190 Darwin, and after Darwtn. 



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 peris- 

 sodactyl 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, and the horse, antelope, 

 and camel were gradually developed, to meet the altered 

 conditions. 



FIG. 84. Palaotherium. (Lower Tertiary of Pahs Basin.) 



The best instance of such progressive modifications 

 in the case of perissodactyl feet is furnished by the 

 fossil pedigree of the existing horse, because here, 

 within the limits of the same continuous family line, 

 we have presented the entire series of modifications. 



There are now known over thirty species of horse- 

 like creatures, beginning from the size of a fox, then 

 progressively increasing in bulk, and all standing in 



