THE TERTIARY PERIOD 



443 



the horse, deer, and rhinoceros families, with their specialized 

 hoofs and grinding teeth, back to a peculiar five-toed animal 

 which had a full set of rather simple teeth, and was no larger 

 than a dog (Fig. 460). This Eocene form seems to be an 

 ancestral or generalized type from which the later hoofed ani- 

 mals diverged and ascended. Furthermore, it resembles in 

 many respects the equally generalized ancestors of the dogs, 

 bears, and cats, although cats and horses, for example, to-day 

 seem to have little in common. 



FIG. 460. A generalized hoofed mammal (Phenacodus) which lived in 

 North America near the beginning of -the Tertiary period. (Painted by 

 C. R. Knight, under the direction of Professor H. F. Osborn. Copyright 

 by the Amer. Mus. of Nat. Hist.) 



Rapid evolution of the mammals. The progress of 

 these generalized mammals of the early Eocene was aston- 

 ishingly rapid. In each later series of deposits the bones of 

 new and more modern varieties are found. Thus, before the 

 middle of the Tertiary period (Miocene), the main divisions 

 of the mammals became entirely distinct and we may easily 

 recognize cats, horses, monkeys, whales, bats, elephants, and 



