190 Darwin, and after Darwin. 
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 t» 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 deveioped, to meet the altered 
con aitions. 
Fic. 84.—/al@otherium. (Lower Tertiary of Paris 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 
