Origin of Species Variorum

character. And the more prepotent the pollen could be rendered through natural selection the greater the advantage would be. We know from the researches of Gärtner that prepotency of this kind always accompanies the sterility which follows from crossing distinct species; but we do not know whether prepotency is a consequence of sterility, or sterility a consequence of prepotency. If the latter view be correct, we may infer that, as the prepotency became stronger through natural selection, from being advantageous to a species in process of formation, so the sterility consequent on prepotency would at the same time be augmented; and the final result would be various degrees of sterility, such as actually occur with our existing species when crossed. This same view might be extended to animals if the female before each birth received several males, so that the sexual element of the prepotent male of her own variety obliterated all effects from the access of previous males of other varieties; but we have no reason to believe, at least with terrestrial animals, that this is the case; as most males and females pair for each birth, and some few for life.

On the whole we may conclude that with animals the sterility of crossed species has not been slowly augmented through natural selection; and as this sterility follows the same general laws in the vegetable as in the animal kingdom, it is improbable, though apparently possible, that crossed plants should have been rendered sterile by a different process from animals. From this consideration, and remembering that species which have never co-existed in the same country, and which therefore could not have profited by having been rendered mutually infertile, yet are