Although I saw the effects of isolation in the case of islands and mountain-ranges, and knew of a few instances of rivers, yet the greater number of your facts were quite unknown to me. I now see that, from the want of knowledge, I did not make nearly sufficient use of the views which you advocate[35].
Now it would be easy to show the justice of these self-criticisms by quoting longer passages from earlier editions of the Origin of Species; but as this, in view of the above passages, is unnecessary, we may next pass on to another point.
The greatest oversight that Wagner made in his otherwise valuable essays on geographical isolation, was in not perceiving that geographical isolation is only one among a number of other forms of isolation: and, therefore, that although it is perfectly true, as he insisted, that polytypic evolution cannot be effected by natural selection alone, it is very far from true, as he further insisted, that geographical isolation is the only means whereby natural selection can be assisted in this matter. Hence it is that, when Darwin said he had not himself "made nearly sufficient use" of geographical isolation as a factor of specific divergence, he quite reasonably added that he could not go so far as Wagner did in regarding such isolation as a condition, sine qua non, to divergent evolution in all cases. Nevertheless, he adds the important words, "I almost wish I could believe in its importance to the same extent with you; for you well show, in a manner which never occurred to me, that it removes many difficulties and objections." These words are important, because they show that Darwin had come to feel the force of the "difficulties and objections" with regard to divergent evolution being possible by means of natural selection alone, and how readily they could be removed by assuming the assistance of isolation. Hence, it is much to be deplored that Wagner presented a single kind of isolation (geographical) as equivalent to the principle of isolation in general. For he thus failed to present the complete—and, therefore, the true—philosophy of the subject to Darwin's mind; and in this, as in certain other respects which I shall notice later on, served rather to confuse than to elucidate the matter as a whole.
To sum up. Although in his later years, as shown by his correspondence, Darwin came to recognize more fully the swamping effects of free intercrossing, and the consequent importance of "separation" for the prevention of these effects, and although in this connexion he likewise came more clearly to distinguish between the "two cases" of monotypic and polytypic evolution, it is evident that he never worked out any of these matters—"thinking it prudent," as he wrote with reference to them in 1878, "now I am growing old, to work at easier subjects[36]." Therefore he never clearly saw, on the one hand, that free intercrossing, far from constituting a "difficulty" to monotypic evolution by natural selection, is the very means whereby natural selection is in this case enabled to operate; or, on the other hand, that, in the case of polytypic evolution, the "difficulty" in question is so absolute as to render such evolution, by natural selection alone, absolutely impossible. Hence, although in one sentence of the Origin of Species he mentions three forms of isolation (besides the geographical form) as serving in some cases to assist natural selection in causing "divergence of character" (i. e. polytypic evolution[37]), on account of not perceiving how great and how sharp is the distinction between the two kinds or "cases" of evolution, he never realized that, where "two or more new species" are in course of differentiation, some form of isolation other than natural selection must necessarily be present, whether or not natural selection be likewise so. The nearest approach which he ever made to perceiving this necessity was in one of his letters to Wagner above quoted, where, after again appealing to the erroneous analogy between monotypic evolution and "unconscious selection," he says:—"But I admit that by this process (i. e. unconscious selection) two or more new species could hardly be formed within the same limited area: some degree of separation, if not indispensable, would be highly advantageous; and here your facts and views will be of great value." But even in this passage the context shows that by "separation" he is thinking exclusively of geographical separation, which he rightly enough concludes (as against Wagner) need certainly not be "indispensable." Had he gone a step further, he must have seen that separation, in some form or another, is "indispensable" to polytypic evolution. Instead of taking this further step, however, two years later he wrote to Semper as follows:—
I went as far as I could, perhaps too far, in agreement with Wagner [i. e. in the last edition of the Origin of Species]; since that time I have seen no reason to change my mind; but then I must add that my attention has been absorbed on other subjects[38].
And he seems to have ended by still failing to perceive that the explanation which he gives of "divergence of character" in the Origin of Species, can only hold on the unexpressed assumption that free intercrossing is in some way prevented at the commencement, and throughout the development, of each diverging type.
Lastly, we have to consider Darwin's opinion touching the important principle of "Independent Variability." This, it will be remembered, is the principle which ensures that when a portion (not too large) of a species is prevented from interbreeding with the rest of the species, sooner or later a divergence of type will result, owing to the fact that the average qualities of the separated portion at the time of its separation cannot have been exactly the same as the average qualities of the specific type as a whole. Thus the state of Amixia, being a state of what Mr. Gulick calls Independent Generation, will of itself—i.e. even if unassisted by natural selection—induce divergence of type, in a ratio that has been mathematically calculated by Delbœuf.
Darwin wrote thus to Professor Weismann in 1872:—
I have now read your essay with very great interest. Your view of the origin of local races through "Amixia" is altogether new to me, and seems to throw an important light on an obscure question[39].
And in the last edition of the Variation of Animals and Plants he adds the following paragraph:—