444 



HISTORICAL GEOLOGY 



many other kinds. True, they were not the same species 

 which exist to-day ; some of the horses, for example, had three 

 toes instead of one, as they now have ; but the types were un- 

 mistakable. Before the close of the Tertiary, the older and 

 more primitive mammals had been exterminated from the 

 northern continents, and the whole animal kingdom had 

 taken on very largely its present aspect. 



FIG. 461. Ancestral Eocene horses (Eohippus) with three and four toes 

 on the feet. (Painted by C. R. Knight, under the direction of Professor 

 H. F. Osborn. Copyright by Amer. Mus. of Nat. Hist.) 



The mammals adopt many modes of life. As mentioned 

 in Chapters on the Mesozoic era, the reptiles when in their 

 prime had occupied the forests, the plains, the marshes, the seas, 

 and all other situations in which animals could well exist. In 

 the Tertiary period we find the mammals stepping into the 

 places relinquished by the reptiles, perhaps after having actu- 

 ally displaced them by sheer victory in competition. Thus 



