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.