It is self-evident, as we have seen in the preceding section, that the greater the number of individuals of the same species on a given area, the less must be the power of natural selection to split that species into two or more allied types; because, the more crowded the population, the greater must be the uniformitarian effect of free intercrossing. This obvious fact has been insisted upon by several previous writers on Darwinism; and the only reason why it has not been recognized by all naturalists is that so few of them have observed the all-important distinction between monotypic and polytypic evolution. The denser the population, and therefore the greater the intercrossing and the severer the struggle for existence within the species, the better will it be for transmutation of the species by natural selection; but the worse it will be for differentiation of the species by this form of homogamy. On the other hand, if physiological selection be entertained as a form of homogamy, the denser the population, the better opportunity it will have of differentiating the species, first, because a greater number of individuals will be present in which the physiological change may arise, and, secondly, because, if it does arise, the severity of the struggle for existence will then give natural selection a better chance of acting rapidly and effectually on each of the isolated sections.
Hence, where the question is whether selective fertility has played any large or general part in the differentiation of specific types, the best criterion we can apply is to ascertain whether it is a general rule that closely allied species occur in intimate association, so that their individual members constitute, as it were, a single population, or, on the other hand, whether they occur rather on different sides of physical barriers. If they occur intimately associated, the form of homogamy to which their differentiation was due must have presumably been the physiological form; whereas, if they are proved to be correlated with physical barriers, the form of homogamy which was concerned in their differentiation must presumably have been the geographical form.
Now, at first this consideration was a trouble to me, because Moritz Wagner had strenuously argued—and supported his argument by a considerable wealth of illustration—that allied species are always found correlated with physical barriers or discontinuous areas. Weismann's answer, indeed, had shown that Wagner's statement was much too general: nevertheless, I was disappointed to find that so much could be said in favour of the geographical (or topographical) form of isolation where closely allied species are concerned. Subsequently, however, I read the writings of Nägeli on this subject, and in them I find a very different state of matters represented.
Seeing as clearly as Wagner that it is impossible under any circumstances for natural selection to cause specific differentiation unless assisted by some other forms of homogamy, but committing the same oversight as Wagner and Weismann in supposing that the only other form of homogamy in nature is geographical isolation, Nägeli, with great force of reasoning, and by many examples, founded his argument against the theory of natural selection on the ground that in the vegetable kingdom closely allied species are most frequently found in intimate association with one another, not, that is to say, in any way isolated by means of physical barriers. This argument is everywhere logically intact; and, as he sustains it by a large knowledge of topographical botany, his indictment against natural selection as a cause of specific differentiation appeared to be insurmountable. And, in point of fact, it was insurmountable; so that the whole problem of the origin of species by differentiation on common areas has hitherto been left in utter obscurity. Nor is there now any escape from this obscurity, unless we entertain the "supplementary factor" of selective fertility. And, apparently, the only reason why this has not been universally recognized, is because Darwinians have hitherto failed to perceive the greatness of the distinction between the differentiation and the transmutation of species; and hence have habitually met such overwhelming difficulties as Nägeli presented by an illogical confounding of these two totally distinct things.
But if the idea of selective fertility had ever occurred to Nägeli as a form of segregation which gives rise to specific differentiation, I can have no doubt that so astute and logical a thinker would have perceived that his whole indictment against natural selection was answered. For it is incredible that he should not have perceived how this physiological form of homogamy (supposing it to arise before or during, and not after the specific differentiation) would perform exactly the same function on a continuous area, as he allowed that "isolation" does on a discontinuous one.
However, be this as it may, there cannot be any question touching the immense value of his facts and arguments as evidence in favour of physiological selection—albeit this evidence was given unconsciously, or, as it were, prophetically. Therefore I will here quote a few examples of both, from his paper Du Développement des Espèces Sociales[24].
After stating the theory of natural selection, he says that if the theory is (of itself) a true explanation of the origin (or divergence) of specific forms, it ought to follow that
two closely allied forms, derived the one from the other, would necessarily occupy two different geographical areas [or topographical stations], since otherwise they would soon become blended. Until they had already become sufficiently consolidated as distinct species to render mutual intercrossing highly improbable, they could not be intermingled without disadvantage [to differentiation]. Had Darwin endeavoured to support his hypothesis by facts, he would, at least in the vegetable kingdom, have found little to favour his cause. I can cite many hundreds of cases, in which species in every stage of development have been found closely mingling with one another, and not in any way isolated. Therefore, I do not think that one can rightly speak of natural selection in the Darwinian sense in the vegetable kingdom; and, in my estimation, there is a great difference between the formation of species by nature and the production of stock by a breeder.... (p. 212).
Of the two kinds of distribution (i. e. growing apart and growing together), Synoicy (or growing together) is by far the most usual in nature. I reckon that out of a hundred allied vegetable forms, at least ninety-five would be found to be synoical (p. 219).