Volume Iii Part 7 (1/2)

[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 distinguis.h.i.+ng 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, s.h.i.+elded 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 distinguis.h.i.+ng 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:--