MAN 177 



look to either side, while in the latter they look directly 

 forward so that the visual fields overlap and, by rotating 

 the head, an object can be scanned by the eyes in alter- 

 nation and the animal can thereby obtain increased per- 

 ception of distance and dimension. The living Tarsiua 

 has utilized this rotary motion to such an extent that it 

 can turn its head through an angle of almost 180° and 

 thus look directly behind itself. But in whatever direc- 

 tion it looks, the two eyes engage in parallel rather than 

 convergent vision. 



Below the primates the brain had been predominantly 

 concerned with smell, while vision, hearing and touch 

 had been merely auxiliary senses. But when the lemu- 

 roids and tarsioids took to Uving in the trees, smell lost 

 most of its usefulness and the other senses became in- 

 creasingly important. This shift in importance is reflected 

 in the structure of the brain: as compared with other 

 mammals, in the primates the areas concerned with 

 vision, hearing, and touch are greatly enlarged relative 

 to the olfactory area— with a proportional increase in 

 those areas concerned with muscle sense and muscular 

 activity, which are functionally related to these senses. 



In the late Eocene or early OHgocene the tarsioid stock 

 gave rise to the monkeys, in which the two eyes are 

 subject to conjugate movements and can be converged 

 upon a nearby object, producing true stereoscopic vision 

 with its wealth of detail concerning distance, size, con- 

 tour, quahty, and solidity. Vision, touch and hearing 

 now became the dominant senses, while the archaic 

 sense of smell was relegated to an inferior position. Some 

 of the monkeys of the New World had larger brains per 

 unit of body weight (one to seventeen) than man (one 

 to thirty-five), but they also had prehensile tails and 

 they used their brains only to stay up in the trees, having 

 no good reason to do otherwise. But the monkeys in the 

 Old World, including the surviving macaques, baboons, 

 and mandrills of southern Asia and Africa, never had a 

 prehensile tail— indeed, they had no tail at all to speak of 



