Again, no adequate allowance has been made in the case of these forms of skull for the influence of modes of nurture in infancy. Dr. Morton, observing that the brachy-kephalic American skull was often unequal sided, and the occiput much flattened, suggests that this is "an exaggeration of the natural form produced by the pressure of the cradle-board in common use among the American natives." Dr. Wilson has noticed the same unsymmetrical character in brachy-kephalic skulls in British barrows, and has suspected some artificial agency in infancy; and says, in reference to the American instances, "I think it extremely probable that further investigation will tend to the conclusion that the vertical or flattened occiput, instead of being a typical characteristic, pertains entirely to the class of artificial modifications of the natural cranium familiar to the American ethnologist." While the points in which the races of men vary are those in which lower animals are most liable to undergo change, the several races display a remarkable constancy in those which are usually less variable. Prichard and Carpenter have well shown this in relation to physiological points, as, for instance, the age of arriving at maturity, the average and extreme duration of life, and the several periods connected with reproduction. The coincidence in these points alone is by many eminent physiologists justly regarded as sufficient evidence of the unity of the species. 4. It may also be affirmed, in relation to the varieties of man, that they do not exceed in amount or extent those observed in the lower animals. If with Frederick Cuvier, Dr. Carpenter, and many other naturalists, we regard the dog as a single species, descended in all probability from the wolf, we can have no hesitation in concluding that this animal far exceeds man in variability.[171] But this is denied by many, not without some show of reason; and we may, therefore, select some animal respecting which little doubt can be entertained. Perhaps the best example is the common hog (_Sus scrofa_), an undoubted descendant of the wild boar, and a creature especially suitable for comparison with man, inasmuch as its possible range of food is very much the same with his, which is not the case with any other of our domesticated animals; and as its headquarters as a species are in the same regions which have supported the greatest and oldest known communities of men. We may exclude from our comparison the Chinese hog, by some regarded as a distinct species (_Sus Indicus_), though no wild original is known, and it breeds freely with the common hog. The color of the domestic hog varies, like that of man, from white to black; and in the black hog the skin as well as the hair partakes of the dark color. The abundance and quality of the hair vary extremely; the stature and form are equally variable, much more so than in man. Blumenbach long ago remarked that