It appears to be comparatively rare for any cause of specific divergence to prove effectual on common areas, unless it sooner or later becomes associated with some degree of cross-infertility. But through this association, the segregating influence of both the causes concerned is, as Mr. Gulick has shown, greatly increased. For instance, if the segregating influence of some degree of cross-infertility be associated with that of any other form of isolation, then, not only will the two segregating influences be added, but multiplied together. And thus, by their mutual action and reaction, divergent evolution is promoted at a rapidly increasing rate.
I will now summarize the main points of the theory of physiological isolation in a categorical form.
1. If no other form of isolation be present, specific divergence can only take place when some degree of cross-infertility has previously arisen between two or more sections of a species.
2. When such cross-infertility has arisen it may cause specific divergence, either (_a_) by allowing independent variability in each of the physiologically isolated groups; (_b_) by becoming associated with any other cause of differentiation already operating; or (_c_) by both these means combined.
3. As some degree of cross-infertility generally obtains between allied species, we are justified in concluding that this has been the most frequent--or, at any rate, the most effective--kind of isolation where the origin of species is concerned; and therefore the kind with which, in the case of species-formation, natural selection, or any other cause of specific divergence, has been most usually associated.
4. Where varietal divergence has begun in the absence of cross-infertility, such divergence seems, as a general rule, to have been incapable of attaining to a specific value.
5. Therefore, in the vast majority of such cases, it must have been those varietal changes of structure, size, colour, &c., which happened to have afterwards been assisted by the reproductive change that were on this account _selected_ as successful candidates for specific differentiation.
6. It follows, that it makes no difference to the general theory of physiological selection in what proportion of cases the physiological change has been the initial change; for, whether prior or subsequent to the varietal changes with which it becomes associated, its presence has been equally important as a condition to specific divergence.