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.
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 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, 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.
Evidence from Special Cases.
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.
Professor Le Conte has adduced the fossil snails of Steinheim as serving to corroborate the theory of physiological selection[26].
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 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.
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 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 protected from intercrossing with their parent form.
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.
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.