That this property of species is useful to them, in the somewhat unusual sense that it keeps them from freely mingling with other species, is true; but, as has been said, this would be a rather peculiar kind of adaptation. If, however, it be claimed that this property is useful to species, as Darwin himself claims, then, as he also points out, it is a useful acquirement that cannot have arisen through natural selection. It is not difficult to show why this must be so. If two varieties were to some extent at the start less fertile, _inter se_, than with their own kind, the only way in which they could become more infertile through selection would be by selecting those individuals in each generation that are still more infertile, but the forms of this sort would, _ex hypothese_, become less numerous than the descendants of each species itself, which would, therefore, supplant the less fertile ones. Darwin’s own statement in regard to this point is as follows:— “At one time it appeared to me probable, as it has to others, that the sterility of first crosses and of hybrids might have been slowly acquired through the natural selection of slightly lessened degrees of fertility, which, like any other variation, spontaneously appeared in certain individuals of one variety when crossed with those of another variety. For it would clearly be advantageous to two varieties or incipient species, if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate.