472 THE CHANGING GENERATIONS 



quadrupeds. The digits, instead of having sharp claws, are protected by flat 

 nails in all but the most primitive forms. In many of the higher primates brachia- 

 tion — swinging through the air from branch to branch by use of the arms — has 

 largely superseded climbing and jumping; and this has been accompanied by a 

 strengthening of the arms and hands at the expense of the legs and feet. On the 

 ground most primates walk on all fours; but all except the lemurs rest in a squat- 

 ting position, and since their hands are thus freed from the supporting function, 

 they can be used to pick up food and other objects. The primitive long tail is 

 retained in the lemurs, tarsioids, and more primitive monkeys, and in some of the 

 latter it has been modified into a prehensile organ. In the higher anthropoids 

 the tail tends to shorten and become rudimentary, and no external signs of it 

 remain in the great apes and man. 



Arboreal life has profoundly affected the nervous system and sense organs. 

 Smell, so important to terrestrial mammals, is of little value to a tree dweller. 

 This sense shows progressive deterioration in primate evolution and is probably 

 more rudimentary in the apes and man than in any mammal of terrestrial descent. 

 On the other hand good eyesight and well-developed touch and muscle senses are 

 of the utmost importance for life in trees. In the higher primates two great im- 

 provements have been made in the visual apparatus. The first was the shifting 

 of the eyes from the sides to the front of the head in the tarsioids and anthropoids, 

 so that the visual fields overlap or coincide. This makes possible stereoscopic vision 

 and accurate distance perception. The second, found only in the anthropoids, 

 was the development of a centrally located "yellow spot" (macula lutea) in the 

 retina, which acts as a color filter to strain out those light rays that cause color 

 aberration. Together with the fovea, or central pit, this gives ability to see a 

 maximum of fine detail and greatly increases acuity of vision. 



The brain has been greatly enlarged and modified in relation to the require- 

 ments of arboreal life and the accompanying changes in the sense organs. Brachia- 

 tion requires good eyesight and accurate judgment of distance. It also demands 

 the most perfect muscular coordination of any type of locomotion except perhaps 

 flight. Correspondingly we find the motor and coordination centers of the brain 

 greatly developed in the higher primates. The volume and variety of sensory 

 impressions are much increased by keen vision and the ability to grasp and handle 

 objects. This is probably correlated with the disproportionate size increase of the 

 cerebral hemispheres, where the sensory areas of the brain are located. Through- 

 out primate evolution the size of the brain has continually increased. 



The changes in sense organs and brain have been accompanied by changes 

 in skull and teeth. The facial region has tended to shorten and become smaller 

 (Fig. 30.6), and the brain case has become larger and rounder. The shortening 

 jaw has lost a part of the original mammalian complement of 44 teeth; in the 

 higher primates, including man, only 32 remain. Primate teeth have stayed quite 

 primitive in form, the molars being low-crowned with rounded cusps, like those 

 of the early ungulates and of the omnivorous pig. The canine tooth is a projecting 

 tusk in most primates, but in man it scarcely rises above the level of the other 

 teeth. 



In primitive placentals the eye socket is not separated from the temple opening, 

 but in more advanced forms a bony bar bridges the space and separates the two 



