o which they belonged over other
communities of the same species; but an individual animal, if rendered
slightly sterile when crossed with some other variety, would not thus in
itself gain any advantage, or indirectly give any advantage to its nearest
relatives or to other individuals of the same variety, leading to their
preservation. I infer from these considerations that, as far as animals are
concerned, the various degrees of lessened fertility which occur with
species when crossed cannot have been slowly accumulated by means of
natural selection.

With plants, it is possible that the case may be somewhat different. With
many kinds, insects constantly carry pollen from neighbouring plants to the
stigmas of each flower; and with some species this is effected by the wind.
Now, if the pollen of a variety, when deposited on the stigma of the same
variety, should become by spontaneous variation in ever so slight a degree
prepotent over the pollen of other varieties, this would certainly be an
advantage to the variety; for its own pollen would thus obliterate the
effects of the pollen of other varieties, and prevent deterioration of
character. And the more prepotent the variety's own pollen could be
rendered through natural selection, the greater the advantage would be. We
know from the researches of Gaertner that, with species which are mutually
sterile, the pollen of each is always prepotent on its own stigma over that
of the other species; but we do not know whether this prepotency is a
consequence of the mutual sterility, or the sterility a consequence of the
prepotency. If the latter view be correct, 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 occurs with existing species. This 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 the effects of the access of previous males belonging to other
varieties; but we have no reason to believe, at least {188} with
terrestrial animals, that this is the ease; 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 thr