THE ANIMAL CLIMAX-THE CHORDATES 



349 



variations which distinguish them as a 

 group. First of all, most of them possess a 

 brain and central nervous system that is 

 developed far and above that of all others. 

 It must be said, however, that some of the 

 lowest primates are probably not as intel- 

 ligent as some of the brightest carnivores. 

 But the group as a whole can be character- 

 ized as having very large brains, the cor- 

 rollary being, of course, that they possess 

 the most integrated and most coordinated 

 bodies of any animals living or extinct. This 

 implies excellent sense organs, particularly 

 those of sight, because in order for the 

 brain to perform its function it must receive 

 accurate impressions. 



The lower mammals have depended to a 

 large extent on their sense of smell to ori- 

 ent themselves, for their vision at best is not 

 very good. But the primates, having taken 

 to the trees, needed good vision to detect 

 their enemies on the ground and this was 

 more important to them than a keen sense 

 of smell. Arboreal life thus appears to have 

 been one of the reasons for the develop- 

 ment of the excellent visual organ of pri- 

 mates. Since the animal could see better, 

 it probably had a greater desire to examine 

 things more closely which, in turn, would 

 be correlated with further brain develop- 

 ment. One other very important aspect of 

 this course of events is that arboreal life 

 had a profound effect on the skeleton. 



As the appendages became well adapted 

 for life in the trees, both the anterior and 

 posterior digits became modified for grasp- 

 ing limbs. The great toes and thumbs op- 

 posed the other four digits and the claw 

 of the carnivores flattened out into nails. 

 The legs became relatively short, so that 

 the arms seemed to be unusually long. The 

 appendages became well developed for 

 supporting the body weight and for carry- 

 ing it with considerable speed from limb 

 to limb through the tree tops. When at rest, 

 the hands were free to handle objects and 

 to bring them close to the keen eyes for 

 closer observation. Freeing the front ap- 



pendages from the burden of supporting 

 the body weight and development of the 

 prehensile hand have probably both been 

 responsible to a large extent for the ad- 

 vance in the primate brain. This superior 

 brain is undoubtedly tlie one reason why 

 these animals dominate all others today. 



Other minor characteristics of the pri- 

 mates are the tooth pattern and skull struc- 

 ture. The early primates were apparently 

 omnivorous just as most primates are today, 

 and a generalized mammalian tooth pattern 

 is characteristic of all of them. They possess 

 two incisors, two premolars and four molars 

 in each half jaw, making 32 in all, consider- 

 ably less than that possessed by the primi- 

 tive mammals. The jaw is short, so that a 

 relatively short face results; this is obvious 

 when the faces of a monkey and a dog are 

 compared (Fig. 15-3). Furthermore, the 

 large nasal chambers essential to lower ani- 

 mals which depend upon smell are much 

 reduced in primates with a corresponding 

 reduction in this sense. Since the brain has 

 grown a great deal in size, it has risen over 

 the face, bringing the latter into a more 

 vertical position. Along with this change 

 has come a gradual shifting in the position 

 of the eyes from the lateral position occu- 

 pied in lower forms to a frontal position. 

 This has resulted in over-lapping images, 

 producing stereoscopic vision, essential to 

 tree-dwellers for judging distances accu- 

 rately, and to men for driving a car. The 

 great advantage of superimposed images is 

 easily demonstrated by viewing an object at 

 a distance first with both eyes and then 

 with only one. The sense of depth is lost 

 with monocular vision. 



Primates are rather shy, avoiding other 

 animals that might be dangerous and pre- 

 ferring tlie seclusion of dense foliaged for- 

 ests to open areas. The more primitive ones 

 remain in the safety of the tree tops most of 

 the time. The more advanced forms, such 

 as the chimpanzee and gorilla, have de- 

 scended to live on the ground, but even 

 these retreat to the trees on occasion. 



