This is a most important point. That so enormous a proportion of vegetable species should have originated in intimate association with their parent or sister types, is clearly unintelligible on the theory of natural selection alone; there obviously must be some other form of homogamy which, whether or not in all places associated with natural selection, is the primary condition to the differentiation. Such I hold with Nägeli, is a logical necessity; and this whether or not I am right in believing the other form of homogamy in question to be selective fertility. But I go further and say, Surely there can be no rational question that this other form of homogamy must have been, at any rate as a highly general rule, the one which I have assigned. For how is it that in these ninety-five per cent. of cases, where vegetable species are growing intimately associated with their nearest allies, there is no hybridizing, or blending and relapsing to the original undifferentiated types? We know well the answer. These are fully differentiated species, and, as such, are protected from mutual intercrossing by the barrier of mutual sterility. But now, if this bar is thus necessary for preserving the specific distinctions when they have been fully developed, much more must it have been so to admit of their development; or, otherwise stated, since we know that this barrier is associated with "synoical" species, and since we clearly perceive that were it withdrawn these species would soon cease to exist, can we reasonably doubt that their existence (or origin) is due to the previous erection of this barrier? If synoical species were comparatively rare, the validity of such reasoning might be open to question; or, even if we should not doubt it in such cases, at any rate we might well doubt the importance or extent of selective fertility as a factor in the origination of species. But the value of Nägeli's writings on the present subject consists in showing that synoical species constitute so overwhelming a majority of the vegetable kingdom, that here, at all events, it appears impossible to rate too highly the importance of the principle I have called physiological selection.
CHAPTER V.
Further Evidences of Physiological Selection.
Evidence from Topographical Distribution of Varieties.
In the last section we have considered the topographical distribution of closely allied species. I now propose to go still further into matters of detail, by considering the case of natural varieties. And here we come upon a branch of our inquiry where we may well expect to meet with the most crucial tests of our theory. For if it should appear that these nascent species more or less resemble fully developed species in presenting the feature of cross-infertility, the theory would be verified in the most direct and conclusive manner possible. These nascent species may be called embryo species, which are actually in course of differentiation from their parent-type; and therefore, if they do not exhibit the feature in relation to that type which the present theory infers to be necessary for the purposes of differentiation, the theory must be abandoned. On the other hand, if they do exhibit this feature, it is just the feature which the theory predicted as one that would be found highly characteristic of such embryo types. Contrariwise, the theory of natural selection can have no reason to form any such anticipation; or rather its anticipation would necessarily require to be the exact opposite. For, according to this theory, the cross-infertility of allied species is due, either to correlation with morphological changes which are being produced by the selection, or else, as Darwin supposed, to "prolonged exposure to uniform conditions of life"; and thus, in either case, the sterility variation ought to be, as a general rule at all events, subsequent to the specific differentiation, and, according to Darwin's view, long subsequent. Thus we ought not to find that the physiological change is ever, on any large or general scale, the initial change; nor ought we to find that it is, on any such scale, even so much as a contemporary change: there ought, in fact, to be no constant or habitual association between divergence of embryo-types and the concurrence of cross-infertility.
Now, it will be my endeavour to prove that there is an extraordinarily general association between varietal divergence and cross-infertility, wherever common areas are concerned; and in as far as this can be proved, I take it that the evidence will make wholly in favour of physiological selection as the prime condition to specific divergence, while at the same time they will make no less wholly, and quite independently, against natural selection as the unaided cause of such divergence.
I shall begin with some further quotations from Nägeli.
Species may be synoical at all stages of relationship. We come across varieties, scarcely distinguishable from one another, growing in the same locality (as, for example, the Cirsium heterophyllum, with smooth or jagged leaves, the Hieracium sylvaticum, with or without caulinary leaves); again, we meet other varieties more accentuated (as the H. hoppeanum, with under ligules of white or red, the Campanula, with white or lilac flowers, &c.), other varieties even more marked, which might almost be elevated to the rank of species (Hieracium alpinum, with hairs and glands, and the new form H. holadenium, which has only glands, Campanula rotundifolia with smooth and hairy leaves), or forms still more distinct, up to well-defined species. I could enumerate endless examples at all stages.
It will be seen that in my definition of synoicy I do not mean to assert that all allied forms are invariably found together, but that they are much more often seen in groups than singly. Take, for instance, nine forms closely related (A to I). A, E, H will be found side by side at one point, B, D at another, C, F at a third, &c. These facts are plainly opposed to the theory of isolation and amixia, and make, on the contrary, in favour of the social development of species (loc. cit., p. 221).
Not to multiply quotations to the same general effect, I will supply but one other, referring to a particular case.