With regard to the size of the limbs, the opponents of a common origin assert that the European is better proportioned. The answer—which is a good one—is that we have no reason to be surprised at the thinness of the extremities in peoples who live mainly on vegetables or have not generally enough to eat. But as against the argument from the extraordinary development of the bust among the Quichuas, the critics who refuse to recognize this as a specific difference are on less firm ground. Their contention that the development among the mountaineers of Peru is explained by the height of the Andes, is hardly serious. There are many mountain-peoples in the world who are quite differently constituted from the Quichuas.[58] The next point is the colour of the skin. The unitarians deny this any specific influence, first because the colour depends on facts of climate, and is not permanent—a very bold assertion; secondly because the colour is capable of infinite gradation, passing insensibly from white to yellow, from yellow to black, without showing a really definite line of cleavage. This proves nothing but the existence of a vast number of hybrids, a fact which the unitarians are continually neglecting, to the great prejudice of their theory. 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.