Very little appears to be known, at least scarcely any intimation is given in the work before us, of the proximate or final cause of these changes. Great as they are, certainly, as far as we can judge, no _nisus formativus_ can account for the enormous horns of the Spanish sheep; nor, looking to the final cause; does there appear any reason why domestic animals should need such overgrown instruments of defence. When, however, we come to the more important anatomical modifications, such as the length and shape of the legs, the bones of the pelvis or of the jaw, the object is more apparent. A greyhound, with the muzzle of a bull-dog, would be an obvious natural inconsistency. We now pass to the physical distinctions of the different races of men. Here we may observe that a much greater importance is to be attached to comparatively slight variations. Considering the surprising external differences that exist in domesticated animals of the same species, the wonder rather is, that the different races of men differ physically so little as they do, than that they differ so much. Here we will take first, the least important shades of difference--the texture of the skin, hair, and complexion; and then pass on to the more prominent diversities of the bony fabric, cranium, &c. "The texture of the body, in which all these varieties have their seat, is the extracorial or exodermal structure, constituting, if I may so speak, the outer coating of the body, external to the true skin, which corresponds to the cuticular and corneous excrescences of animals--a structure which includes horns, hoofs, hair, feathers, and all similar appendages in different orders of animals. This structure displays infinite diversities in colour, constitution, and organization, and is the most variable tissue on the whole body. Many different opinions have, however, been lately maintained, and much research has been made, as to the nature and texture of the parts on which the variety of colour depends."