The acclimatisation of man, as well as of the wild animal, takes place only when he finds the conditions of existence sensibly identical with those in which he has been created. Beyond that, nature punishes him for having overstepped the limits which she had assigned to him, and within which he ought to continue to move his organism in relation to this defined medium. The domestic animal, on the contrary, by reason of this malleability of which we have before spoken, accommodates itself in general very conveniently. And the varieties which it shows on recovering its liberty, are of themselves a proof that it has been under a different sky to that of its original country. Such is, in our opinion, the sole manner of explaining at the present day, in a serious and general point of view, all climateric influences. We must render justice to some monogenists, that they have perfectly understood the real part taken by these influences. Blumenbach calls them _causæ degenerationis_; and here the German anatomist, in defending, like Prichard, the specific unity of the human race, raises himself above the English anthropologist, without, however, reaching what we believe to be the truth. Prichard, inclining to the belief that humanity is entirely descended from the Negroes,[229] acknowledged, consequently, a kind of _causæ perfectionis_, that is to say, an ascending march of phenomena, where his predecessor had only seen an inverse march. Now, this ascending march of phenomena is difficult to reconcile with the notion of the specific unity of man. Every species, in fact, is necessarily constituted by reason of the defined space in which it ought to move. It is unreasonable to suppose that elsewhere the same organism and the same species can meet with more favourable conditions of existence.