THE REJ^iN OF MAMMALS. 



217 



veritable genetic series, to which they appeal as a "demon- 

 stration " of the process of descent with specific modifications. 

 In the Lower Eocene are found the remains of a diminutive 



Fig. 174.— Fore.fojt of Dinoceras. Greatly reduced.— After Marsh. 



ungulate {Eohippus), of the stature of a moderately-sized dog. 

 It has four toes and a rudiment of a fifth in front, and three 



Fig. 175.— Skull of Brontotherium ingens (Marsh). Greatly reduced. A Miocene 



Perissodactyl. 



toes behind ; and has teeth slightly resembling those of the 

 horse, but more simple and shorter in the crown. In this 

 creature it has been supposed that we have a direct ancestor 



