CHAPTER TWELVE 



The History of the Primates 



We come now to the climax of this phylogenetic history— the order Pri- 

 mates, including tree shrews, lemurs, tarsiers, monkeys, apes, and man. 

 But before discussing the history of this group, it may be well to review 

 the classification and the major characteristics of the living members of 

 this order. 



CLASSIFICATION OF THE PRIMATES 



The order Primates is singularly difficult to define because of the absence 

 of salient distinguishing characters, comparable to the chisel-like incisors 

 of rodents, or the hooves with an odd number of toes of the Perissodactyla. 

 Mivart long ago defined the Primates as placental mammals with nails 

 (or claws in some); with clavicles; with orbits encircled by bone; with 

 three kinds of teeth; possessing a brain with a posterior lobe having a 

 fold called the calcarine fissure; a thumb and great toe having a flat nail 

 or none; a large intestine with a blind pouch, the cecum; with penis pen- 

 dulous, and the testes descended into a scrotum; and with two pectoral 

 mammary glands. 



These are largely primitive mammalian characters, and it may be said 

 that, except for a tendency toward expansion of the brain, the Primates 

 are relatively unspecialized mammals. The teeth are adapted to a gen- 

 eralized diet, but this is itself lack of specialization. Thumb and great toe 

 are usually opposable to the other digits, which gives efficiency in grasp- 

 ing objects. While the eyes of most mammals are on the sides of the head, 

 so that each eye sees a different field, those of Primates are placed toward 

 the front, thus permitting binocular, stereoscopic vision. Vision is gener- 

 ally more highly developed than in other mammals, the sense of smell 

 less so. Finally, although enlargement of the brain is a general mammalian 

 characteristic, this is most marked in the Primates. 



Tree Shrews. The tree shrews are certainly the most primitive of Pri- 

 mates; indeed, many zoologists prefer to class them with the Insectivora, 

 from which they are derived. However, they vary from ground shrews in 

 several ways which tend to associate them with Primates. The digits are 

 more mobile and the thumb and great toe are somewhat opposable. These 

 digits, however, are capped by typical claws. The eyes are larger than 



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