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]. [26] _Evolution and its Relations to Religious Thought_, &c. pp. 236-7. 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.