As to the specific character of the hair, Flourens is of opinion that this is no argument against an original unity of race.
After this rapid review of the divergent theories I come to the great scientific stronghold of the unitarians, an argument of great weight, which I have kept to the end—I mean the ease with which the different branches of the human family create hybrids, and the fertility of these hybrids.
The observations of naturalists seem to prove that, in the animal or vegetable world, hybrids can be produced only from allied species, and that, even so, they are condemned to barrenness. It has also been observed that between related species intercourse, although possibly fertile, is repugnant, and usually has to be effected by trickery or force. This would tend to show that in the free state the number of hybrids is even more limited than when controlled by man. We may conclude that the power of producing fertile offspring is among the marks of a distinct species.
As nothing leads us to believe that the human race is outside this rule, there is no answer to this argument, which more than any other has served to hold in check the forces opposed to unity. We hear, it is true, that in certain parts of Oceania the native women who have become mothers by Europeans are no longer fitted for impregnation by their own kind. Assuming this to be true, we might make it the basis of a more profound inquiry; but, so far as the present discussion goes, we could not use it to weaken the general principle of the fertility of human hybrids and the infertility of all others; it has no bearing on any conclusions that may be drawn from this principle.
