xxm. 3 CLASSIFICATION OF PRIMATES 607 



young produced is small, as in other animals with large brains that 

 learn well. There is often only a single pair of teats and in association 

 with the arboreal habit these are pectoral. The arrangements for 

 placentation involve elaborate changes in the uterine mucosa in 

 preparation for reception of the embryo, followed by breakdown at 

 regular intervals (menstruation). This special nature of the uterine 

 mucosa makes possible the efficient haemochorial form of placenta- 

 tion. Besides these arrangements for nutrition of the young the pri- 

 mates also extend parental care for a long time after birth. 



In many features of their life, therefore, the primates show to a high 

 degree the adjustability and power to obtain sustenance from varying 

 environments that is characteristic of all life. The receptors, brain, and 

 hand provide means for doing this in more elaborate ways than are 

 used by any other animals. The monkeys and apes have exploited 

 these powers to a considerable extent and are successful animals, 

 living, as we might say, by their wits, in a wide variety of circumstances. 

 However, non-human primates are unable to adjust to conditions 

 outside the tropical and subtropical regions. Man has made still better 

 use of his talents and by creating his own environment manages to 

 support a population of nearly 3,000 million large individuals, 

 scattered all over the globe. 



3. Divisions of the primates 



Fortunately many of the changes of habit characteristic of primates 

 involved changes in the skull and these can be followed in the fossils. 

 Our knowledge of the evolutionary development of primate life, 

 though far from complete, is less so than might be expected from the 

 rarity of preservation of arboreal skeletons. During the 50 million 

 years since the Eocene the various primate stocks have, of course, 

 divided and subdivided many times, and invaded many special habitats. 

 The forms at present known, living and as fossils, are placed by Simp- 

 son in 150 genera, two-thirds of them extinct, 70 of these being pro- 

 simians. Most of primate evolution has occurred in the Old World; 

 there are no fossil primates known from North America between the 

 Oligocene and modern times. Only ten fossil primates throughout the 

 whole Tertiary are even moderately well known, probably because 

 animals living in trees are seldom preserved as fossils. 



Bitter controversy still rages around the question of the best means 

 of classification of Primates. Earlier zoologists tended to postulate 

 a series of stages successively closer to man, the latest product of 

 evolution. There has been increasing awareness of the unwisdom of 



