6o 4 THE PRIMATES xxm. 2 



Their characters are those of animals raised up from the ground ; the 

 opportunities offered in the trees for the use of hand and brain have 

 no doubt been important influences in the shaping of man. 



The general plan of primate life has thus been to retain the original 

 eutherian conditions, with emphasis on those features important for 

 tree life. In such an existence continual quick reaction to circum- 

 stances is likely to be necessary, the environment is varied, and the 

 mechanical supports it offers are often precarious. Under these condi- 

 tions safety is achieved by quick reactions rather than by stability; 

 thus primate more than any other life tends to be a matter of con- 

 tinual exploration and change. The information that ensures the life 

 of the species is obtained by the individuals and stored in their brains, 

 rather than by selection among large numbers of rapidly breeding 

 individuals. The time taken for development thus increases in the 

 primate series. Growth continues for about 3 years in prosimians, 7 in 

 monkeys, 9 in gibbons, 12 in other apes, and 20 in man. To obtain 

 this information receptors are obviously of first importance, but in the 

 tree-tops one cannot hunt by smell; the eyes and ears therefore became 

 developed, at the expense of the nose. Primates are microsmatic, with 

 reduction of the number and length of the turbinal bones and hence 

 of the long snout that houses them. Consequently the eyes come to 

 face forwards, so that their fields overlap, binocular vision becomes 

 possible, and central areas appear in the retinas. Monkeys are certainly 

 more dependent on vision than are most animals and for this reason 

 they approach the birds in the adoption of colour patterns for sexual 

 recognition and excitation. 



The changes in the receptors were accompanied by conspicuous 

 changes in the brain, which becomes very large in later primates, with 

 cerebral hemispheres reaching far backwards. The olfactory bulbs and 

 rhinopallium become small and the neopallium very large, differen- 

 tiated into areas and provided with a large corpus callosum. The 

 occipital pole, concerned with vision, and the frontal areas, become 

 especially well developed in the apes and man. Stereoscopic eyes with 

 numerous cones would be of no value without a central analyser to 

 allow the animal to discriminate shapes, retain the impression of past 

 situations, and otherwise make use of the available information. The 

 marked differences in the rate of growth of the brain of different 

 primates are shown in Fig. 382, from Schultz's careful measurements. 

 At early stages of development all the primates studied have the same 

 (high) relative brain weight, but in the adults the brain is relatively 

 and absolutelv larger in man than in monkeys or apes. 



