[41] _Life and Letters_, vol. iii. p. 161.

So much, then, for Darwin's opinions. Next in order of time we must consider Moritz Wagner's essays on what he called the "Law of Migration[42]." The merit of these essays was, first, the firm expression of opinion upon the swamping effects of free intercrossing; and, second, the production of a large body of facts showing the importance of geographical isolation in the prevention of these effects, and in the consequent differentiation of specific types. On the other hand, the defect of these essays was, first, not distinguishing between evolution as monotypic and polytypic; and, second, not perceiving that geographical isolation is only one among a number of other forms of isolation. From these two radical oversights--which, however, were shared by all other writers of the time, with the partial exception of Darwin himself, as previously shown--there arose the following and most lamentable errors.

[42] _Die Darwin'sche Theorie und das Migrationsgesetz_ (1868): _Ueber den Einfluss der geographischen Isolirung_, &c. (1870).

Over and over again Moritz Wagner insists, as const.i.tuting the fundamental doctrine of his attempted reform of Darwinism, that evolution by natural selection is impossible, unless natural selection be a.s.sisted by geographical isolation, in order to prevent the swamping effects of intercrossing[43]. Now, if instead of "evolution" he had said "divergence of type," and if instead of "geographical isolation" he had said "prevention of intercrossing," he would have enunciated the general doctrine which it has been the joint endeavour of Mr. Gulick and myself to set forth. But by not perceiving that "evolution" is of two radically different kinds--polytypic and monotypic--he entirely failed to perceive that, while for one of its kinds the _prevention_ of intercrossing is an absolute necessity, for the other of its kinds the _permission_ of intercrossing is a necessity no less absolute. And, again, in missing the fact that geographical isolation is but one of the many ways whereby intercrossing may be prevented, he failed to perceive that, even as regards the case of polytypic evolution, he greatly erred in representing this one form of isolation as being universally a necessary condition to the process. The necessary condition to this process is, indeed, the prevention of intercrossing _by some means or another_; but his unfortunate insistence on geographical separation as the only possible means to this end--especially when coupled with his no less unfortunate disregard of monotypic evolution--caused him to hinder rather than to advance a generalization which he had only grasped in part. And this generalization is, as now so repeatedly stated, that while the form of isolation which we know as natural selection depends for its action upon the intercrossing of all the individuals which it isolates (i. e. selects), when acting alone it can produce only monotypic evolution; but that when it is supplemented by any of the other numerous forms of isolation, it is furnished with the necessary condition to producing polytypic evolution--and this in as many lines of divergent change as there may be cases of this efficient separation.

[43] For instance, speaking of common, or continuous areas, he says:--"In this case a constant variety, or new species, cannot be produced, because the free crossing of a new variety with the old unaltered stock will always cause it to revert to the original type; in other words, will destroy the new form. The formation of a real variety, which Darwin, as we know, regards as the commencement of a new species, will only succeed when a few individuals, having crossed the barrier of their habitat, are able to separate themselves for a long time from the old stock." And the last sentence, given as a summary of his whole doctrine, is--"The geographical isolation of the form, a necessary consequence of migration, is the cause of its typical character."

Nevertheless, while we must lament these shortcomings on the part of Wagner, we ought to remember that he rendered important services in the way of calling attention to the swamping effects of free intercrossing, and, still more, in that of showing the high importance of geographical isolation as a factor of organic evolution. Therefore, although in an elaborate criticism of his views Weismann was easily able to dispose of his generalizations in the imperfect form that they presented, I do not think it was just in Weismann to remark, "if Wagner had confined himself to the statement that geographical isolation materially a.s.sists the process of natural selection, and thus also promotes the origination of new species, he would have met with little or no opposition; but then, of course, in saying this much, he would not have been saying anything new." No doubt, as I have just shown, he _ought_ thus (as well as in other and still more important respects not perceived by Prof. Weismann) to have limited his statement; but, had he done so, it does not follow that he would not have been saying anything new. For, in point of fact, in as far as he said what was true, he did say a great deal that was also new. Thus, most of what he said of the _principle of separation_ (apogamy) was as new as it was true, although, as we have seen, he said it to very little purpose on account of his identifying this principle as a whole with that of but one of its forms. Again, notwithstanding this great error, or oversight, he certainly showed of the particular form in question--viz. geographical isolation--that it was of considerably _more_ importance than had previously been acknowledged.

And this was so far a valuable contribution to the general theory of descent.

Prof. Weismann's essay, to which allusion has just been made[44], was, however, in all respects a great advance upon those of Wagner. It was not only more comprehensive in its view of the whole subject of geographical isolation, but likewise much more adequate in its general treatment thereof. Its princ.i.p.al defects, in my judgement, were, first, the inordinately speculative character of some of its parts, and, second, the restriction of its a.n.a.lysis to but one form of isolation--a defect which it shares with the essays of Wagner, and in quite as high a degree. Furthermore, although this essay had the great merit of enunciating the principle of Amixia, it did so in a very inefficient manner. For not only was this principle adduced with exclusive reference to _geographical_ isolation, but even in regard to this one kind of isolation it was presented in a highly inconsistent manner, as I will now endeavour to show.

[44] _Ueber den Einfluss der Isolirung auf die Artbildung_ (1872).

Weismann was led to perceive the principle in question by the consideration that new specific characters, when they first appear, do not all appear together in the same individuals: they appear one in one individual, another in another, a third in a third, &c.; and it is only in the course of successive generations that they all become blended in the same individuals by free intercrossing. Hence, the eventually emerging constant or specific type is the resultant of all the transitory or varietal types, when these have been fused together by intercrossing. From which Weismann deduces what he considers a general law--namely, that "the constancy of a specific type does not arise suddenly, but gradually; and it is established by the promiscuous crossing of all individuals[45]." From which again it follows, that this constancy must cease so soon as the condition which maintains it ceases--i. e. so soon as free intercrossing is prevented by the geographical isolation of a portion of the species from its parent stock.

[45] _Loc. cit._, p. 43.

Now, to begin with, this statement of the principle in question is not a good statement of it. There was no need while stating the doctrine that separation induces differentiation, to found the doctrine on any such highly speculative basis. In point of fact, there is no real evidence that specific types do attain their constancy in the way supposed; nor, for the purposes of the doctrine in question, is it necessary that there should be. For this doctrine does not need to show how the constancy has been _attained_; it only has to show that the constancy is _maintained_ by free intercrossing, with the result that when free intercrossing is _by any means_ prevented, divergence of character ensues. In short, the correct way of stating the principle is that which has been adopted by Delbuf and Gulick--namely, the average characters of a separated portion of a species are not likely to be the same as those of the whole species; with the result that divergence of type will be set up in the separated portion by intercrossing within that portion. Or the principle may be presented as I presented it under the designation of "Independent Variability"--namely, "a specific type may be regarded as the average mean of all individual variations, any considerable departure from this average mean being, however, checked by intercrossing," with the result that when intercrossing is prevented between a portion of a species and the rest of the species, "this population is permitted to develop an independent history of its own, shielded from intercrossing with its parent form[46]."

[46] _Physiological Selection_, pp. 348, 389.

Not only, however, is Weismann's principle of "Amixia" thus very differently stated from that of my "Independent Variability" (apogamy), or Gulick's "Independent Generation"; but, apparently owing to this difference of statement, the principle itself is not the same. In particular, while Weismann holds with us that when new characters arise in virtue of the mere prevention of intercrossing with parent forms these new characters will be of non-utilitarian kind[47], he appears to think that divergence of character under such circ.u.mstances is not likely to go on to a _specific_ value. Now, it is of importance to observe why he arrives at this conclusion, which is not only so different from that of Delbuf, Gulick, and myself, but apparently so inconsistent with his own recognition of the diversifying effect of "Amixia" as regards the formation of _permanent varieties_. For, as we have already seen while considering Darwin's views on this same principle of "Amixia," it is highly inconsistent to recognize its diversifying effect up to the stage of const.i.tuting 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 Delbuf and Gulick have so well shown. What, then, is the explanation of this apparent inconsistency on Weismann's part? The explanation evidently is that, owing to his erroneous statement of the principle, he misses the real essence of it. For, in the first place, he does not perceive that this essence consists in an initial difference of average characters on the part of the isolated colony as compared with the rest of their species. On the contrary, he loses himself in a maze of speculation about all species having had what he calls "variation-periods," or eruptions of general variability alternating with periods of repose--both being as unaccountable in respect of their causation as they are hypothetical in respect of their occurrence. From these speculations he concludes, that isolation of a portion of a species will then only lead to divergence of character when the isolation happens to coincide with a "variation-period" on the part of the species as a whole, and that the divergence will cease so soon as the "variation-period" ceases. Again, in the second place as previously remarked, equally with Wagner whom he is criticizing, he fails to perceive that _geographical_ isolation is not the only kind of isolation, or the only possible means to the prevention of free intercrossing. And the result of this oversight is, that he thinks amixia can act but comparatively seldom upon sufficiently small populations to become a factor of much importance in the differentiation of species. Lastly, in the third place, owing to his favourite hypothesis that all species pa.s.s 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 part.i.tion (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 Delbuf, or the _c.u.mulative_ 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 Delbuf, Gulick, and myself.

[47] _Loc. cit._, p. 54.

Upon the whole, then, we must say that although Professor Weismann was the first to recognize the diversifying influence of merely indiscriminate isolation _per se_ (apogamy), he did so only in part. He failed to distinguish the true essence of the principle, and by overlaying it with a ma.s.s of hypothetical speculation, concealed even more of it than he revealed.

The general theory of Isolation, as independently worked out by Mr.

Gulick and myself, has already been so fully explained, that it will here be sufficient merely to enumerate its more distinguishing features.

These are, first, drawing the sharpest possible line between evolution as monotypic and polytypic; second, showing that while for the former the peculiar kind of isolation which is presented by natural selection suffices of itself to _transform_ a specific type, in order to work for the latter, or to _branch_ a specific type, natural selection must necessarily be a.s.sisted by some other kind of isolation; third, that even in the absence of natural selection, other kinds of isolation may be sufficient to effect specific divergence through independent generation alone; fourth, that, nevertheless, natural selection, where present, will always accelerate the process of divergence; fifth, that monotypic evolution by natural selection depends upon the _presence_ of intercrossing, quite as much as polytypic evolution (whether with or without natural selection) depends upon the _absence_ of it; sixth, that, having regard to the process of evolution throughout all taxonomic divisions of organic nature, we must deem the physiological form of isolation as the most important, with the exception only of natural selection.

The only difference between Mr. Gulick's essays and my own is, that, on the one hand, he has a.n.a.lyzed much more fully than I have the various forms of isolation; while, on the other hand, I have considered much more fully than he has the particular form of physiological isolation which so frequently obtains between allied _species_. This particular form of physiological isolation I have called "physiological selection,"

and claim for it so large a share in the differentiation of specific types as to find in it a satisfactory explanation of the contrast between natural species and artificial varieties in respect of cross-infertility.

Mr. Wallace, in his _Darwinism_, has done good service by enabling all other naturalists clearly to perceive how natural selection alone produces monotypic evolution--namely, through the free intercrossing of all individuals which have not been eliminated by the isolating process of natural selection itself. For he very lucidly shows how the law of averages must always ensure that in respect of any given specific character, half the individuals living at the same time and place will present the character above, and half below its mean in the population as a whole. Consequently, if it should ever be of advantage to a species that this character should undergo either increase or decrease of its average size, form, colour, &c., there will always be, in each succeeding generation, a sufficient number of individuals--i. e. half of the whole--which present variations in the required direction, and which will therefore furnish natural selection with abundant material for its action, without the need of any other form of isolation. It is to be regretted, however, that while thus so clearly presenting the fact that free intercrossing is the very means whereby natural selection is enabled to effect monotypic evolution, he fails to perceive that such intercrossing must always and necessarily render it impossible for natural selection to effect polytypic evolution. A little thought might have shown him that the very proof which he gives of the necessity of intercrossing where the _trans.m.u.tation_ of species is concerned, furnishes, measure for measure, as good a proof of the necessity of its absence where the _multiplication_ of species is concerned. In justice to him, however, it may be added, that this distinction between evolution as monotypic and polytypic (with the important consequence just mentioned) still continues to be ignored also by other well-known evolutionists of the "ultra-Darwinian" school. Professor Meldola, for example, has more recently said that in his opinion the "difficulty from intercrossing" has been in large part--if not altogether--removed by Mr.

Wallace's proof that natural selection alone is capable of effecting [monotypic] evolution; while he regards the distinction between monotypic and polytypic evolution as mere "verbiage[48]."

[48] _Nature_, vol. xliii. p. 410, and vol. xliv. p. 29.

It is in relation to my presentment of the impossibility of natural selection alone causing polytypic evolution, that Mr. Wallace has been at the pains to show how the permission of intercrossing (panmixia) is necessary for natural selection in its work of causing monotypic evolution. And not only has he thus failed to perceive that the "difficulty" which intercrossing raises against the view of natural selection being of itself capable of causing polytypic evolution in no way applies to the case of monotypic; but as regards this "difficulty,"

where it does apply, he says:--

Professor G. J. Romanes has adduced it as one of the difficulties which can alone be overcome by his theory of physiological selection[49].

[49] _Darwinism_, p. 143.

This, however, is a misapprehension. I have by no means represented that the difficulty in question can alone be overcome by this theory. What I have represented is, that it can be overcome by any of the numerous forms of isolation which I named, and of which physiological selection is but one. And although, _where common areas are concerned_, I believe that the physiological form of isolation is the most important form, this is a very different thing from entertaining the supposition which Mr. Wallace here a.s.signs to me.

I may take this opportunity of correcting a somewhat similar misunderstanding which has been more recently published by Professor W.

A. Herdman, of Liverpool; and as the case which he gives is one of considerable interest in itself, I will quote his remarks in extenso. In his _Opening Address to the Liverpool Biological Society_, Professor Herdman said:--

Some of you will doubtless remember that in last year's address, while discussing Dr. Romanes' theory of physiological selection, I quoted Professor Flemming Jenkin's imaginary case of a white man wrecked upon an island inhabited by negroes, given as an ill.u.s.tration of the supposed swamping effect by free intercrossing of a marked variety with the parent species. I then went on to say in criticism of the result at which Jenkin arrived, viz. that the characteristics of the white man would be stamped out by intercrossing with the black:--

"Two influences have, I think, been ignored, viz. atavism, or reversion to ancestral characters, and the tendency of the members of a variety to breed with one another. Keeping to the case described above, I should imagine that the numbers of intelligent young mulattoes produced in the second, third, fourth, and few succeeding generations would to a large extent intermarry, the result of which would be that a more or less white aristocracy would be formed on the island, including the king and all the chief people, the most intelligent men and the bravest warriors. Then atavism might produce every now and then a much whiter individual--a reversal to the characteristics of the ancestral European--who, by being highly thought of in the whitish aristocracy, would have considerable influence on the colour and other characteristics of the next generation. Now such a white aristocracy would be in precisely the same circ.u.mstances as a favourable variety competing with its parent species," &c.

You may imagine then my pleasure when, a few months after writing the above, I accidentally found, in a letter[50] written by the celebrated African traveller Dr. David Livingstone to Lord Granville, and dated "Unyanyembe, July 1st, 1872," the following pa.s.sage:--

[50] In Appendix to H. M. Stanley's _How I found Livingstone_, 2nd ed. London, 1872, p. 715.

"About five generations ago, a white man came to the highlands of Basango, which are in a line east of the watershed. He had six attendants, who all died, and eventually their headman, called Charura, was elected chief by the Basango. In the third generation he had sixty able-bodied spearmen as lineal descendants. This implies an equal number of the other s.e.x. They are very light in colour, and easily known, as no one is allowed to wear coral beads such as Charura brought except the royal family. A book he brought was lost only lately. The interest of the case lies in its connexion with Mr. Darwin's celebrated theory on the 'origin of species,' for it shows that an improved variety, as we whites modestly call ourselves, is not so liable to be swamped by numbers as some have thought."

Here we have a perfect fulfilment of what I last year, in ignorance of this observation of Livingstone's, predicted as being likely to occur in such a case. We have the whitish aristocracy in a dominant condition, and evidently in a fair way to spread their characteristics over a larger area and give rise to a marked variety, and it had clearly struck Livingstone fourteen years before the theory of physiological selection had been heard of, just as it must strike us now, as an instance telling strongly against the "swamping" argument as used by Flemming Jenkin and Romanes.

Here we have a curious example of one writer supporting the statements of another, while appearing to be under the impression that he is controverting those statements. Both Professor Herdman's imaginary case, and its realization in Livingstone's account, go to show "the tendency of the members of a variety to breed with one another." This is what I have called "psychological selection," and, far from "ignoring" it, I have always laid stress upon it as an obviously important form of isolation or _prevention_ of free intercrossing. But it is a form of isolation which can only occur in the higher animals, and, therefore, the whole of Professor Herdman's criticism is merely a restatement of my own views as already published in the paper which he is criticizing.

For all that his argument goes to prove is, first, the necessity for _some_ form of isolation if the overwhelming effects of intercrossing are to be obviated; and, secondly, the manifest consequence that where the psychological form is unavailable (as in many of the lower animals and in all plants), some other form must be present if divergent evolution is taking place on a common area.

Seeing that so much misunderstanding has been shown with reference to my views on "the swamping effects of intercrossing," and seeing also that this misunderstanding extends quite as much to Mr. Gulick's views as to my own, I will here supply brief extracts from both our original papers, for the double purpose of showing our complete agreement, and of leaving it to be judged whether we can fairly be held responsible for the misunderstanding in question. After having supplied these quotations, I will conclude this historical sketch by considering what Mr. Wallace has said in reply to the views therein presented. I will transcribe but a single pa.s.sage from our papers, beginning with my own.

Any theory of the origin of species in the way of descent must be prepared with an answer to the question, Why have species _multiplied_? How is it that, in the course of evolution, species have not simply become trans.m.u.ted in linear series instead of ramifying into branches? This question Mr. Darwin seeks to answer "from the simple circ.u.mstance that the more diversified the descendants from any one species becomes in structure, const.i.tution, 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 ill.u.s.trate this principle by means of a diagram, showing the hypothetical divergence of character undergone by the descendants of seven species. Thus, he attributes divergence of character exclusively to the influence of natural selection.

Now, this argument appears to me una.s.sailable 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 p.r.o.nounced. It is where they "jostle each other most closely" that natural selection will set a premium upon any members of the species which may depart from the common type. Now, inasmuch as this jostling or overcrowding of individuals is a needful condition to the agency of natural selection in the way of diversifying character, must we not feel that the general difficulty from intercrossing previously considered is here presented in a special and aggravated form? At all events, I know that, after having duly and impartially considered the matter, to me it does appear that unless the swamping effects of intercrossing with the parent form on an overcrowded area is in some way prevented to begin with, natural selection could never have any material supplied by which to go on with. Let it be observed that I regard Mr. Darwin's argument as perfectly sound where it treats of the divergence of _species_, and of their further divergence into _genera_; for in these cases the physiological barrier is known to be already present. But in applying the argument to explain the divergence of individuals into varieties, it seems to me that here, more than anywhere else, Mr. Darwin has strangely lost sight of the formidable difficulty in question; for in this particular case so formidable does the difficulty seem to me, that I cannot believe that natural selection alone could produce any divergence of specific character, so long as all the individuals on an overcrowded area occupy that area together. Yet, if any of them quit that area, and so escape from the unifying influence of free intercrossing, these individuals also escape from the conditions which Mr. Darwin names as those that are needed by natural selection in order to produce divergence. Therefore, it appears to me that, under the circ.u.mstances 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 trans.m.u.tation of species in a linear series, each succeeding member of which might supplant its parent form. But in order to secure _diversity_, _multiplication_, or _ramification_ of species, it appears to me obvious that the primary condition required is that of preventing intercrossing with parent forms at the origin of each branch, whether the prevention be from the first absolute, or only partial.

Now for Mr. Gulick, a portion of whose more lengthy discussion of the subject, however, is all that I need quote:--

Having found that the evolution of the fitted is secured through the prevention of crossing between the better fitted and the less fitted, can we believe that the evolution of a special race, regularly transmitting a special kind of fitness, can be realized without any prevention of crossing with other races that have no power to transmit that special kind of fitness? Can we suppose that any advantage, derived from new powers that prevent severe compet.i.tion with kindred, can be permanently transmitted through succeeding generations to one small section of the species while there is free crossing equally distributed between all the families of the species? Is it not apparent that the terms of this supposition are inconsistent with the fundamental laws of heredity?

Does not inheritance follow the lines of consanguinity; and when consanguinity is widely diffused, can inheritance be closely limited? When there is free crossing between the families of one species, will not any peculiarity that appears in one family either be neutralized by crosses with families possessing the opposite quality, or, being preserved by natural selection, while the opposite quality is gradually excluded, will not the new quality gradually extend to all the branches of the species; so that, in this way or in that, increasing divergence of form will be prevented?

If the advantage of freedom from compet.i.tion 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 acc.u.mulation through Natural Selection of the powers on which the advantage depends. The advantage of divergence of character cannot be retained by those that fail to retain the divergent character; and divergent character cannot be retained by those that are constantly crossing with other kinds; and the prevention of free crossing between those that are equally successful is in no way secured by Natural Selection.

So much, then, as expressive of Mr. Gulick's opinion upon this subject.

To exactly the same effect Professor Lloyd Morgan has recently published his judgement upon it thus:--

That perfectly free intercrossing, between any or all of the individuals of a given group of animals, is, so long as the characters of the parents are blended in the offspring, fatal to divergence of character, is undeniable. Through the elimination of less favourable variations, the swiftness, strength, and cunning of a race may be gradually improved. But no form of elimination can possibly differentiate the group into swift, strong, and cunning varieties, distinct from each other, so long as all three varieties freely interbreed, and the characters of the parents blend in the offspring. Elimination may and does give rise to progress in any given group, _as a group_; it does not and cannot give rise to differentiation and divergence, so long as interbreeding with consequent interblending of characters be freely permitted. Whence it inevitably follows, as a matter of simple logic, that where divergence has occurred, intercrossing and interbreeding must in some way have been lessened or prevented. Thus a new factor is introduced, that of _isolation_ or _segregation_. And there is no questioning the fact that it is of great importance. Its importance, indeed, can only be denied by denying the swamping effects of intercrossing, and such denial implies the tacit a.s.sumption that interbreeding and interblending are held in check by some form of segregation. The isolation explicitly denied is implicitly a.s.sumed[51].