value, as differentiating agents, +of even small degrees of other forms of homogamy<span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span> +when co-operating with physiological selection. But +I also stated my belief that in many cases selective +fertility is presumably of itself capable of splitting +a specific type; and the reason why I still believe +this is, that I do not otherwise understand these phenomena +of prepotency. I cannot believe that in all the +innumerable cases where they arise, they have been +super-induced by some prior morphological changes +going on in some other part of the organism, or by +"prolonged exposure to uniform conditions of life," +on the part of two well-nigh identical forms which +have arisen intimately commingled in exactly the +same environment, and under the operation of a previously +universal intercrossing. Even if such a thing +could be imagined as happening occasionally, I feel +it difficult to imagine that it can happen habitually, +and yet this view must be held by those who would +attribute prepotency to natural selection.</p> + +<p>It must never be forgotten that the relatively +enormous changes as to size, structure, habit, &c., +which are presented by our domesticated plants as +results of artificial selection, do not entail the physiological +character of cross-sterility in any degree, +save possibly in some small number of cases. Although +in wild species any correspondingly small percentage +of cases (where natural selection happens to hit upon +parts of the organism modifications of which produce +the physiological change by way of correlation) would +doubtless be the ones to survive on common areas, +still it is surely incredible that such an accidental +association between natural selection and cross-infertility +is so habitually the means of specific +differentiation as the facts of prepotency (together<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span> +with the observations of Jordan and Nägeli) would +necessarily demand.</p> + +<p>Moreover, this view of the matter is still further +corroborated by certain other facts and considerations. +For example, the phenomena of prepotency (whether +as between varieties or between closely allied species) +are found to occur when the two forms occupy a +common area, i.e. are growing intermingled with +one another. Therefore, but for this physiological +differentiation, there could be absolutely nothing to +prevent free intercrossing. Yet the fact that hybrids +are so comparatively rare in a state of nature—a fact +which Sir Joseph Hooker has pointed out to me as +otherwise inexplicable—proves the efficacy of even +a low degree of such differentiation in preventing +the physiologically-differentiated forms from intercrossing. +Even in cases where there is no difficulty +in producing artificial hybrids or mongrels between +species or varieties growing on common areas, it is +perfectly astonishing what an extremely small percentage +of the hybrid or mongrel forms are found to +occur in nature. And there can be no question that +this is due to the very efficient manner in which +prepotency does its work—efficient, I mean, from +the point of view of the new theory; for upon any +other theory prepotency is a meaningless phenomenon, +which, notwithstanding its frequent occurrence, +plays no part whatever in the process of organic +evolution.</p> + +<p>I attach considerable importance to the phenomena +of prepotency in view of the contrast which is presented +between plants and animals in the relation of +their species to physical barriers. For animals—and<span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span> +especially the higher animals—appear to depend +for their specific differentiations upon such barriers +much more than in the case with plants. This is no +more than we should expect; for, in accordance with +our theory, selective fertility is not so likely to work +alone in the case of the higher animals which mate +together, as in plants which are fertilized through the +agency of wind or insects. In the former case there +is no opportunity given for the first rise of cross-infertility, +in the form of prepotency; and even where +selective fertility has gained a footing in other ways, +the chances against the suitable mating of "physiological +complements" must be much greater than it +is in the latter case. Hence, among the higher animals, +selective fertility ought much more frequently to be +found in association with other forms of homogamy +than it is among plants. And this is exactly what +we find. Thus it seems to me that this contrast +between the comparative absence and presence of +physical barriers, where allied species of plants and of +higher animals are respectively concerned, is entitled +to be taken as a further corroboration of our theory. +For while it displays exactly such a general correlation +as this theory would expect, the correlation is +one which cannot possibly be explained on any other +theory. It is just where physiological selection can +be seen to have the best opportunity of acting (viz. +in the vegetable kingdom) that we find the most +unequivocal evidence of its action; while, on the +other hand, it is just where it can be seen to have +the least opportunity of asserting itself (viz. among +the higher animals) that we find it most associated +with, and therefore assisted by, other forms of homogamy,<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span> +i. e. not only geographical isolation, but also +by sexual preference in pairing, and the several +other forms of homogamy, which Mr. Gulick has +shown to arise in different places as the result of +intelligence.</p> + + +<h3><i>Evidence from Special Cases.</i></h3> + +<p>Hitherto I have been considering, from the most +general point of view, the most widespread facts +and broadest principles which serve to substantiate +the theory of physiological selection. I now pass +to the consideration of one of those special cases in +which the theory appears to have been successfully +applied.</p> + +<p>Professor Le Conte has adduced the fossil snails +of Steinheim as serving to corroborate the theory of +physiological selection<a name="FNanchor_26_26" id="FNanchor_26_26"></a><a href="#Footnote_26_26" class="fnanchor">[26]</a>.</p> + +<p>The facts are these. The snail population of this +lake remain for a long time uniform and unchanged. +Then a small percentage of individuals suddenly began +to vary as regards the form of their shells, and this in +two or three directions at the same time, each affected +individual, however, only presenting one of the variations. +But after all these variations had begun to +affect a proportionally large number of individuals, +some individuals occur in which two or more of the +variations are blended together, evidently, as Weismann +says, by intercrossing of the varieties so blended. +Later still, both the separate varieties and their +blended progeny became more and more numerous, +and eventually a single blended type, comprising +in itself all the initial varieties, supplanted the<span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span> +parent form. Then another long period of stability +ensued until another eruption of new variations took +place; and these variations, after having affected +a greater and greater number of individuals, eventually +blended together by intercrossing and supplanted +their parent form. So the process went on, +comparatively short periods of variation alternating +with comparatively long periods of stability, the +variations, moreover, always occurring suddenly in +crops, then multiplying, blending together, and in +their finally blended type eventually supplanting their +parent form.</p> + +<p>Now, the remarkable fact here is that whenever the +variations arose, they only intercrossed between themselves, +they did not intercross with their parent form; +for, if they had, not only could they never have +survived (having been at first so few in number and +there having been no geographical barriers in the +small lake), but we should have found evidence of +the fact in the half-bred progeny. Moreover, natural +selection can have had nothing to do with the process, +because not only are the variations in the form of the +shells of no imaginable use in themselves; but it +would be preposterous to suppose that at each of these +"variation periods" several different variations should +always have occurred simultaneously, all of which were +of some hidden use, although no one of them ever +occurred during any of the prolonged periods of +stability. How, then, are we to explain the fact that +the individuals composing each crop of varieties, while +able to breed among themselves, never crossed with +their parent form? These varieties, each time that +they arose, were intimately commingled with their<span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span> +parent form, and would certainly have been reabsorbed +into it had intercrossing in that direction +been possible. With Professor Le Conte, therefore, +I conclude that there is only one conceivable answer +to this question. Each crop of varieties must have +been <i>protected from intercrossing with their parent +form</i>.</p> + +<p>They must have been the result of a variation, which +rendered the affected individuals sterile with their +parent form, whilst leaving them fertile amongst themselves. +The progeny of these individuals would then +have dispersed through the lake, physiologically isolated +from the parent population, and especially prone to +develop secondary variations as a direct result of the +primary variation. Thus, as we might expect, two or +three variations arose simultaneously, as expressions +of so many different lines of family descent from the +original or physiological variety; these were everywhere +prevented from intercrossing with their parent +form, yet capable of blending whenever they or their +ever-increasing progeny happened to meet. Thus, +without going into further details, we are able by +the theory of physiological selection to give an explanation +of all these facts, which otherwise remain +inexplicable.</p> + +<hr class="tb" /> + +<p>In view of the evidence which has now been presented, +I will now ask five questions which must be +suitably answered by critics of the theory of physiological +selection.</p> + +<p>1. Can you doubt that the hitherto insoluble problem +of inter-specific sterility would be solved, supposing +cross-infertility were proved to arise before or<span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span> +during the process of specific differentiation, instead +of after that process had been fully completed?</p> + +<p>2. Can you doubt, after duly considering the circumstances +under which allied species of plants have +been differentiated—viz. in ninety-five per cent. of +cases intimately commingled on common areas, and +therefore under identical environments—that cross-infertility +<i>must</i> have arisen before or during the +specific differentiation?</p> + +<p>3. Can you doubt, after duly considering the facts +of prepotency on the one hand and those of Jordan's +physiological varieties on the other, that cross-infertility +<i>does</i> arise before or during the specific differentiation?</p> + +<p>4. If you cannot express a doubt upon any of these +points, can you explain why you refuse to accept the +theory of the origin of species by means of physiological +selection, together with the explanation which +this theory affords of the continued cross-fertility of +domesticated varieties?</p> + +<p>5. Supposing this theory to be true, can you conceive +of any other classes of facts which, either +quantitatively or qualitatively, could more directly or +more effectually prove its truth than those which have +now been adduced?</p> + +<p>On these five heads I entertain no doubt. I am +convinced that the theory of physiological selection is +the only one that can explain the facts of inter-specific +sterility on the one hand, and, on the other hand, the +contrast which these facts display to the unimpaired +fertility of our domesticated varieties.</p> + +<p>In conclusion, it seems desirable once more to insist +that there is no antagonism or rivalry between the<span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span> +theories of natural and of physiological selection. For +which purpose I will quote the final paragraph of my +original paper.</p> + +<blockquote><p>So much, then, for the resemblances and the differences +between the two theories. It only remains to add that the two +are complementary. I have already shown some of the respects +in which the newer theory comes to the assistance of the older, +and this in the places where the older has stood most in need of +assistance. In particular, I have shown that segregation of the +fit entirely relieves survival of the fittest from the difficulty under +which it has hitherto laboured of explaining why it is that sterility +is so constantly found between species, while so rarely found +between varieties which differ from one another even more than +many species; why so many features of specific distinction are +useless to the species presenting them; and why it is that +incipient varieties are not obliterated by intercrossing with parent +forms. Again, we have seen that physiological selection, by +preventing such intercrossing, enables natural selection to +promote diversity of character, and thus to evolve species in +ramifying branches instead of in linear series—a work which I +cannot see how natural selection could possibly perform unless +thus aided by physiological selection. Moreover, we have seen +that although natural selection alone could not induce sterility +between allied types, yet when this sterility is given by physiological +selection, the forms which present it would be favoured in +the struggle for existence; and thus again the two principles are +found playing, as it were, into each other's hands. And here, as +elsewhere, I believe that the co-operation enables the two principles +to effect very much more in the way of species-making +than either of them could effect if working separately. On the +one hand, without the assistance of physiological selection, +natural selection would, I believe, be all but overcome by the +adverse influences of free intercrossing—influences all the more +potent under the very conditions which are required for the +multiplication of species by divergence of character. On the +other hand, without natural selection, physiological selection +would be powerless to create any differences of specific type, +other than those of mutual sterility and trivial details of structure,<span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span> +form, and colour—differences wholly without meaning from a +utilitarian point of view. But in their combination these two +principles appear to me able to accomplish what neither can +accomplish alone—namely, a full and satisfactory explanation of +the origin of species.</p></blockquote> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></p> + + + + +<h2>CHAPTER VI.<br /> +<span class="smcap">A Brief History of Opinions on Isolation +as a Factor of Organic Evolution.</span></h2> + + +<p>This historical sketch must begin with a consideration +of Darwin's opinions on the subject; but as these +were considerably modified from time to time during +a period of thirty years by the publications of other +naturalists, it will be impossible to avoid cross-references +as between his writings and theirs. It +may also be observed that the <i>Life and Letters of +Charles Darwin</i> was not published until the year +1887, so that the various opinions which I shall +quote from the letters, and which show some considerable +approximation in his later years to the +views which have been put forward by Mr. Gulick +and myself, were not before us at the time when our +papers were read.</p> + +<p>The earliest allusion that I can find to geographical +isolation in the writings of Darwin occurs in a +correspondence with Sir Joseph Hooker, as far back +as 1844. He there says:—</p> + +<blockquote><p>I cannot give my reasons in detail; but the most general +conclusion which the geographical distribution of all organic<span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span> +beings appears to me to indicate is, that isolation is the chief +concomitant or cause of the appearance of <i>new</i> forms (I well +know there are some staring exceptions)<a name="FNanchor_27_27" id="FNanchor_27_27"></a><a href="#Footnote_27_27" class="fnanchor">[27]</a>.</p></blockquote> + +<p>And again:—</p> + +<blockquote><p>With respect to original creation or production of new forms, +I have said that isolation appears the chief element<a name="FNanchor_28_28" id="FNanchor_28_28"></a><a href="#Footnote_28_28" class="fnanchor">[28]</a>.</p></blockquote> + +<p>Next, in the earlier editions of the <i>Origin of Species</i> +this view is abandoned, and in its stead we meet +with the opinion that geographical isolation lends +a certain amount of assistance to natural selection, +by preventing free intercrossing. But here we must +note two things. First, the distinction between monotypic +and polytypic evolution is not defined. Secondly, +the levelling effect of free intercrossing in nature, and +hence its antagonism to divergence of character by +natural selection, is not sufficiently recognized; while, +on the other hand, and in consequence of this, the +importance of isolation as a factor of evolution is +underrated—not only in its geographical, but likewise +in all its other forms.</p> + +<p>Taking these two points separately, the only +passages in Darwin's writings, so far at least as I +can find, in which any distinction is drawn between +evolution as monotypic and polytypic, are those in +which he deals with a somewhat analogous distinction +between artificial selection as intentional and unconscious. +He says, for example:—</p> + +<blockquote><p>In the case of methodical selection, a breeder selects for some +definite object, and if the individuals be allowed freely to intercross, +his work will completely fail. But when many men, +without intending to alter the breed, have a nearly common<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span> +standard of perfection, and all try to procure and breed from the +best animals, improvement surely but slowly follows from this +unconscious process of selection, notwithstanding that there is no +separation of selected individuals. Thus it will be under nature<a name="FNanchor_29_29" id="FNanchor_29_29"></a><a href="#Footnote_29_29" class="fnanchor">[29]</a>.</p></blockquote> + +<p>Here we have what may perhaps be regarded as a +glimmering of the distinction between monotypic and +polytypic evolution. But that it is only a glimmering +is proved by the immediately ensuing sentences, which +apply this analogy of unconscious selection <i>not</i> to the +case of monotypic, <i>but</i> to that of polytypic evolution. +So likewise, in the succeeding discussion on "divergence +of character," the analogy is again resorted to for the +purpose of showing how polytypic evolution may occur +in nature.</p> + +<p>Thus far, then, it may be said that we have scarcely +so much as a glimmering of the distinction between +monotypic and polytypic evolution; and as the same +discussion (with but a few verbal alterations) runs +through all the editions of the <i>Origin</i>, it may well be +asked why I should have alluded to such passages in +the present connexion. Well, I have done so because +it is apparent that, during the last years of his life, the +distinction between selection as "methodical" and +"unconscious" enabled Darwin much more clearly to +perceive that between evolution as monotypic and +polytypic. Thus in 1868 he wrote to Moritz Wagner +(who, as we shall presently see, entirely failed to +distinguish between monotypic and polytypic evolution), +expressing his belief—</p> + +<blockquote><p>That in many large areas all the individuals of the same +species have been slowly modified, in the same manner, for +instance, as the English racehorse has been improved, that is,<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span> +by the continued selection of the fleetest individuals, without +any separation. But I admit that by this process two or +more new species could hardly be formed within the same limited +area<a name="FNanchor_30_30" id="FNanchor_30_30"></a><a href="#Footnote_30_30" class="fnanchor">[30]</a>.</p></blockquote> + +<p>Again, in 1876 he wrote another letter to Wagner, +in which the following passage occurs:—</p> + +<blockquote><p>I believe that all the individuals of a species can be slowly +modified within the same district, in nearly the same manner as +man effects by what I have called the process of unconscious +selection. I do not believe that one species will give birth to +two or more new species as long as they are mingled together +within the same district<a name="FNanchor_31_31" id="FNanchor_31_31"></a><a href="#Footnote_31_31" class="fnanchor">[31]</a>.</p></blockquote> + +<p>Two years later he wrote to Professor Semper:—</p> + +<blockquote><p>There are two different classes of cases, it appears to me, +viz. those in which species becomes slowly modified in the +same country, and those cases in which a species splits into two, +or three, or more new species; and, in the latter case, I should +think nearly perfect separation would greatly aid in their +"specification," to coin a new word<a name="FNanchor_32_32" id="FNanchor_32_32"></a><a href="#Footnote_32_32" class="fnanchor">[32]</a>.</p></blockquote> + +<p>Now, these passages show a very much clearer +perception of the all-important distinction between +monotypic and polytypic evolution than any which +occur in the <i>Origin of Species</i>; and they likewise +show that he was led to this perception through what +he supposed to be a somewhat analogous distinction +between "unconscious" and "methodical" selection +by man. The analogy, I need hardly say, is radically +unsound; and it is a curious result of its unsoundness +that, whereas in the <i>Origin of Species</i> it is adduced +to illustrate the process of polytypic evolution, as +previously remarked, in the letters above quoted we<span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span> +find it adduced to illustrate the process of monotypic +evolution. But the fact of this analogy being unsound +does not affect the validity of the distinction between +monotypic and polytypic evolution to which it led +Darwin, in his later years, so clearly to express<a name="FNanchor_33_33" id="FNanchor_33_33"></a><a href="#Footnote_33_33" class="fnanchor">[33]</a>.</p> + +<p>Turning next to the second point which we have to +notice, it is easy to show that in the earlier editions +of his works Darwin did not sufficiently recognize +the levelling effects of free intercrossing, and consequently +failed to perceive the importance of isolation +(in any of its forms) as a factor of organic evolution. +This may be most briefly shown by quoting his own +more matured opinion upon the subject. Thus, with +reference to the swamping effects of intercrossing, he +wrote to Mr. Wallace in 1867 as follows:—</p> + +<blockquote><p>I must have expressed myself atrociously: I meant to say +exactly the reverse of what you have understood. F. Jenkin +argued in the <i>North British Review</i> against single variations +being perpetuated, and has convinced me, though not in quite +so broad a manner as here put. I always thought individual +differences more important; but I was blind, and thought that +single variations might be preserved much oftener than I now +see is possible or probable. I mentioned this in my former<span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span> +note merely because I believed that you had come to a similar +conclusion, and I like much to be in accord with you. I believe +I was mainly deceived by single variations offering such simple +illustrations, as when man selects [i.e. isolates]<a name="FNanchor_34_34" id="FNanchor_34_34"></a><a href="#Footnote_34_34" class="fnanchor">[34]</a>.</p></blockquote> + +<p>Again, somewhere about the same time, he wrote +to Moritz Wagner:—</p> + +<blockquote><p>Although I saw the effects of isolation in the case of islands +and mountain-ranges, and knew of a few instances of rivers, +yet the greater number of your facts were quite unknown to me. +I now see that, from the want of knowledge, I did not make +nearly sufficient use of the views which you advocate<a name="FNanchor_35_35" id="FNanchor_35_35"></a><a href="#Footnote_35_35" class="fnanchor">[35]</a>.</p></blockquote> + +<p>Now it would be easy to show the justice of these +self-criticisms by quoting longer passages from earlier +editions of the <i>Origin of Species</i>; but as this, in view +of the above passages, is unnecessary, we may next +pass on to another point.</p> + +<p>The greatest oversight that Wagner made in his +otherwise valuable essays on geographical isolation, +was in not perceiving that geographical isolation is only +one among a number of other forms of isolation: +and, therefore, that although it is perfectly true, as +he insisted, that polytypic evolution cannot be effected +by natural selection alone, it is very far from true, +as he further insisted, that <i>geographical</i> isolation is +the only means whereby natural selection can be +assisted in this matter. Hence it is that, when +Darwin said he had not himself "made nearly +sufficient use" of geographical isolation as a factor +of specific divergence, he quite reasonably added that +he could not go so far as Wagner did in regarding +such isolation as a condition, <i>sine qua non</i>, to divergent +evolution in all cases. Nevertheless, he adds<span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span> +the important words, "I almost wish I could believe +in its importance to the same extent with you; for +you well show, in a manner which never occurred to +me, that it removes many difficulties and objections." +These words are important, because they show that +Darwin had come to feel the force of the "difficulties +and objections" with regard to divergent evolution +being possible by means of natural selection alone, +and how readily they could be removed by assuming +the assistance of isolation. Hence, it is much to be +deplored that Wagner presented a single kind of +isolation (geographical) as equivalent to the principle +of isolation in general. For he thus failed to present +the complete—and, therefore, the true—philosophy +of the subject to Darwin's mind; and in this, as +in certain other respects which I shall notice later +on, served rather to confuse than to elucidate the +matter as a whole.</p> + +<p>To sum up. Although in his later years, as shown +by his correspondence, Darwin came to recognize +more fully the swamping effects of free intercrossing, +and the consequent importance of "separation" for +the prevention of these effects, and although in this +connexion he likewise came more clearly to distinguish +between the "two cases" of monotypic +and polytypic evolution, it is evident that he never +worked out any of these matters—"thinking it prudent," +as he wrote with reference to them in 1878, +"now I am growing old, to work at easier subjects<a name="FNanchor_36_36" id="FNanchor_36_36"></a><a href="#Footnote_36_36" class="fnanchor">[36]</a>." +Therefore he never clearly saw, on the one hand, +that free intercrossing, far from constituting a "difficulty" +to <i>monotypic</i> evolution by natural selection,<span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span> +is the very means whereby natural selection is in +this case enabled to operate; or, on the other +hand, that, in the case of <i>polytypic</i> evolution, the +"difficulty" in question is so absolute as to render +such evolution, by natural selection alone, absolutely +impossible. Hence, although in one sentence of the +<i>Origin of Species</i> he mentions three forms of isolation +(besides the geographical form) as serving in some +cases to assist natural selection in causing "divergence +of character" (i. e. polytypic evolution<a name="FNanchor_37_37" id="FNanchor_37_37"></a><a href="#Footnote_37_37" class="fnanchor">[37]</a>), on +account of not perceiving how great and how sharp +is the distinction between the two kinds or "cases" +of evolution, he never realized that, where "two or +more new species" are in course of differentiation, +<i>some</i> form of isolation other than natural selection +must <i>necessarily</i> be present, whether or not natural +selection be likewise so. The nearest approach which +he ever made to perceiving this necessity was in one +of his letters to Wagner above quoted, where, after +again appealing to the erroneous analogy between +monotypic evolution and "unconscious selection," he +says:—"But I admit that by this process (i. e. unconscious +selection) two or more new species could +hardly be formed within the same limited area: some +degree of separation, if not indispensable, would be +highly advantageous; and here your facts and views +will be of great value." But even in this passage the +context shows that by "separation" he is thinking +exclusively of <i>geographical</i> separation, which he rightly +enough concludes (as against Wagner) need certainly<span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span> +not be "indispensable." Had he gone a step further, +he must have seen that separation, <i>in some form +or another, is</i> "indispensable" to polytypic evolution. +Instead of taking this further step, however, two years +later he wrote to Semper as follows:—</p> + +<blockquote><p>I went as far as I could, perhaps too far, in agreement with +Wagner [i. e. in the last edition of the <i>Origin of Species</i>]; since +that time I have seen no reason to change my mind; but then +I must add that my attention has been absorbed on other +subjects<a name="FNanchor_38_38" id="FNanchor_38_38"></a><a href="#Footnote_38_38" class="fnanchor">[38]</a>.</p></blockquote> + +<p>And he seems to have ended by still failing to +perceive that the explanation which he gives of +"divergence of character" in the <i>Origin of Species</i>, +can only hold on the unexpressed assumption that +free intercrossing is in some way prevented at the +commencement, and throughout the development, of +each diverging type.</p> + +<p>Lastly, we have to consider Darwin's opinion touching +the important principle of "Independent Variability." +This, it will be remembered, is the principle which +ensures that when a portion (not too large) of a +species is prevented from interbreeding with the rest +of the species, sooner or later a divergence of type +will result, owing to the fact that the average qualities +of the separated portion at the time of its separation +cannot have been exactly the same as the average +qualities of the specific type as a whole. Thus the +state of Amixia, being a state of what Mr. Gulick +calls Independent Generation, will of itself—i.e. even +if unassisted by natural selection—induce divergence +of type, in a ratio that has been mathematically +calculated by Delbœuf.</p> + +<p><span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span> +Darwin wrote thus to Professor Weismann in +1872:—</p> + +<blockquote><p>I have now read your essay with very great interest. Your +view of the origin of local races through "Amixia" is altogether +new to me, and seems to throw an important light on an obscure +question<a name="FNanchor_39_39" id="FNanchor_39_39"></a><a href="#Footnote_39_39" class="fnanchor">[39]</a>.</p></blockquote> + +<p>And in the last edition of the <i>Variation of Animals +and Plants</i> he adds the following paragraph:—</p> + +<blockquote><p>This view may throw some light on the fact that the domestic +animals which formerly inhabited the several districts in Great +Britain, and the half-wild cattle lately kept in several British +parks, differed slightly from one another; for these animals were +prevented from wandering over the whole country and intercrossing, +but would have crossed freely within each district or +park<a name="FNanchor_40_40" id="FNanchor_40_40"></a><a href="#Footnote_40_40" class="fnanchor">[40]</a>.</p></blockquote> + +<p>Now, although I allow that Darwin never attributed +to this principle of Amixia, or Independent +Variability, anything like the degree of importance +to which, in the opinion of Delbœuf, Gulick, Giard, +and myself, it is entitled, the above passage appears +to show that, as soon as the "view" was clearly +"suggested" to his mind, he was so far from being +unfavourably disposed towards it, that he added +a paragraph to the last edition of his <i>Variation</i> for +the express purpose of countenancing it. Nevertheless, +later on the matter appears to have entirely +escaped his memory; for in 1878 he wrote to Semper, +that he did "not see at all more clearly than I did +before, from the numerous cases which he [Wagner] +has brought forward, how and why it is that a long +isolated form should almost always become slightly +modified<a name="FNanchor_41_41" id="FNanchor_41_41"></a><a href="#Footnote_41_41" class="fnanchor">[41]</a>." I think this shows entire forgetfulness<span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span> +of the principle in question, because, if the latter is +good for explaining the <i>initial</i> divergence of type as +between separated stocks of "domesticated animals," +much more must it be competent to explain the +<i>further</i> divergence of type which is "almost always" +observable in the case of "a long isolated form" +under nature. The very essence of the principle +being that, when divergence of type has once begun, +this divergence must <i>ipso facto</i> proceed at an ever-accelerating +pace, it is manifestly inconsistent to entertain +the principle as explaining the first commencement of +divergence, and then to ignore it as explaining the +further progress of divergence. Hence, I can only +conclude that Darwin had forgotten this principle +altogether when he wrote his letter to Semper in 1878—owing, +no doubt, as he says in the sentence which +immediately follows, to his having "not attended +much of late years to such questions."</p> + +<hr class="tb" /> + +<p>So much, then, for Darwin's opinions. Next in +order of time we must consider Moritz Wagner's +essays on what he called the "Law of Migration<a name="FNanchor_42_42" id="FNanchor_42_42"></a><a href="#Footnote_42_42" class="fnanchor">[42]</a>." +The merit of these essays was, first, the firm expression +of opinion upon the swamping effects of free +intercrossing; and, second, the production of a large +body of facts showing the importance of geographical +isolation in the prevention of these effects, and in +the consequent differentiation of specific types. On +the other hand, the defect of these essays was, first, +not distinguishing between evolution as monotypic +and polytypic; and, second, not perceiving that geographical<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span> +isolation is only one among a number of +other forms of isolation. From these two radical +oversights—which, however, were shared by all other +writers of the time, with the partial exception of +Darwin himself, as previously shown—there arose the +following and most lamentable errors.</p> + +<p>Over and over again Moritz Wagner insists, as constituting +the fundamental doctrine of his attempted +reform of Darwinism, that evolution by natural +selection is impossible, unless natural selection be +assisted by geographical isolation, in order to prevent +the swamping effects of intercrossing<a name="FNanchor_43_43" id="FNanchor_43_43"></a><a href="#Footnote_43_43" class="fnanchor">[43]</a>. Now, if instead +of "evolution" he had said "divergence of type," +and if instead of "geographical isolation" he had +said "prevention of intercrossing," he would have +enunciated the general doctrine which it has been the +joint endeavour of Mr. Gulick and myself to set forth. +But by not perceiving that "evolution" is of two +radically different kinds—polytypic and monotypic—he +entirely failed to perceive that, while for one of its +kinds the <i>prevention</i> of intercrossing is an absolute +necessity, for the other of its kinds the <i>permission</i> of +intercrossing is a necessity no less absolute. And, +again, in missing the fact that geographical isolation<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span> +is but one of the many ways whereby intercrossing +may be prevented, he failed to perceive that, even +as regards the case of polytypic evolution, he greatly +erred in representing this one form of isolation as +being universally a necessary condition to the process. +The necessary condition to this process is, indeed, the +prevention of intercrossing <i>by some means or another</i>; +but his unfortunate insistence on geographical separation +as the only possible means to this end—especially +when coupled with his no less unfortunate disregard +of monotypic evolution—caused him to hinder rather +than to advance a generalization which he had only +grasped in part. And this generalization is, as now +so repeatedly stated, that while the form of isolation +which we know as natural selection depends for its +action upon the intercrossing of all the individuals +which it isolates (i. e. selects), when acting alone +it can produce only monotypic evolution; but that +when it is supplemented by any of the other +numerous forms of isolation, it is furnished with +the necessary condition to producing polytypic +evolution—and this in as many lines of divergent +change as there may be cases of this efficient +separation.</p> + +<p>Nevertheless, while we must lament these shortcomings +on the part of Wagner, we ought to remember +that he rendered important services in the +way of calling attention to the swamping effects of +free intercrossing, and, still more, in that of showing +the high importance of geographical isolation as a +factor of organic evolution. Therefore, although in an +elaborate criticism of his views Weismann was easily +able to dispose of his generalizations in the imperfect<span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span> +form that they presented, I do not think it was just in +Weismann to remark, "if Wagner had confined himself +to the statement that geographical isolation materially +assists the process of natural selection, and +thus also promotes the origination of new species, he +would have met with little or no opposition; but then, +of course, in saying this much, he would not have +been saying anything new." No doubt, as I have +just shown, he <i>ought</i> thus (as well as in other and +still more important respects not perceived by Prof. +Weismann) to have limited his statement; but, had +he done so, it does not follow that he would not have +been saying anything new. For, in point of fact, in +as far as he said what was true, he did say a great +deal that was also new. Thus, most of what he said +of the <i>principle of separation</i> (apogamy) was as new +as it was true, although, as we have seen, he said it +to very little purpose on account of his identifying +this principle as a whole with that of but one of its +forms. Again, notwithstanding this great error, or +oversight, he certainly showed of the particular form +in question—viz. geographical isolation—that it was +of considerably <i>more</i> importance than had previously +been acknowledged. And this was so far a valuable +contribution to the general theory of descent.</p> + +<hr class="tb" /> + +<p>Prof. Weismann's essay, to which allusion has just +been made<a name="FNanchor_44_44" id="FNanchor_44_44"></a><a href="#Footnote_44_44" class="fnanchor">[44]</a>, was, however, in all respects a great +advance upon those of Wagner. It was not only +more comprehensive in its view of the whole subject +of geographical isolation, but likewise much more +adequate in its general treatment thereof. Its principal<span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span> +defects, in my judgement, were, first, the inordinately +speculative character of some of its parts, +and, second, the restriction of its analysis to but one +form of isolation—a defect which it shares with the +essays of Wagner, and in quite as high a degree. +Furthermore, although this essay had the great merit +of enunciating the principle of Amixia, it did so in +a very inefficient manner. For not only was this +principle adduced with exclusive reference to <i>geographical</i> +isolation, but even in regard to this one +kind of isolation it was presented in a highly inconsistent +manner, as I will now endeavour to show.</p> + +<p>Weismann was led to perceive the principle in +question by the consideration that new specific characters, +when they first appear, do not all appear +together in the same individuals: they appear one +in one individual, another in another, a third in a +third, &c.; and it is only in the course of successive +generations that they all become blended in +the same individuals by free intercrossing. Hence, the +eventually emerging constant or specific type is the +resultant of all the transitory or varietal types, when +these have been fused together by intercrossing. +From which Weismann deduces what he considers +a general law—namely, that "the constancy of a +specific type does not arise suddenly, but gradually; +and it is established by the promiscuous crossing +of all individuals<a name="FNanchor_45_45" id="FNanchor_45_45"></a><a href="#Footnote_45_45" class="fnanchor">[45]</a>." From which again it follows, +that this constancy must cease so soon as the condition +which maintains it ceases—i. e. so soon as free intercrossing +is prevented by the geographical isolation +of a portion of the species from its parent stock.</p> +<p><span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span></p> +<p>Now, to begin with, this statement of the principle +in question is not a good statement of it. There was +no need while stating the doctrine that separation +induces differentiation, to found the doctrine on any +such highly speculative basis. In point of fact, there +is no real evidence that specific types do attain +their constancy in the way supposed; nor, for the +purposes of the doctrine in question, is it necessary +that there should be. For this doctrine does not +need to show how the constancy has been <i>attained</i>; +it only has to show that the constancy is <i>maintained</i> +by free intercrossing, with the result that when free +intercrossing is <i>by any means</i> prevented, divergence +of character ensues. In short, the correct way of +stating the principle is that which has been adopted +by Delbœuf and Gulick—namely, the average characters +of a separated portion of a species are not +likely to be the same as those of the whole species; +with the result that divergence of type will be set +up in the separated portion by intercrossing within +that portion. Or the principle may be presented +as I presented it under the designation of "Independent +Variability"—namely, "a specific type may +be regarded as the average mean of all individual +variations, any considerable departure from this +average mean being, however, checked by intercrossing," +with the result that when intercrossing +is prevented between a portion of a species and +the rest of the species, "this population is permitted +to develop an independent history of its own, shielded +from intercrossing with its parent form<a name="FNanchor_46_46" id="FNanchor_46_46"></a><a href="#Footnote_46_46" class="fnanchor">[46]</a>."</p> + +<p>Not only, however, is Weismann's principle of<span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span> +"Amixia" thus very differently stated from that +of my "Independent Variability" (apogamy), or +Gulick's "Independent Generation"; but, apparently +owing to this difference of statement, the principle +itself is not the same. In particular, while Weismann +holds with us that when new characters arise in +virtue of the mere prevention of intercrossing with +parent forms these new characters will be of non-utilitarian +kind<a name="FNanchor_47_47" id="FNanchor_47_47"></a><a href="#Footnote_47_47" class="fnanchor">[47]</a>, he appears to think that divergence +of character under such circumstances is not likely to +go on to a <i>specific</i> value. Now, it is of importance +to observe why he arrives at this conclusion, which is +not only so different from that of Delbœuf, Gulick, +and myself, but apparently so inconsistent with his +own recognition of the diversifying effect of "Amixia" +as regards the formation of <i>permanent varieties</i>. For, +as we have already seen while considering Darwin's +views on this same principle of "Amixia," it is highly +inconsistent to recognize its diversifying effect up to +the stage of constituting fixed varieties, and then not +to recognize that, so much divergence of character +having been already secured by the isolation alone, +much more must further divergence continue, and +continue at an ever accelerating pace—as Delbœuf +and Gulick have so well shown. What, then, is the +explanation of this apparent inconsistency on Weismann's +part? The explanation evidently is that, +owing to his erroneous statement of the principle, he +misses the real essence of it. For, in the first place, +he does not perceive that this essence consists in an +initial difference of average characters on the part of +the isolated colony as compared with the rest of their<span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span> +species. On the contrary, he loses himself in a maze +of speculation about all species having had what he +calls "variation-periods," or eruptions of general variability +alternating with periods of repose—both being +as unaccountable in respect of their causation as they +are hypothetical in respect of their occurrence. From +these speculations he concludes, that isolation of a +portion of a species will then only lead to divergence +of character when the isolation happens to coincide +with a "variation-period" on the part of the species +as a whole, and that the divergence will cease so +soon as the "variation-period" ceases. Again, in the +second place as previously remarked, equally with +Wagner whom he is criticizing, he fails to perceive +that <i>geographical</i> isolation is not the only kind of +isolation, or the only possible means to the prevention +of free intercrossing. And the result of this oversight +is, that he thinks amixia can act but comparatively +seldom upon sufficiently small populations to become +a factor of much importance in the differentiation of +species. Lastly, in the third place, owing to his +favourite hypothesis that all species pass through +a "variation-period," he eventually concludes that the +total amount of divergence of type producible by +isolation alone (even in a small population) can never +be greater than that between the extremes of variation +which occur within the whole species at the date +of its partition (p. 75). In other words, the possibility +of change due to amixia alone is taken to be limited +by the range of deviation from the general specific +average, as manifested by different individual variations, +before the species was divided. Thus the +doctrine of amixia fails to recognize the law of<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span> +Delbœuf, or the <i>cumulative</i> nature of divergence of +type when once such divergence begins in a separated +section. Therefore, in this all-important—and, indeed, +essential—respect, amixia differs entirely from the +principle which has been severally stated by Delbœuf, +Gulick, and myself.</p> + +<p>Upon the whole, then, we must say that although +Professor Weismann was the first to recognize the +diversifying influence of merely indiscriminate isolation +<i>per se</i> (apogamy), he did so only in part. He failed +to distinguish the true essence of the principle, and by +overlaying it with a mass of hypothetical speculation, +concealed even more of it than he revealed.</p> + +<hr class="tb" /> + +<p>The general theory of Isolation, as independently +worked out by Mr. Gulick and myself, has already +been so fully explained, that it will here be sufficient +merely to enumerate its more distinguishing features. +These are, first, drawing the sharpest possible line +between evolution as monotypic and polytypic; +second, showing that while for the former the peculiar +kind of isolation which is presented by natural +selection suffices of itself to <i>transform</i> a specific type, +in order to work for the latter, or to <i>branch</i> a specific +type, natural selection must necessarily be assisted by +some other kind of isolation; third, that even in the +absence of natural selection, other kinds of isolation +may be sufficient to effect specific divergence through +independent generation alone; fourth, that, nevertheless, +natural selection, where present, will always +accelerate the process of divergence; fifth, that +monotypic evolution by natural selection depends +upon the <i>presence</i> of intercrossing, quite as much as<span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span> +polytypic evolution (whether with or without natural +selection) depends upon the <i>absence</i> of it; sixth, that, +having regard to the process of evolution throughout +all taxonomic divisions of organic nature, we must +deem the physiological form of isolation as the most +important, with the exception only of natural +selection.</p> + +<p>The only difference between Mr. Gulick's essays +and my own is, that, on the one hand, he has +analyzed much more fully than I have the various +forms of isolation; while, on the other hand, I have +considered much more fully than he has the particular +form of physiological isolation which so frequently +obtains between allied <i>species</i>. This particular form +of physiological isolation I have called "physiological +selection," and claim for it so large a share in the +differentiation of specific types as to find in it a +satisfactory explanation of the contrast between +natural species and artificial varieties in respect of +cross-infertility.</p> + +<hr class="tb" /> + +<p>Mr. Wallace, in his <i>Darwinism</i>, has done good +service by enabling all other naturalists clearly to +perceive how natural selection alone produces monotypic +evolution—namely, through the free intercrossing +of all individuals which have not been eliminated by +the isolating process of natural selection itself. For +he very lucidly shows how the law of averages must +always ensure that in respect of any given specific +character, half the individuals living at the same time +and place will present the character above, and half +below its mean in the population as a whole. Consequently, +if it should ever be of advantage to a species<span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span> +that this character should undergo either increase or +decrease of its average size, form, colour, &c., there +will always be, in each succeeding generation, a sufficient +number of individuals—i. e. half of the whole—which +present variations in the required direction, +and which will therefore furnish natural selection +with abundant material for its action, without the +need of any other form of isolation. It is to be +regretted, however, that while thus so clearly presenting +the fact that free intercrossing is the very +means whereby natural selection is enabled to effect +monotypic evolution, he fails to perceive that such +intercrossing must always and necessarily render it +impossible for natural selection to effect polytypic +evolution. A little thought might have shown him +that the very proof which he gives of the necessity +of intercrossing where the <i>transmutation</i> of species +is concerned, furnishes, measure for measure, as good +a proof of the necessity of its absence where the <i>multiplication</i> +of species is concerned. In justice to him, +however, it may be added, that this distinction between +evolution as monotypic and polytypic (with +the important consequence just mentioned) still continues +to be ignored also by other well-known evolutionists +of the "ultra-Darwinian" school. Professor +Meldola, for example, has more recently said that in +his opinion the "difficulty from intercrossing" has been +in large part—if not altogether—removed by Mr. +Wallace's proof that natural selection alone is capable +of effecting [monotypic] evolution; while he regards +the distinction between monotypic and polytypic +evolution as mere "verbiage<a name="FNanchor_48_48" id="FNanchor_48_48"></a><a href="#Footnote_48_48" class="fnanchor">[48]</a>."</p> +<p><span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span></p> +<p>It is in relation to my presentment of the impossibility +of natural selection alone causing polytypic +evolution, that Mr. Wallace has been at the +pains to show how the permission of intercrossing +(panmixia) is necessary for natural selection in its +work of causing monotypic evolution. And not only +has he thus failed to perceive that the "difficulty" +which intercrossing raises against the view of natural +selection being of itself capable of causing polytypic +evolution in no way applies to the case of monotypic; +but as regards this "difficulty," where it does apply, +he says:—</p> + +<blockquote><p>Professor G. J. Romanes has adduced it as one of the +difficulties which can alone be overcome by his theory of physiological +selection<a name="FNanchor_49_49" id="FNanchor_49_49"></a><a href="#Footnote_49_49" class="fnanchor">[49]</a>.</p></blockquote> + +<p>This, however, is a misapprehension. I have by +no means represented that the difficulty in question +can alone be overcome by this theory. What I have +represented is, that it can be overcome by any of the +numerous forms of isolation which I named, and +of which physiological selection is but one. And +although, <i>where common areas are concerned</i>, I believe +that the physiological form of isolation is the most +important form, this is a very different thing from +entertaining the supposition which Mr. Wallace here +assigns to me.</p> + +<hr class="tb" /> + +<p>I may take this opportunity of correcting a somewhat +similar misunderstanding which has been more +recently published by Professor W. A. Herdman, of +Liverpool; and as the case which he gives is one of<span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span> +considerable interest in itself, I will quote his remarks +in extenso. In his <i>Opening Address to the Liverpool +Biological Society</i>, Professor Herdman said:—</p> + +<blockquote><p>Some of you will doubtless remember that in last year's +address, while discussing Dr. Romanes' theory of physiological +selection, I quoted Professor Flemming Jenkin's imaginary case +of a white man wrecked upon an island inhabited by negroes, +given as an illustration of the supposed swamping effect by +free intercrossing of a marked variety with the parent species. +I then went on to say in criticism of the result at which Jenkin +arrived, viz. that the characteristics of the white man would be +stamped out by intercrossing with the black:—</p> + +<p>"Two influences have, I think, been ignored, viz. atavism, +or reversion to ancestral characters, and the tendency of the +members of a variety to breed with one another. Keeping to +the case described above, I should imagine that the numbers of +intelligent young mulattoes produced in the second, third, fourth, +and few succeeding generations would to a large extent intermarry, +the result of which would be that a more or less white +aristocracy would be formed on the island, including the king +and all the chief people, the most intelligent men and the bravest +warriors. Then atavism might produce every now and then +a much whiter individual—a reversal to the characteristics of +the ancestral European—who, by being highly thought of in +the whitish aristocracy, would have considerable influence +on the colour and other characteristics of the next generation. +Now such a white aristocracy would be in precisely the same +circumstances as a favourable variety competing with its parent +species," &c.</p> + +<p>You may imagine then my pleasure when, a few months after +writing the above, I accidentally found, in a letter<a name="FNanchor_50_50" id="FNanchor_50_50"></a><a href="#Footnote_50_50" class="fnanchor">[50]</a> written by the +celebrated African traveller Dr. David Livingstone to Lord +Granville, and dated "Unyanyembe, July 1st, 1872," the following +passage:—</p> + +<p>"About five generations ago, a white man came to the highlands +of Basañgo, which are in a line east of the watershed.<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span> +He had six attendants, who all died, and eventually their headman, +called Charura, was elected chief by the Basañgo. In +the third generation he had sixty able-bodied spearmen as lineal +descendants. This implies an equal number of the other sex. +They are very light in colour, and easily known, as no one is +allowed to wear coral beads such as Charura brought except the +royal family. A book he brought was lost only lately. The +interest of the case lies in its connexion with Mr. Darwin's +celebrated theory on the 'origin of species,' for it shows that an +improved variety, as we whites modestly call ourselves, is not so +liable to be swamped by numbers as some have thought."</p> + +<p>Here we have a perfect fulfilment of what I last year, in +ignorance of this observation of Livingstone's, predicted as being +likely to occur in such a case. We have the whitish aristocracy +in a dominant condition, and evidently in a fair way to spread +their characteristics over a larger area and give rise to a marked +variety, and it had clearly struck Livingstone fourteen years +before the theory of physiological selection had been heard of, +just as it must strike us now, as an instance telling strongly +against the "swamping" argument as used by Flemming Jenkin +and Romanes.</p></blockquote> + +<p>Here we have a curious example of one writer +supporting the statements of another, while appearing +to be under the impression that he is controverting +those statements. Both Professor Herdman's +imaginary case, and its realization in Livingstone's +account, go to show "the tendency of the members +of a variety to breed with one another." This is +what I have called "psychological selection," and, +far from "ignoring" it, I have always laid stress +upon it as an obviously important form of isolation +or <i>prevention</i> of free intercrossing. But it is a form +of isolation which can only occur in the higher animals, +and, therefore, the whole of Professor Herdman's +criticism is merely a restatement of my own views +as already published in the paper which he is<span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span> +criticizing. For all that his argument goes to prove +is, first, the necessity for <i>some</i> form of isolation if +the overwhelming effects of intercrossing are to be +obviated; and, secondly, the manifest consequence +that where the psychological form is unavailable (as +in many of the lower animals and in all plants), +some other form must be present if divergent evolution +is taking place on a common area.</p> + +<hr class="tb" /> + +<p>Seeing that so much misunderstanding has been +shown with reference to my views on "the swamping +effects of intercrossing," and seeing also that +this misunderstanding extends quite as much to Mr. +Gulick's views as to my own, I will here supply +brief extracts from both our original papers, for the +double purpose of showing our complete agreement, +and of leaving it to be judged whether we can +fairly be held responsible for the misunderstanding +in question. After having supplied these quotations, +I will conclude this historical sketch by considering +what Mr. Wallace has said in reply to the views +therein presented. I will transcribe but a single +passage from our papers, beginning with my own.</p> + +<blockquote><p>Any theory of the origin of species in the way of descent must +be prepared with an answer to the question, Why have species +<i>multiplied</i>? How is it that, in the course of evolution, species +have not simply become transmuted in linear series instead of +ramifying into branches? This question Mr. Darwin seeks to +answer "from the simple circumstance that the more diversified +the descendants from any one species becomes in structure, +constitution, and habits, by so much will they be better enabled +to seize on many and widely diversified places in the economy +of nature, and so be enabled to increase in numbers." And he +proceeds to illustrate this principle by means of a diagram,<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span> +showing the hypothetical divergence of character undergone by +the descendants of seven species. Thus, he attributes divergence +of character exclusively to the influence of natural selection.</p> + +<p>Now, this argument appears to me unassailable in all save +one particular; but this is a most important particular: the +argument wholly ignores the fact of intercrossing with parent +forms. Granting to the argument that intercrossing with parent +forms is prohibited, and nothing can be more satisfactory. The +argument, however, sets out with showing that it is in limited +areas, or in areas already overstocked with the specific form in +question, that the advantages to be derived from diversification +will be most pronounced. It is where they "jostle each other +most closely" that natural selection will set a premium upon +any members of the species which may depart from the common +type. Now, inasmuch as this jostling or overcrowding of +individuals is a needful condition to the agency of natural +selection in the way of diversifying character, must we not feel +that the general difficulty from intercrossing previously considered +is here presented in a special and aggravated form? +At all events, I know that, after having duly and impartially +considered the matter, to me it does appear that unless the +swamping effects of intercrossing with the parent form on an +overcrowded area is in some way prevented to begin with, +natural selection could never have any material supplied by +which to go on with. Let it be observed that I regard Mr. +Darwin's argument as perfectly sound where it treats of the +divergence of <i>species</i>, and of their further divergence into <i>genera</i>; +for in these cases the physiological barrier is known to be +already present. But in applying the argument to explain +the divergence of individuals into varieties, it seems to me that +here, more than anywhere else, Mr. Darwin has strangely lost +sight of the formidable difficulty in question; for in this +particular case so formidable does the difficulty seem to me, +that I cannot believe that natural selection alone could produce +any divergence of specific character, so long as all the individuals +on an overcrowded area occupy that area together. +Yet, if any of them quit that area, and so escape from the +unifying influence of free intercrossing, these individuals also +escape from the conditions which Mr. Darwin names as those<span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span> +that are needed by natural selection in order to produce divergence. +Therefore, it appears to me that, under the circumstances +supposed, natural selection alone could not produce +divergence; the most it could do would be to change the whole +specific type in some one direction, and thus induce transmutation +of species in a linear series, each succeeding member +of which might supplant its parent form. But in order to +secure <i>diversity</i>, <i>multiplication</i>, or <i>ramification</i> of species, +it appears to me obvious that the primary condition required is +that of preventing intercrossing with parent forms at the origin +of each branch, whether the prevention be from the first +absolute, or only partial.</p></blockquote> + +<p>Now for Mr. Gulick, a portion of whose more +lengthy discussion of the subject, however, is all that +I need quote:—</p> + +<blockquote><p>Having found that the evolution of the fitted is secured through +the prevention of crossing between the better fitted and the less +fitted, can we believe that the evolution of a special race, +regularly transmitting a special kind of fitness, can be realized +without any prevention of crossing with other races that have +no power to transmit that special kind of fitness? Can we +suppose that any advantage, derived from new powers that +prevent severe competition with kindred, can be permanently +transmitted through succeeding generations to one small section +of the species while there is free crossing equally distributed +between all the families of the species? Is it not apparent that +the terms of this supposition are inconsistent with the fundamental +laws of heredity? Does not inheritance follow the lines +of consanguinity; and when consanguinity is widely diffused, +can inheritance be closely limited? When there is free crossing +between the families of one species, will not any peculiarity +that appears in one family either be neutralized by crosses +with families possessing the opposite quality, or, being preserved +by natural selection, while the opposite quality is gradually +excluded, will not the new quality gradually extend to all the +branches of the species; so that, in this way or in that, increasing +divergence of form will be prevented?</p> + +<p><span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span> +If the advantage of freedom from competition in any given +variation depends on the possession, in some degree, of new +adaptations to unappropriated resources, there must be some +cause that favours the breeding together of those thus specially +endowed, and interferes in some degree with their crossing +with other variations, or, failing this, the special advantage will +in succeeding generations be lost. As some degree of Independent +Generation is necessary for the continuance of the +advantage, it is evident that the same condition is necessary +for the accumulation through Natural Selection of the powers +on which the advantage depends. The advantage of divergence +of character cannot be retained by those that fail to retain the +divergent character; and divergent character cannot be retained +by those that are constantly crossing with other kinds; and the +prevention of free crossing between those that are equally +successful is in no way secured by Natural Selection.</p></blockquote> + +<p>So much, then, as expressive of Mr. Gulick's +opinion upon this subject. To exactly the same +effect Professor Lloyd Morgan has recently published +his judgement upon it thus:—</p> + +<blockquote><p>That perfectly free intercrossing, between any or all of the +individuals of a given group of animals, is, so long as the +characters of the parents are blended in the offspring, fatal to +divergence of character, is undeniable. Through the elimination +of less favourable variations, the swiftness, strength, and +cunning of a race may be gradually improved. But no form of +elimination can possibly differentiate the group into swift, +strong, and cunning varieties, distinct from each other, so long +as all three varieties freely interbreed, and the characters of +the parents blend in the offspring. Elimination may and does +give rise to progress in any given group, <i>as a group</i>; it does +not and cannot give rise to differentiation and divergence, so +long as interbreeding with consequent interblending of characters +be freely permitted. Whence it inevitably follows, as a matter +of simple logic, that where divergence has occurred, intercrossing +and interbreeding must in some way have been +lessened or prevented. Thus a new factor is introduced, that<span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span> +of <i>isolation</i> or <i>segregation</i>. And there is no questioning the +fact that it is of great importance. Its importance, indeed, can +only be denied by denying the swamping effects of intercrossing, +and such denial implies the tacit assumption that interbreeding +and interblending are held in check by some form of segregation. +The isolation explicitly denied is implicitly assumed<a name="FNanchor_51_51" id="FNanchor_51_51"></a><a href="#Footnote_51_51" class="fnanchor">[51]</a>.</p></blockquote> + +<p>Similarly, and still more recently, Professor +Le Conte writes:—</p> + +<blockquote><p>It is evident, then, as Romanes claims, that natural selection +alone tends to <i>monotypic</i> evolution. Isolation of some sort +seems necessary to <i>polytypic</i> evolution. The tree of evolution +under the influence of natural selection alone grows palm-like +from its terminal bud. Isolation was necessary to the starting +of lateral buds, and thus for the profuse ramification which is its +most conspicuous character<a name="FNanchor_52_52" id="FNanchor_52_52"></a><a href="#Footnote_52_52" class="fnanchor">[52]</a>.</p></blockquote> + +<p>In order to complete this historical review, it only +remains to consider Mr. Wallace's utterances upon the +subject.</p> + +<p>It is needless to say that he stoutly resists the +view of Weismann, Delbœuf, Gulick, and myself, that +specific divergence can ever be due—or, as I understand +him, even so much as assisted—by this principle +of indiscriminate isolation (apogamy). It will be +remembered, however, that Mr. Gulick has adduced +certain general principles and certain special facts +of geographical distribution, in order to prove that +apogamy eventually leads to divergence of character, +provided that the isolated section of the species does +not contain any very large number of individuals. +Now, Mr. Wallace, without making any reference to +this argument of Mr. Gulick, simply states the reverse—namely, +that, as a matter of fact, indiscriminate<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span> +isolation is not found to be associated with divergence +of character. For, he says, "there is an entire +absence of change, where, if this were a <i>vera causa</i>, +we should expect to find it<a name="FNanchor_53_53" id="FNanchor_53_53"></a><a href="#Footnote_53_53" class="fnanchor">[53]</a>." But the only case +which he gives is that of Ireland.</p> + +<p>This, he says, furnishes "an excellent test case, for +we know that it [Ireland] has been separated from +Britain since the end of the glacial epoch: ... yet +hardly one of its mammals, reptiles, or land molluscs +has undergone the slightest change<a name="FNanchor_54_54" id="FNanchor_54_54"></a><a href="#Footnote_54_54" class="fnanchor">[54]</a>." Here, however, +Mr. Wallace shows that he has failed to understand +"the views of those who, like Mr. Gulick, +believe isolation itself to be a cause of modification +of species"; for it belongs to the very essence of these +views that the efficiency of indiscriminate isolation as +a "<i>vera causa</i>" of organic evolution varies inversely +with the number of individuals (i. e. the size of the +species-section) exposed to its influence. Therefore, +far from being "an excellent test case," the case +of Ireland is unsatisfactory. If we are in search of +excellent test cases, in the sense intended by Mr. +Wallace, we ought not to choose a large island, +which from the time of its isolation must have contained +large bulks of each of the geographically +separated species concerned: we ought to choose +cases where as small a number as possible of the +representatives of each species were in the first +instance concerned. And, when we do this, the +answer yielded by any really "excellent test case" is +unequivocal.</p> + +<p>No better test case of this kind has ever been +furnished than that of Mr. Gulick's land-shells,<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span> +which Mr. Wallace is specially considering in the +part of his book where the sentence above quoted +occurs. How, then, does he meet this case? He +meets it by assuming that in all the numerous +adjacent valleys of a small island there must be +as many differences of environment, each of which +is competent to induce slight varietal changes on +the part of its occupants by way of natural selection, +although in no one case can the utility of these +slight changes be surmised. Now, against this explanation +there are three overwhelming considerations. +In the first place, it is purely gratuitous, or offered +merely in order to save the hypothesis that there +<i>can</i> be no other cause of even the most trivial change +in species than that which is furnished by natural +selection. In the second place, as Mr. Gulick writes +to me in a private letter, "if the divergence of +Sandwich Island land molluscs is wholly due to +exposure to different environments, as Mr. Wallace +argues on pages 147-150, then there must be completely +occult influences in the environment that +vary progressively with each successive mile. This +is so violent an assumption that it throws doubt +on any theory that requires such support." In the +third place, the assumption that the changes in +question must have been due to natural selection, +is wholly incompatible with the facts of isolation +elsewhere—namely, in those cases where (as in that +of Ireland) a large section of species, instead of +a small section, has been indiscriminately isolated. +Mr. Wallace, as we have seen, inadvertently alludes +to these "many other cases of isolation" as evidence +against apogamy being <i>per se</i> a cause of specific<span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span> +change. But although, for the reason above stated, +they are without relevancy in this respect, they +appear to me fatal to the explanation which he gives +of specific changes under apogamy where only small +sections of species are concerned. For example, can +it be rationally maintained that there are more +differences of environment between every two of +the many contiguous valleys of a small island, +such as Mr. Gulick describes, than there are in +the incomparably larger area of the whole of +Ireland? But, if not, and if natural selection is +able to work such "occult" wonders in each successive +mile on the Sandwich Islands, why has it so +entirely lost this magic power in the case of Ireland—or +in the "many other cases of isolation" to +which Mr. Wallace refers? On his theory there +is no coherent answer to be given to this question, +while on our theory the answer is given in the +very terms of the theory itself. The facts are +plainly just what the theory requires that they +should be; and therefore, if they were not as they +are, the theory would be deprived of that confirmation +which it now derives from them.</p> + +<p>Thus, in truth, though in an opposite way, the +case of Ireland is, as Mr. Wallace says, "an excellent +test case," when once the theory of apogamy +as a "<i>vera causa</i>" of specific change is understood; +and the effect of applying the test is fully to corroborate +this theory, while at the same time it as +fully negatives the other. For the consideration +whereby Mr. Wallace seeks to explain the inactivity +of natural selection in the case of Ireland is not +"coherent." What he says is, "That changes have<span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span> +not occurred through natural selection, is perhaps +due to the less severe struggle for existence, owing +to the smaller number of competing species<a name="FNanchor_55_55" id="FNanchor_55_55"></a><a href="#Footnote_55_55" class="fnanchor">[55]</a>." But +even with regard to molluscs alone, there is a greatly +larger number of species in Ireland than occurs in +any one valley of the Sandwich Islands; while if we +have regard to all the other classes of animal life, +comparison entirely fails.</p> + +<p>Much more to the point are certain cases which +were adduced long ago by Weismann in his essay +previously considered. Nevertheless, although this +essay was published as far back as 1872, and, +although it expressly deals with the question of +divergence of character through the mere prevention of +intercrossing (Amixia), Mr. Wallace nowhere alludes +to these cases <i>per contra</i>, which are so much more +weighty than his own "test case" of Ireland. Of +such are four species of butterflies, belonging to three +genera<a name="FNanchor_56_56" id="FNanchor_56_56"></a><a href="#Footnote_56_56" class="fnanchor">[56]</a>, which are identical in the polar regions and +in the Alps, notwithstanding that the sparse Alpine +populations have been presumably separated from +their parent stocks since the glacial period; or of +certain species of fresh water crustaceans (<i>Apus</i>), the +representatives of which are compelled habitually to +form small isolated colonies in widely separated +ponds, and nevertheless exhibit no divergence of +character, although apogamy has probably lasted for +centuries. These cases are unquestionably of a very +cogent nature, and appear of themselves to prove +that apogamy alone is not invariably capable of<span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span> +inducing divergence—at any rate, so rapidly as we +might expect. There appears, however, to be +another factor, the presence or absence of which +makes a great difference. This as stated in the text, +is the degree in which a specific type is stable or +unstable—liable or not liable to vary. Thus, for +example, the Goose is what Darwin calls an "inflexible" +type as compared with most other domesticated +birds. Therefore, if a lot of geese were to be indiscriminately +isolated from the rest of their species, the +probability is that in a given time their descendants +would not have diverged from the parent type to such +an extent as would a similar lot of ducks under +similar circumstances: the more stable specific type +would require a longer time to change under the +influence of apogamy alone. Now, the butterflies +and crustaceans quoted by Weismann may be of a +highly stable type, presenting but a small range +of individual variability; and, if so, they would +naturally require a long time to exhibit any change +of type under the influence of apogamy alone. But, +be this as it may, Weismann himself adduces these +cases merely for the sake of showing that there are +cases which seem to tell against the general principle +of modification as due to apogamy alone—i.e. +the general principle which, under the name amixia, +he is engaged in defending. And the conclusion +at which he himself arrives is, that while it would +be wrong to affirm that apogamy <i>must</i> in all +cases produce divergence, we are amply justified +in affirming that in many cases it <i>may</i> have done +so; while there is good evidence to prove that in +not a few cases it <i>has</i> done so, and therefore<span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span> +should be accepted as one of the factors of organic +evolution<a name="FNanchor_57_57" id="FNanchor_57_57"></a><a href="#Footnote_57_57" class="fnanchor">[57]</a>.</p> + +<p>My view from the very first has been that variations +in the way of cross-infertility are of frequent occurrence +(how, indeed, can they be otherwise, looking +to the complex conditions that have to be satisfied +in every case of full fertility?); and, therefore, +however many of such variations are destined to die +out, whenever one arises, "under suitable conditions," +"it must inevitably tend to be preserved as a new +natural variety, or incipient species." Among the +higher animals—which are "comparatively few in +number"—I think it probable that some slight change +of form, colour, habit, &c., must be usually needed +either to "superinduce," or, which is quite a different +thing, to <i>coincide</i> with the physiological change +But in the case of plants and the lower invertebrata. +I see no reason for any frequent concomitance +of this kind; and therefore believe the physiological<span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span> +change to be, "as a general rule," the primordial +change. At the same time, I have always been +careful to insist that this opinion had nothing to do +with "the essence of physiological selection"; seeing +that "it was of no consequence" to the theory in +what proportional number of cases the cross-sterility +had begun <i>per se</i>, had been superinduced by morphological +changes, or only enabled to survive by +happening to coincide with any other form of +homogamy. In short, "the essence of physiological +selection" consists in <i>all</i> cases of the diversifying <i>effect</i> +of cross-infertility, whensoever and howsoever it may +happen in particular cases to have been <i>caused</i>.</p> + +<p>Thus I emphatically reaffirm that "from the first +I have always maintained that it makes no essential +difference to the theory <i>in what proportional +number of cases</i> they [the physiological variations] +have arisen 'alone in an otherwise undifferentiated +species'"; therefore, "even if I am wrong in supposing +that physiological selection can <i>ever</i> act +alone, the <i>principle</i> of physiological selection, as I +have stated it, is not thereby affected. And this +principle is, as Mr. Wallace has re-stated it, 'that +some amount of infertility characterizes the distinct +varieties which are in process of differentiation into +species'—infertility whose absence, 'to obviate the +effects of intercrossing, may be one of the <i>usual</i> +causes of their failure to become developed into +distinct species.'"</p> + +<p>These last sentences are quoted from the correspondence +in <i>Nature</i><a name="FNanchor_58_58" id="FNanchor_58_58"></a><a href="#Footnote_58_58" class="fnanchor">[58]</a>, and to them Mr. Wallace replied +by saying, "if this is not an absolute change of front,<span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span> +words have no meaning"; that "if this is 'the whole +essence of physiological selection,' then physiological +selection is but a re-statement and amplification of +Darwin's views"; that such a "change of front" is +incompatible, not only with my term "physiological +selection," but also with my having "acknowledged +that Mr. Catchpool had 'very clearly put forward the +theory of physiological selection'"; and much more +to the same effect.</p> + +<p>Now, to begin with, it is due to Mr. Catchpool to +state that his only publication upon this subject is +much too brief to justify Mr. Wallace's, inference, that +he supposes variations in the way of cross-infertility +always to arise "alone in an otherwise undifferentiated +species." What Mr. Catchpool's opinion on this +point may be, I have no knowledge; but, whatever it +is, he was unquestionably the first writer who "clearly +stated the leading principles" of physiological selection, +and this fact I am very glad to have "acknowledged." +In my correspondence with Mr. Wallace, +however, I not only named Mr. Catchpool: I also +named—and much more prominently—Mr. Gulick. +For even if I were to grant (which I am far indeed +from doing) that there was any want of clearness in +my own paper touching the point in question, I have +now repeatedly shown that it is simply impossible +for any reader of Mr. Gulick's papers to misunderstand +<i>his</i> views with regard to it. Accordingly, +I replied to Mr. Wallace in <i>Nature</i> by saying:—</p> + +<blockquote><p>Not only have I thus from the first fully recognized the +sundry other causes of specific change with which the physiological +variations may be associated; but Mr. Gulick has gone +into this side of our common theory much more fully, and<span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span> +elaborately calculated out the high ratio in which the differentiating +agency of any of these other causes must be increased +when assisted by—i. e. associated with—even a moderate degree +of the selective fertility, and vice versa. Therefore, it is simply +impossible for Mr. Wallace to show that "our theory" differs +from his in this respect. Yet it is the only respect in which his +reply alleges any difference. (Vol. xliii. p. 127.)</p></blockquote> + +<p>I think it is to be regretted that, in his answer to +this, Mr. Wallace alludes only to Mr. Catchpool, and +entirely ignores Mr. Gulick—whose elaborate calculations +above alluded to were communicated to the +Linnaean Society by Mr. Wallace himself in 1887.</p> + +<p>The time has now come to prove, by means of +quotations, that I have from the first represented +the "principle," or "essence," of physiological selection +to consist in selective fertility furnishing a needful +condition to specific differentiation, in at least +a large proportional number of allied species which +afterwards present the reciprocal character of cross-sterility; +that I have never represented variations +in the way of this selective fertility as necessarily +constituting the initial variations, or as always arising +"alone, in an otherwise undifferentiated species"; +and that, although I have uniformly given it as my +opinion that these variations do <i>in some cases</i> thus +arise (especially among plants and lower invertebrata), +I have as uniformly stated "that it makes no difference +to the theory in what proportional number of +cases they have done so"—or even if, as Mr. Wallace +supposes, they have never done so in any case at all<a name="FNanchor_59_59" id="FNanchor_59_59"></a><a href="#Footnote_59_59" class="fnanchor">[59]</a>.<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span> +These statements (all of which are contradictory +of the only points of difference alleged) have already +been published in my article in the <i>Monist</i> of +October, 1890. And although Mr. Wallace, in his +reply to that article, ignores my references to the +"original paper," it is scarcely necessary to quote the +actual words of the paper itself, since the reader who +is further interested in this controversy can readily +refer to it in the <i>Journal of the Linnaean Society</i> +(vol. xix. pp. 337-411).</p> + +<p>Having arrived at these results with regard to the +theory of Isolation in general and of Physiological +Isolation in particular, I arrive also at the end of this +work. And if, while dealing with the post-Darwinian +period, I have imparted to any general reader the +impression that there is still a great diversity of +expert opinion; I must ask him to note that points +with reference to which disagreement still exists +are but very subordinate to those with regard to +which complete agreement now prevails. The noise +of wrangling disputations which has so filled the +camp of evolutionists since the death of their +captain, is apt to hide from the outside world the +solid unanimity that prevails with regard to all +the larger and more fundamental questions, which +were similarly the subjects of warfare in the past +generation. Indeed, if we take a fair and general<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span> +view of the whole history of Darwinism, what must +strike us as the really significant fact is the astonishing +unanimity which has been so rapidly attained +with regard to matters of such immeasurable importance. +It is now but little more than thirty years +since the publication of the <i>Origin of Species</i>; and +in that period not only have all naturalists unequivocally +embraced the doctrine of descent considered +as a fact; but, in one degree or another, they have +all as unequivocally embraced the theory of natural +selection considered as a method. The only points +with regard to which any difference of opinion still +exist, have reference to the precise causation of that +mighty stream of events which, under the name of +organic evolution, we have now all learnt to accept as +scientifically demonstrated. But it belongs to the +very nature of scientific demonstration that, where +matters of great intricacy as well as of high generality +are concerned, the process of demonstration must be +gradual, even if it be not always slow. It is only by +the labours of many minds working in many directions +that, in such cases, truth admits of being eventually +displayed. Line upon line, precept upon precept, +here a little and there a little—such is the course of +a scientific revelation; and the larger the subject-matter, +the more subtle and the more complex the +causes, the greater must be the room for individual +differences in our reading of the book of Nature. +Now, if all this be true, must we not feel that in the +matter of organic evolution the measure of agreement +which has been attained is out of all proportion to +the differences which still remain—differences which, +although of importance in themselves, are insignificant<span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span> +when compared with those which once divided the +opinions of not a few still living men? And if we are +bound to feel this, are we not bound further to feel +that the very intensity of our disputations over these +residual matters of comparative detail, is really the +best earnest that can be given of the determination +of our quest—determination which, like that of our +fathers, cannot fail to be speedily rewarded by the +discovery of truth?</p> + +<p>Nevertheless, so long as this noise of conflict is +in the Senate, we cannot wonder if the people are +perplexed. Therefore, in conclusion, I may ask it to +be remembered exactly what are the questions—and +the only questions—which still divide the parties.</p> + +<p>Having unanimously agreed that organic evolution +is a fact and that natural selection is a cause, or +a factor in the process, the primary question in debate +is whether natural selection is the only cause, or +whether it has been assisted by the co-operation of +other causes. The school of Weismann maintain that +it is the only cause; and therefore deem it worse +than useless to search for further causes. With this +doctrine Wallace in effect agrees, excepting as regards +the particular case of the human mind. The school +of Darwin, on the other hand—to which I myself +claim to belong—believe that natural selection has +been to a considerable extent supplemented by other +factors; and, therefore, although we further believe +that it has been the "main" factor, we agree with +Darwin himself in strongly reprobating all attempts +to bar <i>a priori</i> the progress of scientific investigation +touching what, if any, these other factors may be. +Lastly, there are several more or less struggling<span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span> +schools, chiefly composed of individual members who +agree with each other only to the extent of holding +that the causal agency of natural selection is not so +great as Darwin supposed. The Duke of Argyll, +Mr. Mivart and Mr. Geddes may be named in this +connexion; together with the self-styled neo-Lamarckians, +who seek to magnify the Lamarckian +principles at the expense of the distinctively Darwinian.</p> + +<p>This primary difference of opinion leads deductively +to certain secondary differences. For if a man starts +with the premiss that natural selection must necessarily +be the "exclusive" cause of organic evolution, +he is likely to draw conclusions which another man +would not draw who starts with the premiss that +natural selection is but the "main" cause. Of these +subordinate differences the most important are those +which relate to the possible transmission of acquired +characters, to the necessary (or only general) utility +of specific characters, and to the problem touching the +inter-sterility of allied species. But we may well +hope that before another ten years shall have passed, +even these still outstanding questions will have been +finally settled; and thus that within the limits of an +ordinary lifetime the theory of organic evolution will +have been founded and completed in all its parts, to +stand for ever in the world of men as at once the +greatest achievement in the history of science, and the +most splendid monument of the nineteenth century.</p> + +<p>In the later chapters of the foregoing treatise I have +sought to indicate certain matters of general principle, +which many years of study specially devoted to this +great movement of contemporary thought have led<span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span> +me to regard as almost certainly sound in themselves, +and no less certainly requisite as complements of the +Darwinian theory. I will now conclude by briefly +summarizing these matters of general principle in the +form of twelve sequent propositions. And, in doing +so, I may ask it to be noticed that the system which +these propositions serve to express may now claim, +at the least, to be a strictly logical system. For the +fact that, not merely in its main outlines, but likewise +in its details, it has been independently constructed +by Mr. Gulick, proves at any rate this much; seeing +that, where matters of such intricacy are concerned, +nothing but accurate reasoning from a common +foundation of <i>data</i> could possibly have yielded so +exact an agreement. The only difference between us +is, that Mr. Gulick has gone into much further detail +than I have ever attempted in the way of classifying +the many and varied forms of isolation; while I have +laid more special stress upon the physiological form, +and found in it what appears to me a satisfactory +solution of "the greatest of all the difficulties in the +way of accepting the theory of natural selection as +a complete explanation of the origin of species"—namely, +"the remarkable difference between varieties +and species when crossed."</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></p> + + + + +<h2>GENERAL CONCLUSIONS.</h2> + +<div class="smcap"> +<p>1. Natural Selection is primarily a theory +of the cumulative development of adaptations +wherever these occur; and therefore +is only incidentally, or likewise, a theory +of the origin of species in cases where allied +species differ from one another in respect of +peculiar characters, which are also adaptive +characters.</p> + +<p>2. Hence, it does not follow from the +theory of natural selection that all +species—much less all specific characters—must +necessarily have owed their origin to +natural selection; since it cannot be proved +deductively from the theory that no "means +of modification" other than natural selection +is competent to produce such slight +degrees of modification as go to constitute +diagnostic distinctions between closely +allied species; while, on the other hand, +there is an overwhelming mass of evidence +to prove the origin of "a large proportional +number of specific characters" by causes of +modification other than natural selection.<span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span></p> + +<p>3. Therefore, and upon the whole, as +Darwin so emphatically held, "Natural +selection has been the main, but not the +exclusive means of modification."</p> + +<p>4. Even if it were true that all species +and all specific characters must necessarily +owe their origin to natural selection, it +would still remain illogical to define the +theory of natural selection as indifferently +a theory of species or a theory of adaptations; +for, even upon this erroneous supposition, +specific characters and adaptive +characters would remain very far indeed +from being conterminous—most of the more +important adaptations which occur in +organic nature being the common property +of many species.</p> + +<p>5. In no case can natural selection have +been the cause of mutual infertility +between allied, or any other, species—<i>i.e.</i> of +the most general of all "specific characters."</p> + +<p>6. Without Isolation, or the prevention of +free intercrossing, organic evolution is in +no case possible. Therefore, it is isolation +that <i>has</i> been "the exclusive means of +modification," or, more correctly, the universal +condition to it. Therefore, also, +Heredity and Variability being given, the +whole theory of organic evolution becomes +a theory of the causes and conditions which +lead to Isolation.<span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span></p> + +<p>7. Isolation may be either discriminate +or indiscriminate. When discriminate, it +has reference to resemblances between individuals +constituting the isolated colony +or group; when indiscriminate, it has no +such reference. In the former case there +arises Homogamy, and in the latter case +there arises Apogamy.</p> + +<p>8. Except where very large populations +are concerned, indiscriminate isolation +always tends to become increasingly discriminate; +and, in the measure that it does so, +apogamy passes into homogamy, by virtue of +Independent Variability.</p> + +<p>9. Natural Selection is one among many +other forms of discriminate isolation, and +presents in this relation the following +peculiarities:—(<i>a</i>) The isolation is with +reference to superiority of fitness; (<i>b</i>) is +effected by death of the excluded individuals; +and (<i>c</i>) unless assisted by some +other form of isolation, can only effect +monotypic as distinguished from polytypic +evolution.</p> + +<p>10. It is a general law of organic evolution +that the number of possible directions in +which divergence may occur can never be +more than equal to the number of cases of +efficient isolation; but, excepting natural +selection, any one form of isolation need +not necessarily require the co-operation<span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span> +of another form in order to create an +additional case of isolation, or to cause +polytypic as distinguished from monotypic +evolution.</p> + +<p>11. Where common areas and polytypic evolution +are concerned, the most general and +most efficient form of isolation has been +the physiological, and this whether the +mutual infertility has been the antecedent +or the consequent of morphological changes +on the part of the organisms concerned, and +whether or not these changes are of an +adaptive character.</p> + +<p>12. This form of isolation—which, in +regard to incipient species, I have called +Physiological Selection—may act either +alone or in conjunction with other forms of +isolation on common areas: in the former +case its agency is of most importance among +plants and the lower classes of animals; +in the latter case its importance consists +in its greatly intensifying the segregative +power of whatever other form of isolation +it may be with which it is associated.</p> +</div> + +<hr class="full" /> +<p class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></p> + + + + +<h2><i>APPENDICES</i></h2> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></p> + + + +<h2>APPENDIX A.<br /> +<span class="smcap">Mr. Gulick's Criticism of Mr. Wallace's Views on +Physiological Selection.</span></h2> + + +<p>I have received from Mr. Gulick the results of his +consideration of Mr. Wallace's criticism. As these results +closely resemble those which I have myself reached, and +as they were independently worked out on the other side of +the globe, I deem it desirable to publish them here for the +sake of comparison.</p> + +<p>In his covering letter Mr. Gulick writes:—</p> + +<blockquote><p>Mr. Wallace has most certainly adopted the fundamental principles +of our theory, and in an arbitrary way attempted to claim +the results produced by these principles as the effects of natural +selection. He takes our principles, which in the previous +chapter he has combated; but he makes such disjointed use of +them that I am not willing to recognize his statement as an +intelligible exposition of our theory.... I have endeavoured to +indicate at what points Mr. Wallace has deserted his own principles, +and at what points he has failed to make the best use of +ours. To bring out these points distinctly has been no easy +task; but if you regard this paper on <i>The Preservation and Accumulation +of Cross-infertility</i> as giving any help in elucidating +the true principles, and in showing Mr. Wallace's position in +regard to them, I shall be satisfied. Please make any use of it +that may seem desirable, and then forward it to Professor +Dana.</p></blockquote> + +<p><span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span> +The following is a general summary of Mr. Gulick's +results:—</p> + +<blockquote><p>Mr. Wallace's criticism of the theory of Physiological Selection +is unsatisfactory; (l) because he has accepted the fundamental principle +of that theory on pages 173-9, in that he +maintains that without the cross-infertility the incipient species +there considered would be swamped; (2) because he assumes +that physiological selection pertains simply to the infertility +of first crosses, and has nothing to do with the infertility of +mongrels and hybrids; (3) because he assumes that infertility +between first crosses is of rare occurrence between species of +the same genus, ignoring the fact that in many species of plants +the pollen of the species is pre-potent on the stigma of the same +species when it has to compete with the pollen of other species +of the same genus; (4) because he not only ignores Mr. Romanes' +statement that cross-infertility often affects "a whole race or +strain," but he gratuitously assumes that the theory of Physiological +Selection excludes this "racial incompatibility" (which +Mr. Romanes maintains is the more probable form), and bases his +computation on the assumption that the cross-infertility is not +associated with any other form of segregation; (5) because he +claims to show that "all infertility not correlated with some +<i>useful</i> variation has a constant tendency to effect its own +elimination," while his computation only shows that, if the cross-infertility +is not associated with some form of <i>positive</i> segregation, +it will disappear<a name="FNanchor_60_60" id="FNanchor_60_60"></a><a href="#Footnote_60_60" class="fnanchor">[60]</a>; and (6) because he does not observe +that the positive segregation may be secured by the very form of +the physiological incompatibility.... Without here entering +into any computation, it is evident that, e.g. the prepotency of +pollen of each kind with its own kind, if only very slight, will +prevent cross-fertilization as effectually as a moderate degree of +instinctive preference in the case of an animal.</p></blockquote> + +<p><span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span> +The paper likewise indicates a point which, in studying +Mr. Wallace's theory, I have missed. It will be remembered +that the only apparent difference between his theory +and mine has been shown to consist in this—that while +I was satisfied to state, in a general way, that natural selection +is probably able to increase a selective fertility which +has already been begun by other causes, Mr. Wallace +has sought to exhibit more in detail the precise conditions +under which it can do so. Now, Mr. Gulick shows that +the particular conditions which Mr. Wallace describes, even +if they do serve to promote an increase of cross-infertility, +are conditions which preclude the possibility of natural selection +coming into play at all. So that if, under these particular +conditions, a further increase of cross-infertility does +take place, it does not take place in virtue of natural selection. +To me it appears that this criticism is sound; and, if so, +it disposes of even the one very subordinate addition to +our theory which Mr. Wallace "claims" as the most +"distinctive" part of his.</p> + +<p>The following is the criticism in question:—</p> + +<blockquote><p>On pages 173-186 Mr. Wallace maintains that "Natural +selection is, in some probable cases at all events, able to +accumulate variations in infertility between incipient species" +(p. 174); but his reasoning does not seem to me conclusive. +Even if we grant that the increase of this character [cross-infertility] +occurs by the steps which he describes, <i>it is not +a process of accumulation by natural selection</i>. In order to be a +means of cumulative modification of varieties, races, or species, +selection, whether artificial or adaptational [i.e. natural], must +preserve certain forms of an intergenerating stock, to the +exclusion of other forms of the same stock. Progressive +change in the size of the occupants of a poultry-yard may be +secured by raising only bantams the first, only common fowls +the second, and only Shanghai fowls the third year; but this +is not the form of selection that has produced the different +races of fowls. So in nature, rats may drive out and supplant<span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span> +mice; but this kind of selection modifies neither rats nor mice. +On the other hand, if certain variations of mice prevail over +others, through their superior success in escaping their pursuers, +then modification begins. Now, turning to page 175, we +find that, in the illustrative case introduced by Mr. Wallace, +the commencement of infertility between the incipient species +is in the relations to each other of two portions of a species +that are locally segregated from the rest of the species, and +partially segregated from each other by different modes of +life. These two local varieties, being by the terms of his +supposition better adapted to the environment than the freely +interbreeding forms in other parts of the general area, increase +till they supplant these original forms. Then, in some limited +portion of the general area, there arise two still more divergent +forms, with greater mutual infertility, and with increased adaptation +to the environment, enabling them to prevail throughout +the whole area. The process here described, if it takes place, +is not modification by natural selection.</p></blockquote> + +<p>On the other hand, it <i>is</i> modification by physiological +selection. For, among the several other forms of isolation +which are called into requisition, the physiological (i.e. +ever accumulating cross-infertility) is supposed to play an +important part. That the modification is not modification +by natural selection may perhaps be rendered more +apparent by observing, that in as far as <i>any</i> other mode +of isolation is involved or supposed, so far is the <i>possible</i> +agency of natural selection eliminated <i>as between the two +or more otherwise isolated sections of a species</i>; and yet it is +modes of isolation other than that furnished by natural selection +(i.e. perishing of the less fit), that Mr. Wallace here +supposes to have been concerned—including, as I have +before shown, the physiological form, to which, indeed, he +really assigns most importance of all. Or, as Mr. Gulick +states the matter in his independent criticism:—</p> + +<blockquote><p>In the supposed case pictured by Mr. Wallace, the principle +by which the two segregating forms are kept from crossing,<span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span> +and so are eventually preserved as permanently distinct forms, +is no other than that which Mr. Romanes and myself have +discussed under the terms Physiological Selection and Segregate +Fecundity. Not only is Mr. Wallace's exposition of the divergence +and the continuance of the same in accord with these +principles which he has elsewhere rejected, but his whole +exposition is at variance with his own principle, which, in the +previous chapter, he vigorously maintains in opposition to my +statement that many varieties and species of Sandwich Island +land molluscs have arisen, while exposed to the same environment, +in the isolated groves of the successive valleys of the same +mountain range. If he adhered to his own theory, "the greater +infertility between the two forms in one portion of the area" +would be attributed to a difference between the <i>environment</i> presented +in that portion and that presented in the other portions; +and the difficulty would be to consistently show how this +greater infertility could continue unabated when the varieties +thus characterized spread beyond the environment on which +the character depends. But, without power to continue, the +process which he describes would not take place. Therefore, in +order to solve the problem of the <i>origin</i> and <i>increase</i> of +infertility between species, he tacitly gives up his own theory, +and adopts not only the theory of Physiological Selection but +that of Intensive Segregation<a name="FNanchor_61_61" id="FNanchor_61_61"></a><a href="#Footnote_61_61" class="fnanchor">[61]</a> through Isolation, though he +still insists on calling the process natural selection; for on +page 183 he says, "No form of infertility or sterility between +the individuals of a species can be increased by natural selection +unless correlated with some useful variation, while all +infertility not so correlated has a constant tendency to effect +its own elimination." Even this claim he seems to unwittingly +abandon when on page 184 he says: "The moment it [a +species] becomes separated either by geographical or selective +isolation, or by diversity of station or of habits, then, while +each portion must be kept fertile <i>inter se</i>, there is nothing +to prevent infertility arising between the two separated +portions."</p></blockquote> + +<p><span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span> +The criticism proceeds to show yet further inconsistencies +and self-contradictions in Mr. Wallace's treatment of this +subject; but it now seems needless to continue. Nor, +indeed, should I have quoted this much but for the sake +of so fully justifying my own criticism by showing the +endorsement which it has received from a completely independent +examination.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></p> + + + + +<h2>APPENDIX B.<br /> +<span class="smcap">An Examination by Mr. Fletcher Moulton of Mr. +Wallace's Calculation touching the Possibility Of +Physiological Selection ever acting alone.</span></h2> + + +<p>We have seen that the only important point of difference +between Mr. Wallace's more recent views and my own on +the problem of inter-specific sterility, has reference to the +question whether variations in the way of cross-infertility can +<i>ever</i> arise and act "alone, in an otherwise undifferentiated +species," or whether they can <i>never</i> so arise and act. It +is Mr. Wallace's opinion that, even if they ever do arise +alone, at all events they can never act in differentiating a +specific type, seeing that the chances against their suitable +mating must be so great: only if they be from the first +associated with some other form of homogamy, which will +have the effect of determining their suitable mating, does +he think that they can act in the way supposed by our +theory of "selective fertility"<a name="FNanchor_62_62" id="FNanchor_62_62"></a><a href="#Footnote_62_62" class="fnanchor">[62]</a>. On the other hand, as<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span> +previously and frequently stated, I have so strong a belief +in the segregating power of physiological selection, or +selective fertility, that I do not think it is necessary for +this principle to be <i>always</i> associated with some other form +of homogamy. From the first, indeed, I have laid great +stress (as, also, has Mr. Gulick) on the re-enforcing influence +which association with any other form of homogamy must +exercise upon the physiological form, and vice versa; but +I have also said that, in my opinion, the physiological form +may in many cases be able to act entirely alone, or without +assistance derived from any other source. The question +here is, as we have already so fully seen, a question of but +secondary importance; since, whether or not the physiological +form of homogamy ever acts alone, even Mr. Wallace +now allows, or rather argues, that it acts in combination—and +this so habitually, as well as with so much effect, that it +constitutes a usual condition to the origination of species. +Nevertheless, although the only relevancy of his numerical +computation of chances—whereby he thinks that he overturns +my theory <i>in toto</i>—is such relevancy as it bears to this +question of secondary importance, I have thought it desirable +to refer the question, together with Mr. Wallace's views upon +it, to the consideration of a trained mathematician.</p> + +<p>As this "subordinate question" depends entirely on +numerical computations involving the doctrine of chances, +I should first of all like to remark, that in reference to +biological problems of the kind now before us, I do not +myself attach much importance to a merely mathematical +analysis. The conditions which such problems involve are +so varied and complex, that it is impossible to be sure about +the validity of the <i>data</i> upon which a mathematical analysis is<span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span> +founded. Nevertheless, for the sake of meeting these +criticisms upon their own ground, I will endeavour to show +that, even as mathematical calculations, they are quite untrustworthy. +And, in order to do this effectually, I will quote +the results of a much more competent, as well as a much more +thorough, inquiry. I applied to Mr. Moulton for this +purpose, not only because he is one of the ablest mathematicians +of my acquaintance; but also because his interest +in biology, and his knowledge of Darwinian literature, +render him well fitted to appreciate exactly, and in all their +bearings, the questions which were submitted to his consideration. +I need only add that his examination was +completely independent, and in no way influenced by me. +Having previously read my paper on <i>Physiological Selection</i>, +Mr. Gulick's paper on <i>Divergent Evolution</i>, and Mr. Wallace's +book on <i>Darwinism</i>, he was in possession of all the materials; +and I merely requested the favour of his opinion upon the +whole case from a mathematical point of view. The +following is his reply; and I give it <i>in extenso</i>, because it +serves to place in another light some of the general considerations +which it has already been my endeavour to present<a name="FNanchor_63_63" id="FNanchor_63_63"></a><a href="#Footnote_63_63" class="fnanchor">[63]</a>.</p> + +<p>After some introductory remarks on Mr. Wallace's +"adoption of the theory of physiological selection pure +and simple," and "the pure caricature of it which he +puts forward as" mine, the letter proceeds thus:—</p> + +<blockquote><p>The reason why it is so easy to attack your theory is that +it is so easy to confuse the survival of an <i>individual</i> with the<span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span> +survival of a <i>peculiarity</i> of <i>type</i>. No one has ever said that +an <i>individual</i> is <i>assisted</i> by the possession of selective fertility: +that is a matter which cannot affect his chance of <i>life</i>. +Nor has any one said that the possession of selective fertility +in an <i>individual</i> will <i>of itself</i> increase the chance of his having +<i>progeny</i> that will survive, and in turn become the progenitors +of others that will survive. Taken by itself, the fact that an +<i>individual</i> is capable of fertility with some only of the opposite +sex lessens the chance of his having progeny. Whether +or not he is more or less favourably situated than his <i>confreres</i> +for the battle of life must be decided by the <i>total sum</i> +of his peculiarities; and the question whether or not this +selective fertility will be a hindrance must be decided by +considerations depending on the other peculiarities associated +with it.</p> + +<p>But when we come to consider the survival or permanence +of a <i>type</i> or <i>peculiarity</i>, the case is quite different. It then +becomes not only a favourable circumstance, but, in my opinion, +almost a necessary condition, that the peculiarity should be +associated with selective fertility<a name="FNanchor_64_64" id="FNanchor_64_64"></a><a href="#Footnote_64_64" class="fnanchor">[64]</a>.</p> + +<p>Take the case of the Jews. I don't think that intermarriage +with other nations would lessen their fertility, or diminish the +number of their progeny; nor is there any reason to think that +this progeny would be unequal to the struggle for existence. +But no one doubts that the abandonment of their voluntary +isolation (which operates so far as this is concerned as a selective +fertility), would lead to the disappearance of the familiar +Jewish type. All the world would get some of it; but as a whole +it would be "swamped."</p> + +<p>Now although no doubt Wallace would admit all this, he +fails to give it the weight it ought to have. In discussing the +question of its operation he considers too exclusively the case +of the individual.</p> + +<p>Of course, a type can only be perpetuated through the medium +of individuals, and all that his argument amounts to is, that<span class="pagenum"><a name="Page_161" id="Page_161">[Pg 161]</a></span> +selective fertility would be so fatal to individuals that <i>no</i> type +which presents it could be formed or perpetuated—a conclusion +which is not only absurd in itself, but contradicted by +his own subsequent adoption of your theory. Besides, apart +from calculations (with which I will deal when I write next), +such reasoning brings its own refutation. Selective fertility is +not in the same category as some of the other influences to +which an important share has been ascribed in the formation +of the existing types. <i>It exists as a recognized phenomenon.</i> +Hence all these numerical proofs that it would lead to extinction, +because it is so disadvantageous to the possessor, prove +too much. They would show that the degree of selective +fertility which so frequently characterizes species is a most +onerous gift; and that, were it not present, there would be +a vastly increased chance of fertility, which would render the +races fitter and lead to their increased survival. Why then +has it not been got rid of?</p> + +<p>The two answers which no doubt would be given seem to +me to support rather than to make against your theory. In +the first place, Wallace might say that this infertility is an +advantage because it keeps pure a type which is specially +fitted to its surroundings, as shown by its continued existence. +But if this be so, and it is necessary to protect the <i>developed</i> +type, how much more necessary to protect the <i>incipient</i> type! +In the second place, he might say that this selective fertility +is not so disadvantageous when the species has been formed, +because the individual can choose his mate from his like; +whereas, when it is beginning to be formed, he must mate +blindly, or without what you call "psychological selection." +But this seems to me to be wholly inapplicable to at least half +the animal, and to all the vegetable kingdom. Moreover, with regard +to the other half of the animal kingdom, it merely raises the +question,—How soon will such an incipient type recognize itself? +Seeing it is probable that many families [broods] will belong to +the same [incipient] type, I should not be surprised if it were +found that this sexual recognition and preference sets in very +early.</p> + +<p>But this leads me to the question of your letter. I understand +you to want me to examine and criticize the attempted<span class="pagenum"><a name="Page_162" id="Page_162">[Pg 162]</a></span> +numerical arguments against or for your theory. Now it seems +to me that it will be best to take, in the first instance, the +vegetable kingdom, and with regard to it I cannot see how +there can be any numerical argument against the theory. For +we often have species side by side with others nearly allied, +but much more numerous. The condition of these is precisely +analogous to that of your incipient species. They are exposed +to fertilization from, say, ten times as numerous individuals of +the allied species. They reject this in favour of that from the +relatively few individuals of their own. Yet the two species +are in competition. I could go through the numerical arguments +of your assailant word for word, applying them to +such a case as this, and they would triumphantly show that +the specific fertility of the rarer kind would lead to its certain +extinction. Yet we know that this is not so.</p> + +<p>Indeed, the too triumphant character of the logic used against +you seems to me to be capable of being turned to your use. +If cross-infertility is so intensely disadvantageous to the individuals +presenting it, it cannot have been <i>that</i> which made +these individuals and their progeny survive. It is therefore +a burden which they have carried. But we find that it is +more or less present in all the closely allied types that occur +on common areas: therefore it must be a necessary feature +in the formation of such types; for it cannot be an accident +that it is present in so many. In other words, it must be +the price which the individual and his progeny pay for their +formation into a type. And this is your theory pure and +simple.</p> + +<p>The more I consider the matter, the more I feel that it is +impossible to decide as to the sufficiency of selective fertility +to explain the formation of species, if we consider merely the +effect it would have on the number of individuals, as contrasted +with what it would be if no such peculiarity had developed +itself. Indeed, I may say that on pondering over +the matter I have come to the conclusion, that mere fertility +is probably a comparatively unimportant factor in the preservation +of the species, after a certain sufficient degree of fertility +is attained. I do not wish to be misunderstood. To a certain +point fertility is not only advantageous but necessary, in<span class="pagenum"><a name="Page_163" id="Page_163">[Pg 163]</a></span> +order to secure survival of the type; but I feel that little +reliance can be placed on calculations based on the numerical +co-efficient of fertility (i. e. the ratio of the number of offspring +to the number of parents) in determining the relative chance of +type-survival.</p> + +<p>Take, for instance, the oak tree. It produces thousands of +acorns, almost the whole of which die without producing any +progeny. Have we any reason to believe that if the number +of acorns borne by oak trees were diminished, even so much +as to one-tenth, the race of oaks would perish? It may +of course be said that, if all other things are equal, the probabilities +of survival must be increased by increased fertility +of this kind; but I feel convinced that when numerical fertility +has attained to a high point in circumstances in which +actual increase of the race cannot take place to any substantial +extent, the numerical value of this fertility sinks down +into a factor of the second or third order of importance—that +is to say, into the position of a factor whose effects are only to +be considered when we have duly allowed for the full effects +of all the main factors. Until we have done that, we gain +little or nothing in the way of accuracy of conclusion by taking +into consideration the minor factors. It may be very well to +neglect the effect of the attraction of Jupiter in our early researches +on the motion of the Moon; and our doing so will +not prevent the results being approximate and having considerable +value, because we are retaining the two main factors that +establish the motion, viz. the effects of the Earth and the Sun. +But if we exclude the effect of one of these main factors, our +results would be worthless; and it would not be rendered substantially +less so by the fact that we had taken Jupiter into +account in arriving at them.</p> + +<p>You must not imagine, however, that I think it wholly profitless +to see whether there would be any substantial effect on +numerical fertility were <i>selective</i> fertility to manifest itself. But +if we want to derive any assistance from calculation, it must +be by applying it with a good deal more precision and definiteness +than anything that Wallace shows. And, in the first +place, it is useless to confuse the vegetable and animal kingdoms. +In the former you have union unaffected by choice; in the latter,<span class="pagenum"><a name="Page_164" id="Page_164">[Pg 164]</a></span> +so far at all events as the higher animals are concerned, you +have "psychological selection." In order to give you a specimen +of what can safely be done by calculation if you take +a problem of sufficient definiteness, I have chosen the case of +a flowering plant in which a certain proportion of the race +have developed the peculiarity of being sterile with the remainder, +while retaining the normal fertility of the race in +unions among themselves. In order to give the greatest advantage +to your critics, I have assumed that such flowers as +possess the peculiarity are not self-fertilizable; for it is clear that +if we suppose that they are self-fertilizable, the fertility need +be very slightly affected.</p> + +<p>As I have excluded self-fertilization, it is necessary, if we are +to get any trustworthy results, that one should consider the +mode in which fertilization will be produced. I have taken +the case of fertilization by insects, and have assumed that each +flower is visited a certain number of times by insects during +the period when fertilization is possible; and, further, that the +insects which visit it have on the average visited a certain +number of flowers of the same species before they came there. +Of course nothing but observation can fix these latter numbers; +but I should not be surprised at finding that they are of +considerable magnitude<a name="FNanchor_65_65" id="FNanchor_65_65"></a><a href="#Footnote_65_65" class="fnanchor">[65]</a>. In order to make the results a little<span class="pagenum"><a name="Page_165" id="Page_165">[Pg 165]</a></span> +more intelligible, I have grouped them under the numbers which +represent the average number of flowers that an insect visits +in a journey. This is a little more than twice as great as the +number which represents the number of flowers he has on the +average visited before coming to the individual whose fertility +we are considering.</p> + +<p>I send you the formula and the calculation on which it is +based in an Appendix; but as I know you have a holy horror +of algebraical formulae, I give you here a few numerical +results.</p> + +<p>The cases I have worked out are those in which the number +of insects visiting each flower is 5, or 10, or 15; and I have +also taken 5, 10, and 15, to represent the number of flowers +which an insect visits each journey. This makes nine cases +in all; and I have applied these to two instances—viz. one +in which one-fifth of the whole race have developed cross-infertility, +and the other in which one-tenth only have done so. +Taking first the instance where one-fifth have developed the +peculiarity, I find that if on the average five insects visit +a flower, and each insect on the average visits five flowers on +a journey, the fertility is diminished by about one-tenth. If, +however, the average number of flowers the insect visits is ten, +the reduction of fertility is less than one per cent. And it +becomes inappreciable if the average number is fifteen. If on +the average ten insects visit each flower, then, if each insect +visits on the average five flowers on a journey, the reduction +of fertility is a little over one per cent.; but if it visits ten or +fifteen the reduction is inappreciable. If fifteen insects visit the +flower on an average, then, if these insects on the average visit<span class="pagenum"><a name="Page_166" id="Page_166">[Pg 166]</a></span> +five or more flowers on a journey, the reduction of fertility +is inappreciable.</p> + +<p>By the term inappreciable I mean that it is not substantially +greater than one-tenth of one per cent.—i.e. not more +than one-thousandth.</p> + +<p>Of course, if the proportion of individuals acquiring the +peculiarity is less, the effect on the fertility under the above +hypothesis will be greater; and it will not be counteracted so +fully unless the number of insect visits is larger, or unless the +insects visit more flowers on a journey. Thus if only one-tenth +of the race have developed the peculiarity, then, if each flower +is visited on the average by five insects who visit five flowers +on each trip, the fertility will be reduced about one-third. +If, however, the insects visit on the average ten flowers per +trip, it will be only diminished about one-tenth; and if they +visit fifteen on each trip, it will be only diminished about +one-fortieth. If in the same case we suppose that each +flower receives ten insect visits, then, if the insects visit on an +average five flowers per trip, the fertility will be diminished +about one-eighth. If they visit ten on a trip, it will be diminished +about one-hundredth, and the diminution is inappreciable +if they visit fifteen on a trip. Similarly, if a flower receives +fifteen insect visits, the diminution is about one-twenty-fifth, +if insects visit on the average five flowers on a trip; and is +inappreciable if they visit ten or fifteen.</p> + +<p>These figures will show you that it is exceedingly possible +that a peculiarity like this, the effect of which at first sight +would seem to be so prejudicial to fertility, may in fact have +little or no influence upon it; and if you set against this the +overwhelming importance of such a peculiarity in segregating +the type so as to give it a chance of becoming a fixed species, +you will, I think, feel that your hypothesis has nothing to +fear from a numerical examination.</p> + +<p>I have not examined the case of fertilization by other means; +nor have I examined the case of fertilization in animals, where +psychological selection can come in. To obtain any useful +results, one would have to consider very carefully the circumstances +of each case; and at present, at all events, I do not +think it would be useful to do so. Nor have I attempted to<span class="pagenum"><a name="Page_167" id="Page_167">[Pg 167]</a></span> +show the converse of the problem—viz. the effect of swamping +where cross-fertilization is possible. I shall be very glad to +examine any one of these cases if you want me to do so; +but I should prefer to leave it until I hear from you again.</p> + +<p>If you contrast the results that I have given above with +those given on pages 181 to 183 of Wallace's book, you will +see the enormous difference. His calculations can only apply +to the animal kingdom in those cases in which there is only +a union between one individual of each sex; and before you +can deal with the question of such animals, you will have to +take into consideration many elements besides that of mere +fertility, if you wish to get any tolerably accurate result<a name="FNanchor_66_66" id="FNanchor_66_66"></a><a href="#Footnote_66_66" class="fnanchor">[66]</a>.</p></blockquote> + +<p>The above analysis leaves nothing to be added by me. +But, in conclusion, I may once more repeat that the particular +point with which it is concerned is a point of very subordinate +importance. For even if Mr. Wallace's computation +of chances had been found by Mr. Moulton to have been an +adequate computation—and, therefore, even if it had been +thus proved that physiological homogamy must always be +associated with some other form of homogamy in order to +produce specific divergence—still the importance of selective +fertility as a factor of organic evolution would not have +been at all diminished. For such a result would merely +have shown that, not only "in many cases" (as I originally +said), but actually in all cases, the selective fertility which +I hold to have been so generally concerned in the differentiation +of species has required for this purpose the co-operation +of some among the numerous other forms of homogamy. +But inasmuch as, by hypothesis, no one of these other or +co-operating factors would of itself have been capable of +effecting specific divergence in any of the cases where its +association with selective fertility is concerned, the mathematical<span class="pagenum"><a name="Page_168" id="Page_168">[Pg 168]</a></span> +proof that such an association is <i>always</i>—and not +merely <i>often</i>—necessary, would not have materially affected +the theory of the origin of species by means of physiological +selection. We have now seen, however, that a competent +mathematical treatment proves the exact opposite; and, therefore, +that Mr. Wallace's criticism fails even as regards the +very subordinate point in question.</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_169" id="Page_169">[Pg 169]</a></p> + + + + +<h2>APPENDIX C.<br /> +<span class="smcap">Some Extracts from the Author's Note-books.</span></h2> + + +<p><i>Bearing of Weismannism on Physiological Selection.</i>—If +in view of other considerations I could fully accept Professor +Weismann's theory of heredity, it would appear to me in no +small measure to strengthen my own theory of physiological +selection. For Weismann's theory supposes that all changes +of specific type must have their origin in variations of a +continuous germ-plasm. But <i>the more the origin of species is +referred directly to variations arising in the sexual elements, +the greater is the play given to the principles of physiological +selection</i><a name="FNanchor_67_67" id="FNanchor_67_67"></a><a href="#Footnote_67_67" class="fnanchor">[67]</a>; while, on the other hand, the less standing-ground +is furnished to the theory that cross-infertility between allied +species is due to "external conditions of life," "prolonged +exposure to uniform change of conditions," "structural +modifications re-acting on the sexual functions"; or, in +short, that "somatogenetic" changes of any kind can of +themselves induce the "blastogenetic" change of cross-infertility +between progeny of the same parental stock.</p> + + +<p><i>Cross-infertility and Diversity of Life.</i>—Observe that one +great consequence of duly recognizing the importance of intercrossing +is indefinitely to raise our estimate of the part played +by the principle of cross-infertility in diversifying organic +nature. For whenever in any line of descent the bar of<span class="pagenum"><a name="Page_170" id="Page_170">[Pg 170]</a></span> +sterility arises, there the condition is given for a new crop of +departures (species of a genus); and when genera are formed +by the occurrence of this bar, there natural selection and all +other equilibrating causes are supplied with new material for +carrying on adaptational changes in new directions. Thus, +owing to cross-infertility, all these causes are enabled to +work out numberless adaptations in many directions (i. e. +lines of descent) simultaneously.</p> + + +<p><i>Cross-infertility and Stability.</i>—The importance of sterility +as a diagnostic feature is obvious if we consider that more +than any other feature it serves to give <i>stability</i> to the type; +and unless a type is stable or constant, it cannot be ranked +as a species. That Darwin himself attributes the highest +importance to this feature as diagnostic, see <i>Forms of Flowers</i>, +pp. 58, 64.</p> + + +<p><i>Cross-infertility and Specific Differentiation.</i>—In their +elaborate work on the many species of the genus Hieracium, +Nägeli and Peter are led to the general conclusion that the +best defined species are always those which display absolute +sterility <i>inter se</i>; while the species which present most +difficulty to the systematist are always those which most +easily hybridize. Moreover, they find, as another general +rule applicable to the whole genus, that there is a constant +correlation between inability to hybridize and absence of +intermediate varieties, and, conversely, between ability to +hybridize and the presence of such varieties.</p> + + +<p><i>Cross-infertility in Domesticated Cattle.</i>—Mr. J. W. Crompton, +who has had a large experience as a professional cattle-breeder, +writes to me (March 2, 1887)—</p> + +<blockquote><p>"That form of barrenness, very common in some districts, +which makes heifers become what are called 'bullers'—that +is, irregularly in 'season,' wild, and failing to conceive—is +certainly produced by excess of iron in their drinking-water, +and I suspect also by a deficiency of potash in the soil."</p></blockquote><p><span class="pagenum"><a name="Page_171" id="Page_171">[Pg 171]</a></span></p> + +<p>He also informs me that pure white beasts of either sex +are so well known by experienced breeders to be comparatively +infertile together, that they are never used for breeding +purposes, so that "in some parts of the country, where a +tendency to sterility had become so confirmed in the white +race that they utterly died out," only the coloured breeds are +now to be found. He goes on to say that if "a lot of white +heifers were put to a lot of white bulls, I think you would +probably get a fertile breed of pure white cattle.... I think, +in short, that domestication has produced just what your +theory suggests, a new variety inclined to prove sterile with +its parent stock."</p> + +<p>Commenting on the origin of domesticated cattle, Professor +Oscar Schmidt remarks (<i>Doctrine of Descent</i>, p. 139)—</p> + +<blockquote><p>"Rütimeyer's minute researches on domestic cattle have shown +that, in Europe at least, three well-defined species of the diluvial +period have contributed to their formation—<i>Bos primigenius</i>, +<i>longifrons</i>, and <i>frontosus</i>. These species once lived geographically +separate, but contemporaneously; and they and their +specific peculiarities have perished, to rise again in our domestic +races. These races breed together with unqualified fertility. +In the form of skull and horns they recall one or other of the +extinct species; but collectively they constitute a new main species. +That from their various breeds, the three or any one of the +aboriginal species would ever emerge in a state of pristine +purity, would be an utterly ludicrous assertion."</p></blockquote> + +<p>Now, seeing that these "aboriginal species," although living +"contemporaneously," were "geographically separate," we +can well understand that their divergence of type from a +common ancestor did not require, as a condition to their +divergence, that any cross-sterility should have arisen between +them. The geographical isolation was enough to secure +immunity from mutual intercrossing, and therefore, as our +present theory would have expected as probable, morphological +divergence occurred without any corresponding physiological<span class="pagenum"><a name="Page_172" id="Page_172">[Pg 172]</a></span> +divergence, as must almost certainly have been the +case if such polytypic evolution had occurred on a common +area. Indeed, one of the two lines of experimental verification +of our theory consists in selecting cases where nearly +allied species are separated by geographical barriers, and +proving that, in such cases, there is no cross-sterility.</p> + + +<p><i>Fertility of Domesticated Varieties.</i>—Some writers have +sought to explain the contrast between domesticated varieties +and natural species in respect of fertility when crossed, by +the consideration that it is only those natural species which +have proved themselves so far flexible as to continue fertile +under changed conditions of life that can have ever allowed +themselves to become domesticated. But although this +condition may well serve to explain the unimpaired fertility +under domestication of such species as for this very reason have +ever become domesticated, I fail to see how it explains the +further and altogether different fact, that this fertility continues +unimpaired between all the newly differentiated morphological +types which have been derived from the original specific type. +It is one thing that this type should continue fertile after +domestication: it is quite another thing that fertility should +continue as between all its modified descendants, even +although the amount of modification may extend much +further than that which usually obtains between different +natural species.</p> + + +<p><i>Testing for Cross-infertility</i> among varieties growing on +the same area is a much more crucial line of verification than +testing for unimpaired fertility between allied species which +occupy different areas, because while in the former case we +are dealing with "incipient species" with a view to ascertaining +whether the divergence which they have already undergone +is accompanied by physiological isolation, in the latter case +we can never be sure that two allied species, which are now +widely disconnected geographically, have always been so<span class="pagenum"><a name="Page_173" id="Page_173">[Pg 173]</a></span> +disconnected. They may both have originated on the same +area; or one may have diverged from the other before it +migrated from that area; or even if, when it migrated, it was +unchanged, and if in its new home it afterwards split into two +species by physiological selection, the newer species would +probably prove infertile, not only with its parent type, but +also with its grand-parent in any other part of the world.</p> + + +<p><i>Seebohm on Isolation.</i>—Seebohm is so strongly influenced +by the difficulty from "the swamping effects of free intercrossing," +that he is driven by it to adopt Asa Gray's hypothesis +of variations as teleological. Indeed, he goes as far as +Wagner, for he maintains that in no case can there be +divergence or multiplication of species without isolation. +He makes the important statement that "the more the +geographical distribution of birds is studied, the more doubtful +it seems to be that any species of bird has ever been differentiated +without the aid of geographical isolation" (<i>Charadriidae</i>, +p. 17). If this is true, it makes in favour of physiological +selection by showing the paramount importance of the +swamping effects of intercrossing, and consequent importance +of isolation. But it makes against physiological +selection by showing that the geographical form of isolation +is sufficient to explain all the cases of specific differentiation +in birds. But I must remember that the latter point rests +largely on negative inference, and that birds, owing to +their highly locomotive habits, are the class of animals where +physiological selection is likely to be most handicapped.</p> + + +<p><i>Herbert on Hybridization.</i>—Herbert tells us that when he +first astonished the Horticultural Society by laying before them +the results of his experiments on hybridization, his brother +botanists took serious alarm. For it appeared to them that +this "intermixture of species would confuse the labours +of botanists, and force them to work their way through +a wilderness of uncertainty." Therefore he was bluntly told<span class="pagenum"><a name="Page_174" id="Page_174">[Pg 174]</a></span> +by several of these gentlemen, "I do not thank you for your +mules." Now, although naturalists have travelled far and +learnt much since those days, it appears to me that a modern +evolutionist might still turn to the horticulturist with the same +words. For assuredly he has no reason to thank the +horticulturist for his mules, until he has found a satisfactory +answer to the question why it is that natural species differ so +profoundly as regards their capacity for hybridizing.</p> + + +<p><i>Advance on Herbert's Position.</i>—- If it be said that all my +work amounts to showing what Herbert said long ago—viz. +that the only true or natural distinction between organic types +is the sexual distinction—I answer that my work does much +more than this. For it shows that the principle of sterility +is the main condition to the differentiation, not merely of +species and genera, but also to the evolution of adaptations +everywhere, in higher as well as in lower taxonomic divisions. +Moreover, even though naturalists were everywhere to consent +to abandon specific designations, and, as Herbert advises, to +"entrench themselves behind genera," there would still remain +the facts of what are now called specific differences (of +the secondary or morphological kind), and by whatever name +these are called, they alike demand explanation at the hands +of the evolutionist.</p> + + +<p><i>Fritz Müller on Cross-infertility.</i>—Fritz Müller writes, +"Every plant requires, for the production of the strongest +possible and most prolific progeny, a certain amount of +difference between male and female elements which unite. +Fertility is diminished as well when this degree is too low +(in relatives too closely allied) as when it is too high (in +those too little related)." Then he adds, as a general rule, +"Species which are wholly sterile with pollen of the same +stock, and even with pollen of nearly allied stocks, will +generally be fertilized very readily by the pollen of another +species. The self-sterile species of the genus Abutilon,<span class="pagenum"><a name="Page_175" id="Page_175">[Pg 175]</a></span> +which are, on the other hand, so much inclined to hybridization, +afford a good example of this theory, which appears +to be confirmed also by Lobelia, Passiflora, and Oncidium" +(<i>American Naturalist</i>, vol. viii, pp. 223-4, 1874).</p> + + +<p><i>Different groups of plants exhibit remarkable differences in +the capability of their constituent species to hybridize.</i>—In so +far as these differences have reference only to first crosses, +they have no bearing either for or against my theory. Only +in so far as the differences extend to the production of fertile +hybrids does any question arise for me. First of all, therefore, +I must ascertain whether (or how far) there is any correlation +between groups whose species manifest aptitude to form first +crosses, and groups where first crosses manifest aptitude +to produce fertile hybrids. Next, whatever the result of this +inquiry should be, if I find that certain natural groups of +plants exhibit comparatively well-marked tendencies to form +fertile hybrids, the question will arise, Are these tendencies +correlated with <i>paucity</i> of species? If they are, the fact +would make strongly in favour of physiological selection. +For the fact would mean that in these natural groups, owing +to "the nature of the organisms" included under them, less +opportunity is given to physiological selection in its work of +differentiating specific types than is given by other natural +groups where the nature of the organism renders them more +prone to mutual sterility. But in prosecuting this branch +of verification, I must remember to allow for possibilities of +differential degrees of geographical isolation in the different +groups compared.</p> + +<p>On this subject Focke writes me as follows:—"In a +natural group (family, order, genus) showing considerable +variability in the structure of the flower, we may expect +to find [or do find] a greater number of mules than in +a group whose species are only distinguished by differences +in the shape of the leaves, or in growth, &c. I do not<span class="pagenum"><a name="Page_176" id="Page_176">[Pg 176]</a></span> +know, however, which in this connexion of things is the +cause and which the effect. A useful ancestral structure of +the flower may be conserved by an otherwise varying progeny, +on condition that the progress of diversity be not +disturbed by frequent intercrossings. [Therefore, if this +condition be satisfied, the structure of the flower in different +members of the group will continue constant: here the cause +of <i>constancy</i> in the flower (however much variability there +may be in the leaves, &c.) is its original <i>inability</i> to hybridize.] +On the other hand, in species or groups ready to +hybridize [or capable of hybridizing], the fixation of a new +specific type will require some change in the structure of the +flower, and a change considerable enough to alter the conditions +of fertilization. [Here the reason of the <i>in</i>constancy +of the flower in different members of the group is the +original <i>aptitude</i> of their ancestral forms to hybridize.] +Perhaps there is something in this suggestion, but certainly +there are other efficient physiological relations, which are +at present unknown. Your theory of physiological selection +may serve to explain many difficult facts."</p> + + +<p><i>The Importance of Prepotency.</i>—A. Kerner shows by means +of his own observations on sundry species of plants which +hybridize in the wild state, that they do so very much more +frequently if both, or even if only one of the parent forms be +rare in the neighbourhood. This fact can only be explained +by supposing that, even in species most prone to hybridizing +under Nature, there is some degree of prepotency of pollen +of the same species over that of the other species; so that +where both species are common, it is correspondingly rare +that the foreign pollen gets a chance. But if there were no +prepotency, the two species would blend; and this Kerner +supposes must actually take place wherever two previously +separated species, thus physiologically circumstanced, happen +to be brought together. (Kerner's paper is published in<span class="pagenum"><a name="Page_177" id="Page_177">[Pg 177]</a></span> +<i>Oester. Bot. Zeitschrift</i>, XXI, 1871, where he alludes to +sundry other papers of his own advocating similar views.)</p> + +<p>The relation of these observations to Jordan's <i>espèces affines</i> +is obvious. We have only to suppose that some such slight +and constant difference characterizes the sexual elements of +these allied varieties as demonstrably characterizes their +morphology, and we can understand how pollen-prepotency +would keep the forms distinct—such forms, therefore, being +so many records of such prepotency.</p> + +<p>Both from Kerner's work, and still more from that of +Jordan and Nägeli, I conclude that (at all events in plants) +prepotency is the way in which physiological selection +chiefly acts. That is to say, <i>sudden</i> and <i>extreme</i> variations in +the way of sexual incompatibility are probably rare, as compared +with some degree of prepotency. According as this +degree is small or great so will be the amount of the +corresponding separation. This view would show that in +plants the principle of physiological selection is one of +immensely widespread influence, causing (on the same +areas) more or less permanent varieties much below specific +rank. And when we remember on how delicate a balance +of physiological conditions complete correspondency of pollen +to ovules depends, we may be prepared to expect that the +phenomenon of prepotency is not of uncommon occurrence.</p> + + +<p><i>Self-fertilization and Variability.</i>—It occurred to Count +Berg Sagnitz that, if physiological selection is a true +principle in nature, vegetable species in which self-fertilization +obtains ought to be more rich in constant +varieties than are species in which cross-fertilization rules. +For, although even in the latter case physiological isolation +may occasionally arise, it cannot be of such habitual or +constant occurrence as it must be in the former case. +Acting on this idea, Count Berg Sagnitz applied himself to +ascertain whether there is any general correlation between the<span class="pagenum"><a name="Page_178" id="Page_178">[Pg 178]</a></span> +habit of self-fertilization and the fact of high variability; and +he says that in all the cases which he has hitherto investigated, +the correlation in question is unmistakable.</p> + + +<p><i>Additional Hypothesis concerning Physiological Selection.</i>—In +reciprocal crosses <i>A</i> × <i>B</i> is often more fertile than +<i>B</i> × <i>A</i>. If hybrid <i>AB</i> is more fertile with <i>A</i>, and hybrid +<i>BA</i> with <i>B</i>, than vice versa, there would be given a good +analogy on which to found the following hypothesis.</p> + +<p>Let <i>A</i> and <i>B</i> be two intergenerating groups in which +segregate fecundity is first beginning. Of the hybrids, <i>AB</i> +will be more fertile with <i>A</i>, and <i>BA</i> with <i>B</i>, than vice versa. +The interbreeding of <i>AB</i> with <i>A</i> will eventually modify +sexual characters of <i>A</i> by assimilating it to those of <i>AB</i>, +while the interbreeding of <i>BA</i> with <i>B</i> will similarly modify +sexual characters of <i>B</i> by assimilating it to those of <i>BA</i>. +Consequently, <i>A</i> will become more and more infertile with +<i>B</i>, while <i>B</i> becomes more and more infertile with <i>A</i>. Fewer +and fewer hybrids will thus be produced till mutual sterility +is complete.</p> + +<p>To sustain this hypothesis it would be needful to prove +experimentally, (1) that hybrid forms <i>AB</i> are more fertile +with <i>A</i> than with <i>B</i>, while hybrid forms <i>BA</i> are more fertile +with <i>B</i> than with <i>A</i> [or, it may be possible that the opposite +relations would be found to obtain, viz. that <i>AB</i> would be +more fertile with <i>B</i>, and <i>BA</i> with <i>A</i>]; (2) that, if so, +effect of intercrossing <i>AB</i> with <i>A</i> is to make progeny more +fertile with <i>A</i> than with <i>B</i>, while effect of intercrossing <i>BA</i> +with <i>B</i> is to make progeny more fertile with <i>B</i> than with <i>A</i>.</p> + +<p>Such experiments had best be tried with species where +there is already known to be a difference of fertility between +reciprocal crosses (e.g. Matthiola annua and M. glabra, see +<i>Origin of Species</i>, p. 244).</p> + +<hr class="chap" /> +<p class="pagenum"><a name="Page_179" id="Page_179">[Pg 179]</a></p> + + + + +<h2>INDEX</h2> + + + +<ul class="index"> +<li class="ifrst">A.</li> + +<li class="indx"><span class="smcap">Allen</span>, Mr. J. A., on variation under nature, <a href="#Page_34">34</a>.</li> + +<li class="indx">Amixia, <a href="#Page_12">12</a>-<a href="#Page_28">28</a>, <a href="#Page_110">110</a>-<a href="#Page_115">115</a>, <a href="#Page_117">117</a>-<a href="#Page_133">133</a>.</li> + +<li class="indx">Apogamy, <a href="#Page_5">5</a>, <a href="#Page_6">6</a>, <a href="#Page_10">10</a>, <a href="#Page_18">18</a>, <a href="#Page_28">28</a>.</li> + + +<li class="ifrst">B.</li> + +<li class="indx"><span class="smcap">Belt</span>, on physiological selection, <a href="#Page_44">44</a>.</li> + +<li class="indx"><span class="smcap">Berg Sagnitz</span>, Count, on self-fertilization and variability, <a href="#Page_177">177</a>.</li> + +<li class="indx">Breeding, separate and segregate, <a href="#Page_5">5</a>.</li> + +<li class="indx">Butterflies of polar regions and Alps, <a href="#Page_133">133</a>.</li> + + +<li class="ifrst">C.</li> + +<li class="indx"><span class="smcap">Catchpool</span>, Mr., on physiological selection, <a href="#Page_44">44</a>, <a href="#Page_137">137</a>.</li> + +<li class="indx">Cross-infertility, <a href="#Page_46">46</a>;</li> +<li class="isub1">and varietal divergence, <a href="#Page_82">82</a>;</li> +<li class="isub1">and diversity of life, <a href="#Page_169">169</a>;</li> +<li class="isub1">and stability, <a href="#Page_170">170</a>;</li> +<li class="isub1">and specific differentiation, <a href="#Page_170">170</a>;</li> +<li class="isub1">in domesticated cattle, <a href="#Page_170">170</a>;</li> +<li class="isub1">testing for, <a href="#Page_172">172</a>;</li> +<li class="isub1">Fritz Müller on, <a href="#Page_174">174</a>.</li> + + +<li class="ifrst">D.</li> + +<li class="indx"><i>Darwin</i>, Charles, on isolation, <a href="#Page_2">2</a>, <a href="#Page_106">106</a>;</li> +<li class="isub1">on diversity under nature, <a href="#Page_31">31</a>;</li> +<li class="isub1">on the fertility of varieties, <a href="#Page_50">50</a>;</li> +<li class="isub1">on the origin of cross-infertility, <a href="#Page_51">51</a>;</li> +<li class="isub1">on distribution, <a href="#Page_68">68</a>;</li> +<li class="isub1">on prepotency, <a href="#Page_89">89</a>;</li> +<li class="isub1">on geographical isolation, <a href="#Page_101">101</a>, <a href="#Page_108">108</a>;</li> +<li class="isub1">on methodical selection, <a href="#Page_102">102</a>;</li> +<li class="isub1">on modification in large areas, <a href="#Page_103">103</a>;</li> +<li class="isub1">on the swamping effects of intercrossing, <a href="#Page_105">105</a>;</li> +<li class="isub1">on independent variability, <a href="#Page_109">109</a>;</li> +<li class="isub1">on domestic animals, <a href="#Page_110">110</a>.</li> + +<li class="indx"><span class="smcap">Delbœuf</span>, law of independent variability, <a href="#Page_13">13</a>.</li> + +<li class="indx">Differentiation under natural selection, <a href="#Page_37">37</a>.</li> + +<li class="indx">Diversity of life and cross-infertility, <a href="#Page_169">169</a>.</li> + +<li class="indx">Domesticated cattle and cross-infertility, <a href="#Page_170">170</a>, <a href="#Page_172">172</a>.</li> + + +<li class="ifrst">E.</li> + +<li class="indx">Evidences of physiological selection, <a href="#Page_62">62</a>.</li> + +<li class="indx">Evolution, monotypic and polytypic, <a href="#Page_21">21</a>, <a href="#Page_75">75</a>, <a href="#Page_102">102</a>, <a href="#Page_107">107</a>, <a href="#Page_112">112</a>, <a href="#Page_129">129</a>.</li> + +<li class="indx">Experimental research in physiological selection, <a href="#Page_85">85</a>.</li> + + +<li class="ifrst">F.</li> + +<li class="indx">Fertility of domesticated varieties, <a href="#Page_172">172</a>.</li> + +<li class="indx"><span class="smcap">Focke</span>, Herr, on hybridization, <a href="#Page_175">175</a>.</li> + + +<li class="ifrst">G.</li> + +<li class="indx"><span class="smcap">Galton</span>, Mr. Francis, law of regression, <a href="#Page_39">39</a>.</li> + +<li class="indx">General conclusions, <a href="#Page_144">144</a>.</li> + +<li class="indx">Geographical distribution and physiological selection, <a href="#Page_65">65</a>.</li> + +<li class="indx"><span class="smcap">Giard</span>, M., on apogamy, <a href="#Page_14">14</a>.</li> + +<li class="indx"><span class="pagenum"><a name="Page_180" id="Page_180">[Pg 180]</a></span><span class="smcap">Grabham</span>, Dr., on mollusca of Madeira, <a href="#Page_135">135</a>.</li> + +<li class="indx"><span class="smcap">Gulick</span>, Rev. J., on natural Selection as a mode of isolation, <a href="#Page_9">9</a>;</li> +<li class="isub1">on divergence, <a href="#Page_11">11</a>;</li> +<li class="isub1">on segregate breeding, <a href="#Page_19">19</a>;</li> +<li class="isub1">on geographical distribution, <a href="#Page_27">27</a>;</li> +<li class="isub1">on the prevention of intercrossing, <a href="#Page_127">127</a>;</li> +<li class="isub1">on Mr. Wallace's criticisms, <a href="#Page_151">151</a>.</li> + + +<li class="ifrst">H.</li> + +<li class="indx"><span class="smcap">Herbert</span>, on hybridization, <a href="#Page_173">173</a>;</li> +<li class="isub1">advance on his position, <a href="#Page_174">174</a>.</li> + +<li class="indx"><span class="smcap">Herdman</span>, Prof., on physiological isolation, <a href="#Page_123">123</a>.</li> + +<li class="indx">Historical sketch of opinions on isolation, <a href="#Page_101">101</a>.</li> + +<li class="indx">Homogamy, <a href="#Page_5">5</a>, <a href="#Page_6">6</a>;</li> +<li class="isub1">forms of, <a href="#Page_7">7</a>, <a href="#Page_19">19</a>, <a href="#Page_29">29</a>.</li> + +<li class="indx">Hybridization, <span class="smcap">Herbert</span> on, <a href="#Page_173">173</a>;</li> +<li class="isub1">in plants, <a href="#Page_175">175</a>.</li> + +<li class="indx">Hypothesis, additional, concerning physiological selection, <a href="#Page_178">178</a>.</li> + + +<li class="ifrst">I.</li> + +<li class="indx">Independent variability, <a href="#Page_12">12</a>-<a href="#Page_29">29</a>.</li> + +<li class="indx">Isolation, defined, <a href="#Page_2">2</a>;</li> +<li class="isub1">forms of, <a href="#Page_3">3</a>, <a href="#Page_6">6</a>;</li> +<li class="isub1">geographical, <a href="#Page_3">3</a>;</li> +<li class="isub1">discriminate and indiscriminate, <a href="#Page_5">5</a>;</li> +<li class="isub1">physiological, <a href="#Page_9">9</a>, <a href="#Page_41">41</a>, <a href="#Page_58">58</a>;</li> +<li class="isub1">its importance, <a href="#Page_39">39</a>;</li> +<li class="isub1">sketch of opinions on, <a href="#Page_101">101</a>;</li> +<li class="isub1">general conclusions, <a href="#Page_144">144</a>;</li> +<li class="isub1"><span class="smcap">Seebohm</span> on, <a href="#Page_173">173</a>.</li> + + +<li class="ifrst">J.</li> + +<li class="indx"><span class="smcap">Jordan</span>, M., on cross sterile varieties of plants, <a href="#Page_86">86</a>;</li> +<li class="isub1">his researches summarized, <a href="#Page_87">87</a>.</li> + + +<li class="ifrst">K.</li> + +<li class="indx"><span class="smcap">Kerner</span>, Prof. A., on prepotency, <a href="#Page_176">176</a>.</li> + + +<li class="ifrst">L.</li> + +<li class="indx"><span class="smcap">Lankester</span>, Prof. Ray, on divergent evolution, <a href="#Page_15">15</a>.</li> + +<li class="indx"><span class="smcap">Le Conte</span>, Prof., on fossil snails of steinheim, <a href="#Page_95">95</a>;</li> +<li class="isub1">on isolation, <a href="#Page_129">129</a>.</li> + +<li class="indx"><span class="smcap">Livingstone</span>, Dr. David, Quoted, <a href="#Page_123">123</a>.</li> + + +<li class="ifrst">M.</li> + +<li class="indx"><span class="smcap">Meldola</span>, Prof., on difficulty from intercrossing, <a href="#Page_121">121</a>.</li> + +<li class="indx">Misunderstandings of Physiological selection, <a href="#Page_59">59</a>.</li> + +<li class="indx">Monotypic evolution, see Evolution.</li> + +<li class="indx"><span class="smcap">Morgan</span>, Prof. Lloyd, on sterility, <a href="#Page_56">56</a>;</li> +<li class="isub1">on isolation, <a href="#Page_128">128</a>.</li> + +<li class="indx"><span class="smcap">Moulton</span>, Mr. Fletcher, an examination of Mr. Wallace's calculations on physiological selection, <a href="#Page_157">157</a>.</li> + +<li class="indx"><span class="smcap">Müller</span>, Fritz, on cross-infertility, <a href="#Page_174">174</a>.</li> + + +<li class="ifrst">N.</li> + +<li class="indx"><span class="smcap">Nägeli</span>, on isolation, <a href="#Page_76">76</a>;</li> +<li class="isub1">on synoicy, <a href="#Page_78">78</a>, <a href="#Page_82">82</a>.</li> + +<li class="indx">Natural selection, a form of discriminate isolation, <a href="#Page_9">9</a>, <a href="#Page_10">10</a>, <a href="#Page_23">23</a>;</li> +<li class="isub1">leads to monotypic evolution, <a href="#Page_24">24</a>-<a href="#Page_29">29</a>;</li> +<li class="isub1">difficulties of, <a href="#Page_41">41</a>, <a href="#Page_51">51</a>.</li> + + +<li class="ifrst">P.</li> + +<li class="indx">Panmixia, <a href="#Page_12">12</a>.</li> + +<li class="indx">Physiological selection, <a href="#Page_9">9</a>, <a href="#Page_41">41</a>;</li> +<li class="isub1">summarized, <a href="#Page_58">58</a>;</li> +<li class="isub1">misunderstandings of, <a href="#Page_59">59</a>;</li> +<li class="isub1">evidences of, <a href="#Page_81">81</a>-<a href="#Page_119">119</a>;</li> +<li class="isub1">and Weismannism, <a href="#Page_169">169</a>;</li> +<li class="isub1">additional hypothesis, <a href="#Page_178">178</a>.</li> + +<li class="indx">Polytypic evolution, see Evolution.</li> + +<li class="indx">Prepotency, <a href="#Page_89">89</a>;</li> +<li class="isub1">importance of, <a href="#Page_176">176</a>.</li> + + +<li class="ifrst">S.</li> + +<li class="indx"><span class="smcap">Schmidt</span>, Prof. Oscar, on domesticated cattle, <a href="#Page_171">171</a>.</li> + +<li class="indx"><span class="smcap">Seebohm</span> on isolation, <a href="#Page_173">173</a>.</li> + +<li class="indx">Segregation, <a href="#Page_28">28</a>.</li> + +<li class="indx">Selection, physiological, see Physiological selection.</li> + +<li class="indx">Self-fertilization and variability, <a href="#Page_177">177</a>.</li> + +<li class="indx">Snails of Sandwich Islands, <a href="#Page_16">16</a>, <a href="#Page_130">130</a>;</li> +<li class="isub1">fossil of Steinheim, <a href="#Page_95">95</a>.</li> + +<li class="indx">Specific differentiation and cross-infertility, <a href="#Page_170">170</a>.</li> + +<li class="indx">Stability and cross-infertility, <a href="#Page_170">170</a>.</li> + +<li class="indx"><span class="pagenum"><a name="Page_181" id="Page_181">[Pg 181]</a></span>Synoicy, <a href="#Page_78">78</a>.</li> + + +<li class="ifrst">T.</li> + +<li class="indx">Topographical distribution and physiological selection, <a href="#Page_74">74</a>;</li> +<li class="isub1">of varieties, <a href="#Page_81">81</a>.</li> + +<li class="indx">Transformation, serial and divergent, <a href="#Page_21">21</a>, <a href="#Page_121">121</a>.</li> + + +<li class="ifrst">V.</li> + +<li class="indx">Variability and self-fertilization, <a href="#Page_177">177</a>.</li> + +<li class="indx">Variation in birds, <a href="#Page_34">34</a>.</li> + +<li class="indx">Varieties, topographical distribution of, <a href="#Page_81">81</a>.</li> + + +<li class="ifrst">W.</li> + +<li class="indx"><span class="smcap">Wagner</span>, Maritz, <a href="#Page_3">3</a>;</li> +<li class="isub1">on geographical isolation, <a href="#Page_76">76</a>;</li> +<li class="isub1">quoted, <a href="#Page_103">103</a>;</li> +<li class="isub1">law of migration, <a href="#Page_111">111</a>.</li> + +<li class="indx"><span class="smcap">Wallace</span>, Mr. A. R., <a href="#Page_3">3</a>, <a href="#Page_17">17</a>;</li> +<li class="isub1">quoted, <a href="#Page_34">34</a>, <a href="#Page_47">47</a>, <a href="#Page_51">51</a>, <a href="#Page_57">57</a>,<a href="#Page_130">130</a>-<a href="#Page_136">136</a>;</li> +<li class="isub1">criticized by Gulick, <a href="#Page_152">152</a>.</li> + +<li class="indx"><span class="smcap">Weismann</span>, Prof., on geographical isolation, <a href="#Page_76">76</a>, <a href="#Page_114">114</a>-<a href="#Page_118">118</a>.</li> + +<li class="indx">Weismannism and physiological selection, <a href="#Page_169">169</a>.</li> +</ul> + +<hr class="full" /> + + + + +<h2>TITLE LIST OF OPEN COURT PUBLICATIONS +ARRANGED ALPHABETICALLY BY AUTHORS</h2> + + +<p>ANESAKI, M.</p> + +<p>345. BUDDHIST AND CHRISTIAN GOSPELS, Being Gospel Parallels +from Pali Texts. Now first compared from the originals +by Albert J. Edmunds. Edited with parallels and notes from +the Chinese Buddhist Triptaka by <i>M. 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(c) SCIENCE A +RELIGIOUS REVELATION. 5c. (3d.) <i>Paul Carus.</i></p> + +<p>290. THE SURD OF METAPHYSICS, An Inquiry into the Question +<span class="smcap">Are there Things-in-themselves?</span> <i>Paul Carus.</i> Cloth, $1.25 +net. (5s. 6d. net.)</p> + +<p>303. KANT AND SPENCER, A Study of the Fallacies of Agnosticism. +<i>Paul Carus.</i> Cloth, 50c net. (2s. 6d. net.)</p> + +<p>312. KANT'S PROLEGOMENA TO ANY FUTURE METAPHYSICS. +Edited by <i>Paul Carus</i>. Cloth, 75c net. (3s. 6d. net.)</p> + +<p>215. THE GOSPEL OF BUDDHA, According to Old Records, told by +<i>Paul Carus</i>. Cloth, $1.00. (5s.)</p> + +<p>254. BUDDHISM AND ITS CHRISTIAN CRITICS. <i>Paul Carus.</i> +$1.25. (6s. 6d.)</p> + +<p>261. GODWARD, A Record of Religious Progress. <i>Paul Carus.</i> 50c. +(2s. 6d.)</p> + +<p>278. THE HISTORY OF THE DEVIL AND THE IDEA OF EVIL, +From the Earliest Times to the Present day. <i>Paul Carus.</i> Illustrated. +$6.00. (30s.)</p> + +<p>280. 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KARMA, Eine buddhistische Erzählung. <i>Paul Carus.</i> Illustrated. +35c.</p> + +<p>246. THE CROWN OF THORNS, a Story of the Time of Christ. +<i>Paul Carus.</i> Illustrated. Cloth 75c net. (3s. 6d. net.)</p> + +<p>247. THE CHIEF'S DAUGHTER, a Legend of Niagara. <i>Paul Carus.</i> +Illustrated. Cloth, $1.00 net. (4s. 6d.)</p> + +<p>267. SACRED TUNES FOR THE CONSECRATION OF LIFE. +Hymns of the Religion of Science. <i>Paul Carus.</i> 50c.</p> + +<p>281. GREEK MYTHOLOGY. <i>Paul Carus.</i> In preparation.</p> + +<p>282. EROS AND PSYCHE, A Fairy-Tale of Ancient Greece, Retold +after Apuleius, by <i>Paul Carus</i>. Illustrated. $1.50 net. (6s. net.)</p> + +<p>295. THE NATURE OF THE STATE. <i>Paul Carus.</i> Cloth 50c net. +(2s. 6d. net)</p> + +<p>224. GOETHE AND SCHILLER'S XENIONS. Selected and translated +by <i>Paul Carus</i>. Paper, 50c. (2s. 6d.)</p> + +<p>243. 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Trans., +Stanford, London, 1873).</p></div> + +<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> I may here most conveniently define the senses in which all the +following terms will be used throughout the present discussion:—<i>Species</i> +of isolation are, as above stated, homogamy and apogamy, or isolation +as discriminate and indiscriminate. <i>Forms</i> of isolation are modes of +isolation, such as the geographical, the sexual, the instinctive, or any +other of the numerous means whereby isolation of either species may be +secured. <i>Cases</i> of isolation are the instances in which any of the forms +of isolation may be at work: thus, if a group of <i>n</i> intergenerants be +segregated into five groups, <i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>, <i>e</i>, then, before the segregation there +would have been one case of isolation, but after the segregation there +would be five such cases.</p></div> + +<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> <i>Divergent Evolution through Cumulative Segregation</i> (<i>Zool. Journal, +Linn. Soc.</i>, vol. xx. pp. 189-274).</p></div> + +<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> The passage proceeds to show that in view of this consideration we +have a strong additional reason for rejecting the <i>a priori</i> dogma that all +specific characters must necessarily be useful characters. For it is evident +that any divergence of specific character which is brought about in this +way need not present any utilitarian significance—although, of course, +natural selection will ensure that it shall never be deleterious.</p></div> + +<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> <i>Revue Scientifique</i>, Nov. 23, 1889.</p></div> + +<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> <i>Nature</i>, Oct. 10, 1889, p. 568.</p></div> + +<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> e. g. p. 81.</p></div> + +<div class="footnote"><p><a name="Footnote_10_10" id="Footnote_10_10"></a><a href="#FNanchor_10_10"><span class="label">[10]</span></a> See Chapter xxiii. vol. ii. p. 262. (Edition of 1888.)</p></div> + +<div class="footnote"><p><a name="Footnote_11_11" id="Footnote_11_11"></a><a href="#FNanchor_11_11"><span class="label">[11]</span></a> This term may here be taken as equivalent to Isolation.</p></div> + +<div class="footnote"><p><a name="Footnote_12_12" id="Footnote_12_12"></a><a href="#FNanchor_12_12"><span class="label">[12]</span></a> <i>Zool. Journal Lin. Soc.</i>, vol. xix. pp. 337-411.</p></div> + +<div class="footnote"><p><a name="Footnote_13_13" id="Footnote_13_13"></a><a href="#FNanchor_13_13"><span class="label">[13]</span></a> <i>Ibid.</i>, vol. xx. pp. 202-212.</p></div> + +<div class="footnote"><p><a name="Footnote_14_14" id="Footnote_14_14"></a><a href="#FNanchor_14_14"><span class="label">[14]</span></a> <i>Zool. Journal Lin. Soc.</i>, vol. xxiii. p. 313.</p></div> + +<div class="footnote"><p><a name="Footnote_15_15" id="Footnote_15_15"></a><a href="#FNanchor_15_15"><span class="label">[15]</span></a> <i>See Nineteenth Century</i>, January, 1887, pp. 61, 62.</p></div> + +<div class="footnote"><p><a name="Footnote_16_16" id="Footnote_16_16"></a><a href="#FNanchor_16_16"><span class="label">[16]</span></a> <i>Nicaragua</i>, p. 207.</p></div> + +<div class="footnote"><p><a name="Footnote_17_17" id="Footnote_17_17"></a><a href="#FNanchor_17_17"><span class="label">[17]</span></a> <i>Nature</i>, vol. xxxi. p. 4.</p></div> + +<div class="footnote"><p><a name="Footnote_18_18" id="Footnote_18_18"></a><a href="#FNanchor_18_18"><span class="label">[18]</span></a> <i>Zool. Journal, Lin. Soc.</i>, vol. xix. pp. 337-411 (1886); and for +Mr. Gulick's papers, <i>ibid.</i>, vol. xx. pp. 189-274 (1887), vol. xxiii. +pp. 312-380 (1889). Mr. Gulick has recently drawn my attention, in +a private letter, to the fact that as early as 1872 a paper of his was +read at the British Association, bearing the title <i>Diversity of Evolution +under one set of External Conditions</i>, and that here the principle of +physiological segregation is stated. Although it does not appear that +Mr. Gulick then appreciated the great importance of this principle, it +entitles him to claim priority.</p></div> + +<div class="footnote"><p><a name="Footnote_19_19" id="Footnote_19_19"></a><a href="#FNanchor_19_19"><span class="label">[19]</span></a> <i>Darwinism</i>, p. 169.</p></div> + +<div class="footnote"><p><a name="Footnote_20_20" id="Footnote_20_20"></a><a href="#FNanchor_20_20"><span class="label">[20]</span></a> <i>Origin of Species</i>, p. 136.</p></div> + +<div class="footnote"><p><a name="Footnote_21_21" id="Footnote_21_21"></a><a href="#FNanchor_21_21"><span class="label">[21]</span></a> <i>Darwinism</i>, p. 152.</p></div> + +<div class="footnote"><p><a name="Footnote_22_22" id="Footnote_22_22"></a><a href="#FNanchor_22_22"><span class="label">[22]</span></a> <i>Origin of Species</i>, pp. 44, 45.</p></div> + +<div class="footnote"><p><a name="Footnote_23_23" id="Footnote_23_23"></a><a href="#FNanchor_23_23"><span class="label">[23]</span></a> <i>Origin of Species</i>, ed. 6, pp. 134, 135.</p></div> + +<div class="footnote"><p><a name="Footnote_24_24" id="Footnote_24_24"></a><a href="#FNanchor_24_24"><span class="label">[24]</span></a> <i>Archives des Sciences physiques et naturelles</i> (Genève), vol. liii. (1875), +pp. 211-236.</p></div> + +<div class="footnote"><p><a name="Footnote_25_25" id="Footnote_25_25"></a><a href="#FNanchor_25_25"><span class="label">[25]</span></a> <i>Remarques sur le fait de l'existence en société à l'état sauvage des +espèces végétales affines et sur d'autres faits relatifs à la question de +l'espèce</i>, par Alexis Jordan; lues au congrès de l'Association Française +pour l'Avancemeat des Sciences, 2<sup>m</sup>e session, Lyon, séance de 28 Août, +1873.</p></div> + +<div class="footnote"><p><a name="Footnote_26_26" id="Footnote_26_26"></a><a href="#FNanchor_26_26"><span class="label">[26]</span></a> <i>Evolution and its Relations to Religious Thought</i>, &c. pp. 236-7.</p></div> + +<div class="footnote"><p><a name="Footnote_27_27" id="Footnote_27_27"></a><a href="#FNanchor_27_27"><span class="label">[27]</span></a> <i>Life and Letters</i>, vol. ii. p. 28.</p></div> + +<div class="footnote"><p><a name="Footnote_28_28" id="Footnote_28_28"></a><a href="#FNanchor_28_28"><span class="label">[28]</span></a> <i>Ibid.</i></p></div> + +<div class="footnote"><p><a name="Footnote_29_29" id="Footnote_29_29"></a><a href="#FNanchor_29_29"><span class="label">[29]</span></a> <i>Origin of Species</i>, p. 80, 6th ed. (1872).</p></div> + +<div class="footnote"><p><a name="Footnote_30_30" id="Footnote_30_30"></a><a href="#FNanchor_30_30"><span class="label">[30]</span></a> <i>Life and Letters</i>, vol. iii. p. 158.</p></div> + +<div class="footnote"><p><a name="Footnote_31_31" id="Footnote_31_31"></a><a href="#FNanchor_31_31"><span class="label">[31]</span></a> <i>Ibid.</i> p. 159.</p></div> + +<div class="footnote"><p><a name="Footnote_32_32" id="Footnote_32_32"></a><a href="#FNanchor_32_32"><span class="label">[32]</span></a> <i>Ibid.</i> p. 160.</p></div> + +<div class="footnote"><p><a name="Footnote_33_33" id="Footnote_33_33"></a><a href="#FNanchor_33_33"><span class="label">[33]</span></a> The analogy is radically unsound because unconscious selection +differs from methodical selection only in the <i>degree</i> of "separation" +which it effects. These two forms of selection do not necessarily differ +from one another in regard to the <i>number</i> of characters which are being +simultaneously diversified; for while it may be the object of methodical +selection to breed for modification of a single character alone, it may, +on the other hand, be the result of unconscious selection to diversify an +originally uniform stock, as Darwin himself observes with regard to +horse-breeding. The real distinction between monotypic and polytypic +evolution is, not at all with reference to the <i>degree</i> of isolation (i. e. +<i>amount</i> of "separation"), but to the <i>number of cases</i> in which any +efficient degree of it occurs (i. e. whether in but a single case, or in two +or more cases).</p></div> + +<div class="footnote"><p><a name="Footnote_34_34" id="Footnote_34_34"></a><a href="#FNanchor_34_34"><span class="label">[34]</span></a> <i>Life and Letters</i>, vol. iii. pp. 157-8.</p></div> + +<div class="footnote"><p><a name="Footnote_35_35" id="Footnote_35_35"></a><a href="#FNanchor_35_35"><span class="label">[35]</span></a> <i>Ibid.</i> pp. 157-8.</p></div> + +<div class="footnote"><p><a name="Footnote_36_36" id="Footnote_36_36"></a><a href="#FNanchor_36_36"><span class="label">[36]</span></a> <i>Life and Letters</i>, vol. iii. p. 161.</p></div> + +<div class="footnote"><p><a name="Footnote_37_37" id="Footnote_37_37"></a><a href="#FNanchor_37_37"><span class="label">[37]</span></a> Page 81. The three forms of isolation mentioned are, "from +haunting different stations, from breeding at slightly different seasons, or +from the individuals of each variety preferring to pair together."</p></div> + +<div class="footnote"><p><a name="Footnote_38_38" id="Footnote_38_38"></a><a href="#FNanchor_38_38"><span class="label">[38]</span></a> <i>Life and Letters</i>, vol. iii. p. 159.</p></div> + +<div class="footnote"><p><a name="Footnote_39_39" id="Footnote_39_39"></a><a href="#FNanchor_39_39"><span class="label">[39]</span></a> <i>Life and Letters</i>, vol. iii. p. 155.</p></div> + +<div class="footnote"><p><a name="Footnote_40_40" id="Footnote_40_40"></a><a href="#FNanchor_40_40"><span class="label">[40]</span></a> <i>Variation</i>, &c., vol. ii. p. 262.</p></div> + +<div class="footnote"><p><a name="Footnote_41_41" id="Footnote_41_41"></a><a href="#FNanchor_41_41"><span class="label">[41]</span></a> <i>Life and Letters</i>, vol. iii. p. 161.</p></div> + +<div class="footnote"><p><a name="Footnote_42_42" id="Footnote_42_42"></a><a href="#FNanchor_42_42"><span class="label">[42]</span></a> <i>Die Darwin'sche Theorie und das Migrationsgesetz</i> (1868): <i>Ueber +den Einfluss der geographischen Isolirung</i>, &c. (1870).</p></div> + +<div class="footnote"><p><a name="Footnote_43_43" id="Footnote_43_43"></a><a href="#FNanchor_43_43"><span class="label">[43]</span></a> For instance, speaking of common, or continuous areas, he says:—"In +this case a constant variety, or new species, cannot be produced, +because the free crossing of a new variety with the old unaltered stock +will always cause it to revert to the original type; in other words, will +destroy the new form. The formation of a real variety, which Darwin, +as we know, regards as the commencement of a new species, will only +succeed when a few individuals, having crossed the barrier of their +habitat, are able to separate themselves for a long time from the old +stock." And the last sentence, given as a summary of his whole +doctrine, is—"The geographical isolation of the form, a necessary +consequence of migration, is the cause of its typical character."</p></div> + +<div class="footnote"><p><a name="Footnote_44_44" id="Footnote_44_44"></a><a href="#FNanchor_44_44"><span class="label">[44]</span></a> <i>Ueber den Einfluss der Isolirung auf die Artbildung</i> (1872).</p></div> + +<div class="footnote"><p><a name="Footnote_45_45" id="Footnote_45_45"></a><a href="#FNanchor_45_45"><span class="label">[45]</span></a> <i>Loc. cit.</i>, p. 43.</p></div> + +<div class="footnote"><p><a name="Footnote_46_46" id="Footnote_46_46"></a><a href="#FNanchor_46_46"><span class="label">[46]</span></a> <i>Physiological Selection</i>, pp. 348, 389.</p></div> + +<div class="footnote"><p><a name="Footnote_47_47" id="Footnote_47_47"></a><a href="#FNanchor_47_47"><span class="label">[47]</span></a> <i>Loc. cit.</i>, p. 54.</p></div> + +<div class="footnote"><p><a name="Footnote_48_48" id="Footnote_48_48"></a><a href="#FNanchor_48_48"><span class="label">[48]</span></a> <i>Nature</i>, vol. xliii. p. 410, and vol. xliv. p. 29.</p></div> + +<div class="footnote"><p><a name="Footnote_49_49" id="Footnote_49_49"></a><a href="#FNanchor_49_49"><span class="label">[49]</span></a> <i>Darwinism</i>, p. 143.</p></div> + +<div class="footnote"><p><a name="Footnote_50_50" id="Footnote_50_50"></a><a href="#FNanchor_50_50"><span class="label">[50]</span></a> In Appendix to H. M. Stanley's <i>How I found Livingstone</i>, 2nd ed. +London, 1872, p. 715.</p></div> + +<div class="footnote"><p><a name="Footnote_51_51" id="Footnote_51_51"></a><a href="#FNanchor_51_51"><span class="label">[51]</span></a> <i>Animal Life and Intelligence</i>, pp. 98, 99 (1890-1891).</p></div> + +<div class="footnote"><p><a name="Footnote_52_52" id="Footnote_52_52"></a><a href="#FNanchor_52_52"><span class="label">[52]</span></a> <i>The Factors of Evolution</i> (1891).</p></div> + +<div class="footnote"><p><a name="Footnote_53_53" id="Footnote_53_53"></a><a href="#FNanchor_53_53"><span class="label">[53]</span></a> <i>Darwinism</i>, p. 151.</p></div> + +<div class="footnote"><p><a name="Footnote_54_54" id="Footnote_54_54"></a><a href="#FNanchor_54_54"><span class="label">[54]</span></a> <i>Ibid.</i></p></div> + +<div class="footnote"><p><a name="Footnote_55_55" id="Footnote_55_55"></a><a href="#FNanchor_55_55"><span class="label">[55]</span></a> <i>Loc. cit.</i>, p. 151.</p></div> + +<div class="footnote"><p><a name="Footnote_56_56" id="Footnote_56_56"></a><a href="#FNanchor_56_56"><span class="label">[56]</span></a> Namely, <i>Lycaena denzelii</i>, <i>L. pheretes</i>, <i>Argynnis pales</i>, <i>Erebia +mante</i>.</p></div> + +<div class="footnote"><p><a name="Footnote_57_57" id="Footnote_57_57"></a><a href="#FNanchor_57_57"><span class="label">[57]</span></a> Since the above was written, I have heard of some cases which seem +to present greater difficulties to our theory than those above quoted. +These refer to some of the numerous species of land mollusca which +inhabit the isolated rocks near Madeira (Dezertas). My informant is +Dr. Grabham, who has himself investigated the matter, and reports +as follows:— +</p><p> +"It is no uncommon thing to meet with examples of the same species, +sub-fossil, recent, and living upon one spot, and presenting no variation +in the long record of descent." Then, after naming these examples, he +adds, "All seem to vary immediately on attaining new ground, assuming +many aspects in different districts." +</p><p> +Unquestionably these statements support, in a very absolute manner, +Mr. Wallace's opinion, while making directly against my own. It is +but fair, however, to add that the cases are not numerous (some half-dozen +at the most, and all within the limits of a single genus), and that, +even in the opinion of my informant himself, the facts have not hitherto +been sufficiently investigated for any decisive judgement to be formed +upon them.</p></div> + +<div class="footnote"><p><a name="Footnote_58_58" id="Footnote_58_58"></a><a href="#FNanchor_58_58"><span class="label">[58]</span></a> Vol. xliii. p. 127.</p></div> + +<div class="footnote"><p><a name="Footnote_59_59" id="Footnote_59_59"></a><a href="#FNanchor_59_59"><span class="label">[59]</span></a> This refers to what I understand Mr. Wallace to say in the <i>Nature</i> +correspondence is the supposition on which his own theory of the origin +of species by cross-infertility is founded. But in the original statement +of that theory itself, it is everywhere "supposed" that when species are +originated by cross-infertility, the <i>initial</i> change <i>is</i> the physiological +change. In his original statement of that theory, therefore, he literally +went further than I had gone in my "original paper," with reference to +supposing the physiological change to be the initial change. I do not +doubt that this is due to some oversight of expression; but it is curious +that, having made it, he should still continue his endeavour to fix exactly +the same oversight upon me.</p></div> + +<div class="footnote"><p><a name="Footnote_60_60" id="Footnote_60_60"></a><a href="#FNanchor_60_60"><span class="label">[60]</span></a> "Positive segregation" is Mr. Gulick's term for forms of homogamy +other than that which is due to selective fertility. Of these other, +or "positive" forms, natural selection is one; but as it is far from +being the <i>only</i> one, the criticism points out that utility is not the +<i>only</i> conserving principle with which selective fertility may be associated.</p></div> + +<div class="footnote"><p><a name="Footnote_61_61" id="Footnote_61_61"></a><a href="#FNanchor_61_61"><span class="label">[61]</span></a> By Intensive Segregation Mr. Gulick means what I have called Independent +Variability.</p></div> + +<div class="footnote"><p><a name="Footnote_62_62" id="Footnote_62_62"></a><a href="#FNanchor_62_62"><span class="label">[62]</span></a> His sentence, "all fertility not correlated with some <i>useful</i> variation +has a constant tendency to effect its own elimination," still further +restricts the possible action of physiological selection to cases where at +least one of the other forms of homogamy with which it is associated is +natural selection. Or, in other words, it is represented that physiological +selection must always be associated with natural selection, even if it be +likewise associated with any other form of exclusive breeding. But as +this further limitation appears to me self-evidently unjustifiable (seeing +that utility is not the only possible means of securing effective isolation) +I here neglect it, and take the wider ground marked out above. It is +needless to say that this is giving Mr. Wallace every possible advantage, +by not holding him to his still narrower ground.</p></div> + +<div class="footnote"><p><a name="Footnote_63_63" id="Footnote_63_63"></a><a href="#FNanchor_63_63"><span class="label">[63]</span></a> In our <i>Nature</i> correspondence of 1890-1891, Mr. Wallace remarked: +"If Dr. Romanes will carefully work out numerically (as I have +attempted to do) a few cases showing the preservative and accumulative +agency of pure physiological selection within an otherwise undifferentiated +species, he will do more for his theory than volumes of general disquisition +or any number of assertions that it <i>does</i> possess this power." +Several months before this was written I had already in my hands +Mr. Moulton's letter, with its accompanying calculations.</p></div> + +<div class="footnote"><p><a name="Footnote_64_64" id="Footnote_64_64"></a><a href="#FNanchor_64_64"><span class="label">[64]</span></a> As, for example, in the case of sexuality in general. It is not to +the advantage of such individual male Arthropoda as perish after the +performance of the sexual act that they should perform it; but its performance +is necessary for the perpetuation of their species.—G. J. R.</p></div> + +<div class="footnote"><p><a name="Footnote_65_65" id="Footnote_65_65"></a><a href="#FNanchor_65_65"><span class="label">[65]</span></a> In this anticipation Mr. Moulton is right. The well-known botanist, +Mr. Bennett, read a most interesting paper on the subject before the +British Association in 1881. His results have since been corroborated +by other observers. In particular, Mr. R. M. Christy has recorded the +movements of 76 insects while visiting at least 2,400 flowers. (<i>Entomologist</i>, +July 1883, and <i>Zool. Journal Lin. Soc.</i>, August 1883.) The +following is an analysis of his results. In the case of butterflies, in +twelve observations on nearly as many species, there are recorded +altogether 99 visits to fifteen species of flowers; and of these 99 visits 94 +were constant to the same species, leaving only 5 visits to any other, +or second species. In the case of the hive-bee, there were 8 individuals +observed: these visited altogether 258 flowers, and all the visits paid by +the same individual were paid to the same species in each of the eight +cases. Lastly, as regards bumble-bees, there were altogether observed +55 individuals belonging to four species. These paid altogether 1751 +visits to 94 species of flowers. Of these 1751 visits, 1605 were paid to +one species, 131 to two species, 16 to three, 6 to four, and 1 to five. +Adding all these results together, we find that 75 insects (butterflies +and bees) visited 117 species of flowers: of these visits, 1957 were +constant to one species of flower; 136 were paid also to a second +species, 16 also to a third, 6 also to a fourth, and 1 also to a fifth. Or, +otherwise stated, while 1957 were absolutely constant, from such absolute +constancy there were only 159 deviations. Moreover, if we eliminate +three individual humble bees, which paid nearly an equal number of visits +to two species (and, therefore, would have ministered to the work of +physiological selection almost as well as the others), the 159 deviations +become reduced to 72, or about four per cent. of the whole.—G. J. R.</p></div> + +<div class="footnote"><p><a name="Footnote_66_66" id="Footnote_66_66"></a><a href="#FNanchor_66_66"><span class="label">[66]</span></a> Here follows the Appendix presenting the calculations on which the +above results are founded; but it seems unnecessary to reproduce it +on the present occasion.—G. J. R.</p></div> + +<div class="footnote"><p><a name="Footnote_67_67" id="Footnote_67_67"></a><a href="#FNanchor_67_67"><span class="label">[67]</span></a> <i>Doctrine of Descent and Darwinism</i>, Eng. trans. p. 139.</p></div> + +<hr class="full" /> + +<div class="transnote"> +<h3>Transcriber's Notes</h3> + +<p>In paragraph 4 of page 171 "peculiarites" has been corrected to +"peculiarities"</p> + +<p>Variable spacing in the following abbreviation was left as it was in +the original: "i. e." (22 instances) and "i.e." (14 instances).</p> + +<p>Different hyphenation patterns were left as in the original text:</p> + + +<div class="center"> +<table border="1" cellpadding="4" cellspacing="0" summary=""> +<tr><td align="left">prepotent (1 instance)</td><td align="left">pre-potent (1 instance)</td></tr> +<tr><td align="left">presupposes (1)</td><td align="left">pre-supposes (1)</td></tr> +<tr><td align="left">reacting(5)</td><td align="left">re-acting (1)</td></tr> +<tr><td align="left">restatement (1)</td><td align="left">re-statement (2)</td></tr> +<tr><td align="left">superinduced (2)</td><td align="left">super-induced (1)</td></tr> +</table></div> +</div> + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of Darwin, and After Darwin (Vol 3 of 3), by +George John Romanes + +*** END OF THIS PROJECT GUTENBERG EBOOK DARWIN, AFTER DARWIN (VOL 3 OF 3) *** + +***** This file should be named 37777-h.htm or 37777-h.zip ***** +This and all associated files of various formats will be found in: + https://www.gutenberg.org/3/7/7/7/37777/ + +Produced by Marilynda Fraser-Cunliffe, LN Yaddanapudi and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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