It has been shown that divergent or polytypic evolution on common areas is inexplicable by natural selection alone. Hence the question arises: What form of isolation has, under such circumstances, rendered possible divergent evolution? In answer to this question the theory of physiological selection suggests that variations in the reproductive function occur in such a way as to isolate more or less perfectly from each other different sections of a species. While cross-fertility remains unimpaired among the members of each section, there is more or less cross-infertility when members of either section mate with those of the other. Thus a physiological barrier is interposed between the two sections; and any divergences of structure, colouring, or instinct arising in the members of either section will not in any way be affected by such divergences as arise among the members of the other. In support of this suggestion, it has been shown in the preceding chapter that the very general association of cross-infertility with specific differentiation points most strongly to the inference that the former has usually been an indispensable condition to the occurrence of the latter. It cannot be denied that in many cases the specific distinction is now maintained by means of that sexual isolation which cross-infertility confers: it is therefore probable that such isolation has been instrumental in securing its initial attainment. This probability is strengthened by the observed fact that the general association in question is conspicuously absent in the case of domesticated varieties, notwithstanding that their multitudinous and diverse varietal characters usually equal, and frequently surpass, specific characters in their degrees of divergence. Since, then, it would seem to be impossible for divergent evolution on common areas to take place in the absence of some mode of isolation; since cross-infertility appears to be the only possible mode under the given circumstances; and since among domesticated varieties, where isolation is otherwise secured by artificial means, cross-infertility is usually absent, the logical foundations of the theory of physiological selection would seem to be securely laid. We may therefore pass to more special lines of evidence. _Evidence from Geographical Distribution._