On the sole of the foot we find the region of the heel, the ball of the great toe, in this case resembling the ball of a thumb, the roots and tips of the toes, together with pads consisting of muscles, tendons, and skin. The several divisions of these padded balls are separated from each other by furrows which are longitudinal, oblique, and transverse, and more or less distinct from each other. The black skin which covers the sole of the foot is thick and horny, but provided with a series of papillæ. The whole skin of an aged animal is of a deep black colour, somewhat glossy, and covered with intersecting wrinkles. [Illustration: Fig. 4.--The same animal at a still earlier age.] The young male gorilla does not essentially differ from the old male in its general and external appearance. Its skull is, however, without the crest which characterizes the latter animal, and is still of a rounded form in the region of the crown and occiput. At this age the head is not so high at the back and on the top as in aged males. The orbits are less prominent, the general aspect of the face is not so decidedly prognathous, and the bridge of the nose is shorter. The lines of the body in the young male are softer and less exaggerated, and the expression of the face is less ferocious than in an aged male. The horny callosities on the hands and feet are altogether wanting or only faintly indicated, and the hands, fingers, and toes have not arrived at the powerful development which we observe in the older animal. (Comp. Figs. 3 and 4.) Considerable differences may be observed in the whole structure of the adult female gorilla. The animals of this sex are smaller and weaker than males of the same age. The skull of the female is smaller and more rounded than that of the male, and the great bony crests are also absent. The orbits are less prominent, and a front view of the head gives the impression of a trapezoidal form. The coronal arch rises above this trapezoid. In the male, on the contrary, the crown seems to lengthen above and behind into a pyramidal form. In the aged female the bridge of the nose is generally shorter than in the aged male, but even in this particular there is great variation in different individuals. Sometimes the bridge of the nose in a female is much depressed, and then the interval between the orbits and the end of the nose is shorter: I intentionally avoid the term _tip_ of the nose, on account of the blunted form of this organ. Even when the bridge of the nose is more prominent, the interval between its end and the orbits is sometimes very short.