The skull of the chimpanzee is longer than that of the orang, the most prominent feature in the top view being the extreme protuberance of the orbits, which are surrounded by a supraorbital and circumorbital bony ridge, which is also strongly developed in the Neanderthal skull as well as in the _Pithecanthropus_ or Trinil skull but, so far as we know, is entirely lacking in that of Piltdown. As in the orang and the gorilla, a crest develops along the middle of the top of the skull for the insertion of the powerful muscles of the jaws, a crest which is wholly wanting in the gibbon and probably wanting in all the true ancestors of man. [Illustration: FIG. 21. Contrast of the projecting face (prognathism), retreating forehead, and small brain-case of a young gorilla, as compared with the vertical face, prominent nose, high forehead, and large brain-case of a high race of man. After Klaatsch.] The gorilla illustrates in the extreme the specializations which are begun in the chimpanzee, and which are attributable to a life partly arboreal, partly terrestrial, with the skull and jaws used as powerful fighting organs. The head is lengthened by the forward growth of the muzzle into an extreme prognathism. The limbs and body of the gorilla show a departure from the primitive, slender-limbed, arboreal type of apes and are partly adapted to a bipedal, ground-dwelling habit. As regards psychic evolution,(3) Elliot Smith observes that the arboreal mode of life of the early ancestors of man developed quick, alert, and agile movements which stimulated the progressive development of the posterior and lateral portions of the brain. The sense of smell had been well developed in a previous terrestrial life, but once these creatures left the earth and took to the trees, guidance by the olfactory sense was less essential, for life amidst the branches of the trees is most favorable to the high development of the senses of vision, touch, and hearing. Moreover, it demands an agility and quickness of movement that necessitate efficient motor centres in the brain to co-ordinate and control such actions as tree life calls for. The specialization of sight awakens curiosity to examine objects with greater minuteness and guides the hands to more precise and skilled movements. [Illustration: FIG. 22. Side view of a human brain of high type, showing the chief areas of muscular control and of the sensory impressions of sight and hearing, also the prefrontal area in which the higher mental faculties are centred. Modified after M. Allen Starr.] The anatomy of man is full of remote reminders of this original arboreal existence, which also explains the very large and early development of the posterior portions of the brain, in which the various senses of sight, touch, and hearing are located.