The characteristics by which the races of men are gradually distinguished are partly taken from the formation of the hair, partly from the colour of the skin, and partly from the formation of the skull. In regard to the last character, two extremes are distinguished, namely, long heads and short heads. In _long-headed men_ (Dolichocephali), whose strongest development is found in Negroes and Australians, the skull is extended, narrow, and compressed on the right and left. In _short-headed men_ (Brachycephali), on the other hand, the skull is compressed in an exactly opposite manner, from the front to the back, is short and broad, which is especially striking in the case of the Mongolians. _Medium-headed men_ (Mesocephali), standing between the two extremes, predominate especially among Americans. In every one of these three groups we find men with _slanting teeth_ (Prognathi), whose jaws, like those of the animal snout, strongly project, and whose front teeth therefore slope in front, and men with _straight teeth_ (Orthognathi), whose jaws project but little, and whose front teeth stand perpendicularly. During the last ten years a great deal of time and trouble have been devoted to the careful examination and measurement of the forms of skulls, which have, however, not been rewarded by corresponding results. For within a single species, as for example within the Mediterranean species, the form of the skull may vary so much that both extremes are met with in the same species. Much better starting-points for the classification of the human species are furnished by the nature of the hair and speech, because they are much more strictly hereditary than the form of the skull.