THE ANIMAL ANCESTRY OF MAN 77 



Taken collectively, the lower Primates were represented in 

 Eocene times by a great number of genera and species, 

 founded mostly on fragmentary jaws but in some cases 

 known also from various other parts of the skeleton. In the 

 several instances in which the structure of the hind feet 

 is known the great toe is very large, provided with a flat 

 nail and set off at an open angle from the other digits, 

 which were long and slender. In all the recent primates 

 this kind of great toe is a sign of tree-climbing and an inten- 

 sive study of the skeleton of many different types of Primates 

 from Eocene to recent times can lead only to the con- 

 clusions that the ancestral stock of the entire order acquired 

 many of its peculiar characters in the trees (Gregory, 1920, 

 1927, 1928) and that this momentous series of events, of far 

 greater importance to mankind than any celebrated in 

 secular history, took place at a very early date in the history 

 of the placental mammals, perhaps even before the close 

 of the Cretaceous period. 



With this brief review of the earlier fossil records of the 

 rise of the Primates before us, let us return to the considera- 

 tion of the evolution of their locomotor apparatus. 



In such a specialized swift-running type of mammal 

 as the horse, the limbs have become modified into slender, 

 suddenly extensible compound levers, and in full 

 flight the body is catapulted forward by the sledge-hammer 

 strokes of the solid hoofs. In this case the middle metacarpal 

 bones of the forefeet and the middle metatarsals of the 

 hind feet become greatly elongated, while the remaining 

 metacarpals and metatarsals become more or less reduced 

 and the digits below these have even disappeared entirely. 

 In the line leading to man, on the other hand, the process 

 of digital reduction was avoided, because long before the 

 lateral digits could be reduced through running on the 

 ground, our ancestors took to the trees, where all five digits 

 of the hands and feet were needed for climbing. It is also 

 to this early ascent into the trees that the Primates, including 

 man, doubtless owe the retention of other relatively primi- 

 tive mammalian features in many parts of the skeleton. 

 For although arboreal life eventually takes its toll in the 

 way of specializations, leading finally to cul-de-sacs from 



