CONCLUSION. 189 



The liiub of the modern race-horse is a nearly perfect piece of 

 machinery, especially adapted to great speed on dry, level, gronnd. The 

 limb of an antelope, or deer, is likewise well fitted for rapid motion on 

 a plain, but the foot itself is adapted to rough mountain work, as well, 

 and it is to this advantage, in part, that the Artiodactyls owe their present 

 supremacy. 



The plantigrade, pentadactyl, foot of the primitive Ungulate, and 

 even the Perissodactyl foot that succeeded it, both belong to the past 

 humid period of the world's history. As the surface of the earth slowly 

 dried up, in the gradual desiccation still in progress, .new types of feet 

 became a necessity, an'd the horse, antelope, and camel, were gradually 

 developed, to meet the altered conditions. 



The Proboscidians and Perissodactyls now living, except the horse, 

 are doomed to early extinction, but the Artiodactyls, with their greater 

 ])0wer of adaptation, will replace them, and perhifps develop new forms. 



The genealogy of the special Ungulate lines which ended in the 

 horse, tapir, and rhinoceros, of the Perissodactyls, and of the pig, camel, 

 and deer, among the Artiodactyls, has already been marked out by the 

 author elsewhere, ^ and need not here be repeated, especially as the 

 subject will be fully discussed in a future volume. 



Extinction of Large Mammals. 



During the Mesozoic period, all the mammals appear to have been 

 small, and it is not probable that any of large size existed, as reptilian 

 life then reig-ned supreme. With the dawn of the Tertiary, a new era 

 began, and mammalian life first found the conditions for its full and 

 rapid development. 



In the lower Eocene, the largest land mammal was Coryphodon, more 

 than the equal, in size and power, of any of the I'eptiles of that time. 

 Dinoceras and its allies, in the middle Eocene, were much larger, and were 



' Introduction artd Succession of Vertebrate Life in America, 1877. See also 

 New Equine Mammals, etc., 1874, and Polydactyle Horses Recent and Extinct, 1879. 



