THE HUMAN FORM 321 



of defensive weapons. Thanks to these, the jaws entirely ceased 

 to bite and tear, as they had already ceased to seize, and 

 limited themselves to the mastication of food. On account of 

 this less arduous work, they became shorter and lighter. In 

 the larger Apes the muscles that raise the lower jaw 

 are very powerful, being inserted in the temporal fossa 

 during youth ; but, as the animal grows older, they creep 

 gradually up the lateral walls of the skull, as in Carnivora, 

 till they finally meet at the vertex, where they cause 

 the development of a median crest at the point of their 

 attachment. Henceforth this crest prevents any ex- 

 pansion of the skull, whose bones are definitely sutured 

 along the median line. When the muscles attached to it contract 

 they even tend to compress the walls of the skull laterally, and 

 thus to compress, and so arrest the development of, the brain. 

 This is probably one reason why old Monkeys are more 

 capricious, more evilly disposed, and more stupid than young 

 ones. In Man the muscles that raise the lower jaw have ceased 

 to migrate in this way. They are inserted in the temporal 

 fossa, like those of the young Monkeys, and their contraction 

 can exercise no pressure upon the brain ; on the contrary, 

 they tend to separate the frontal and parietal bones and so to 

 relieve the brain, thus favouring its development. The head is 

 so balanced on the vertebral column that it projects to 

 an equal extent before and behind ; and it likewise develops 

 in height, a fact which has important consequences. The 

 frontal development of the skull and the brain naturally 

 bring forward the base of the nose, whereas the retraction of the 

 jaws permits of the nostrils opening freely above them — hence, 

 the nasal salient so characteristic of Man. The same retraction 

 gives freedom of movement to the lips, now no longer strained 

 forward over projecting teeth, and it becomes possible for them 

 to smile. As the skull grows in height, it dominates the ears, 

 already immobile in Monkeys, and, as it widens at the 

 same time, it brings the eyes, more or less laterally placed in 

 most Mammals, to a frontal position. Thus all the characteristic 

 features of the human face are consequent on the development 

 of the brain, in itself stimulated by the new importance of the 

 hand. In the same way the characteristic features of the 

 Vertebrates have been determined by the predominance 

 assumed by the nervous system, so that the evolution of the 



