THE ANIMAL ANCESTRY OF MAN 85 



like. After the orang had begun to diverge, the common 

 stock contained the ancestors of the chimpanzee, gorilla, 

 man and possibly of other species now known chiefly from 

 fossil teeth and jaws of the Dryopithecus group. The common 

 stock was probably intermediate in size between the siamang 

 and the smaller species of chimpanzee. By Upper Miocene 

 times in India there was already a wide range in size, as 

 indicated by the fossil teeth of anthropoids, some being 

 but httle larger than those of siamangs, others nearly as big 

 as those of gorillas. 



The known African anthropoids, the chimpanzee, the 

 gorilla and the extinct Australopithecus (Dart), show the 

 most unmistakable marks of close kinship with each other. 

 Among recent forms the chimpanzee on the whole probably 

 retains the greater number of primitive characters. The 

 range of variabihty in existing chimpanzees is very great, 

 especially in regard to external features, details of skull 

 form, size of teeth, degree of wrinkling of enamel on the 

 molars, and many other characters. In some chimpanzees 

 the basic patterns of the premolars and molars rather 

 closely] approximate the primitive human type, but the 

 canine teeth exhibit the opposite tendency toward enlarge- 

 ment. In many races of anthropoids there seems to be a 

 tendency to gigantism, the body weight mounting to many 

 fold greater than that of the primitive anthropoid stock 

 represented by the tiny fossil lower jaw of Propliopithecus. 

 Very heavy bodies are not favorable for extreme agility 

 in the trees unless a cautious swinging movement is adopted, 

 as in the orang. Hence in order to maintain this agility it 

 was necessary for the chimpanzee to acquire a surprisingly 

 high degree of muscular strength. 



The typical chimpanzees are forest animals that appar- 

 ently spend most of their time in the trees and have therefore 

 had time to become specialized considerably beyond the 

 stage of the "common ancestor" of the higher apes and 

 man. For instance, at least in many chimpanzees the thumb 

 is reduced. When either the chimpanzee or the gorilla walks 

 on the ground it commonly assumes a position which is 

 superficially like that of a quadruped, but on closer inspec- 

 tion we see that these animals differ profoundly from true 



