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since we know that this barrier is associated with "synoical" species, and since we clearly perceive that were it withdrawn these species would soon cease to exist, can we reasonably doubt that their existence (or origin) is due to the previous erection of this barrier?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._