The adversaries of the unity doctrine pretend that the European is better proportioned. They are answered that the excessive leanness of the extremities among those nations which subsist principally on vegetable diet, or whose alimentation is imperfect, is not at all surprising; and this reply is certainly valid. But a much less conclusive reply is made to the argument drawn from the excessive development of bust among the mountaineers of Peru (Quichuas) by those who are unwilling to recognize it as a specific characteristic; for to pretend, as they do, that it can be explained by the elevation of the Andes, is not advancing a very serious reason.[135] There are in the world many mountain populations who are constituted very differently from the Quichuas.[136] The color of the skin is another argument for diversity of origin. But the opposite party refuse to accept this as a specific characteristic, for two reasons: first, because, they say, this coloration depends upon climatic circumstances, and is not permanent--which is, to say the least of it, a very bold assertion; secondly, because color is liable to indefinite gradations, by which white insensibly passes into yellow, yellow into black, so that it is impossible to find a line of demarcation sufficiently decided. This fact simply proves the existence of innumerable hybrids; an observation to which the advocates for unity are constantly inattentive. With regard to the specific differences in the formation of the pile, Mr. Flourens brings his great authority in favor of the original unity of race.[137] I have now passed rapidly in review the more or less inconsistent arguments of the advocates of unity; but their strongest one still remains. It is of great force, and I therefore reserved it for the last--the facility with which the different branches of the human family produce hybrids, and the fecundity of these hybrids themselves. The observations of naturalists seem to have well established the fact that half-breeds can spring only from nearly related species, and that even in that case they are condemned to sterility. It has been further observed that, even among closely allied species, where fecundation is possible, copulation is repugnant, and obtained, generally, either by force or ruse, which would lead us to suppose that, in a state of nature, the number of hybrids is even more limited than that obtained by the intervention of man. It has, therefore, been concluded that, among the number of specific characteristics, we must place the faculty of producing prolific offspring.