The first is, that, as Mr. Wallace remarks, "every species has come into existence coincident both in space and time with a pre-existing and closely allied species." I regard this as important evidence that physiological selection is one of the natural causes concerned. For the general fact implied is that every species has come into existence on an area occupied by its parent type, and therefore under circumstances which render it imperative that intercrossing with that type should be prevented. In the case of monotypic evolution by natural selection alone, intercrossing with the parent type is prevented through the gradual extinction of that type by successive generations of the developing type. But in the case of polytypic evolution, intercrossing with the parent type can only be prevented by some form of isolation other than natural selection; and here it is evident that cross-infertility with the parent type must be as efficient to that end as any other form of isolation that can be imagined. Consequently we might almost have expected beforehand that in a large proportional number of cases cross-infertility should have been the means employed. And the fact that this is actually the case so far corroborates the only theory which is able to explain it. The second point is this. 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.