The first advance from arboreal to terrestrial life is marked by the power of walking more or less erect on the hind limbs and thus releasing the arms; this power is developed to a greater or less degree in all the anthropoid apes; with practice they become expert walkers. The additional freedom which the erect attitude gives to the arms and to the movements of the hands and the separate movements of the fingers is especially noticeable in the gibbon. The cultivation of the powers of the hand reacts upon the further growth and specialization of the brain; thus the brain and the erect attitude react upon each other. In the gibbon there is a marked increase in the size of those portions of the brain which supply the centres of touch, vision, and hearing. [Illustration: FIG. 23. The evolution of the brain. Outlines (side view) of typical human and prehuman brains, showing the early development of the posterior portions of the brain and the relatively late development of the anterior portions, the seat of the higher mental faculties.] Discussion as to how the ancestors of man were fashioned has chiefly dealt with the rival claims of four lines of structural evolution: first, the assumption of the _erect attitude_; second, the development of the _opposable thumb_; third, the _growth of the brain_; and fourth, the acquisition of the _power of speech_. The argument for the erect attitude suggested by Lamarck, and ably put by Munro(4) in 1893, indicates that the cultivation of skill with the hands and fingers lies at the root of man's mental supremacy. Elliot Smith's argument that the steady growth and specialization of the brain itself has been the chief factor in leading the ancestors of man step by step upward indicates that such an advance as the erect attitude was brought about because the brain had made possible the skilled movements of the hands. [Illustration: FIG. 24. The evolution of the brain. Outlines (top view) of typical human and prehuman brains, showing the narrow forebrain of the primitive type and the successive expansion of the seat of the higher mental faculties in the successive races.]