The Gorilla and Chimpanzee are closely related. The Gorilla is not only the largest ape of the four, but in shape and build the most man-like, which is accounted for by the fact that it is less strictly arboreal than the others, and confines itself largely to the ground. Its skull is superficially much less human in appearance than that of the Chimpanzee, due to the strongly developed crests, which serve as attachments for the powerful muscles of the lower jaw. The teeth of the Chimpanzee are more uniform in size, and the skull smoother. There is a marked difference in temperament between the two species, the Gorilla being fierce and gloomy and quite untamable, while the Chimpanzee is of a pleasant and lively disposition, and can, as is well known, be trained to wear clothes, eat with a fork and knife, etc. The Orang-Utan is found inhabiting forest ground in Borneo and Sumatra, and living largely in trees, in which it builds nests as temporary sleeping-places. It is a clumsy-looking animal, supporting itself on the knuckles of its hands when travelling along the ground, and moving but slowly. It has twelve pairs of ribs, the same number as in man, and one fewer than in the Gorilla and Chimpanzee. Practically all the important anatomical distinctions between man and the anthropoids are reducible to two causes: the change from the arboreal existence to a life on the ground and in the open country, and the great development of the intelligence. When the change in the mode of life occurred, it is obvious that an erect carriage would possess an advantage over the stooping gait of the ape, in which it is neither definitely a biped nor a quadruped. The erect position necessitated a better-developed heel, stronger calf and hip muscles, and a more parallel position and stronger development of the great toe. It also brought about a shortening of the fore-arms and a widening of the pelvis. Much has been made of the difference in the foot, between the condition of a grasping organ, with an opposable great toe, and that seen in man. But the change is just what we should devise in order to make the foot stronger for propelling the body on the ground. Moreover, the difference is more obvious than fundamental, and it is well known that in armless persons, who develop the possibilities of their feet, the latter members can still be turned into wonderfully efficient 'hands.' [Illustration: FIG. 137.--Brains of (_F_) Cercopithecus (Eastern Monkey); (_H_) Australian Bushman; (_L_) Chimpanzee; (_M_) European.]