28 EVOLUTION OF THE HORSE 
small one, in order to keep up a proper amount of activity. On 
the other hand a large animal is better able than a small one to 
defend itself against its enemies and rivals. Consequently, as 
long as food is abundant, the larger animals have the advan- 
tage over their smaller brethren, and by the laws of natural se- 
lection the race tends to become continually larger until a limit 
is reached, when sufficient food becomes difficult to obtain, the 
RESTORATION OF THE FOUR-TOED HORSE 
Oldest known Ancestor of the Modern Horse; only 16 inches high 
Photo from original watercolor by C. R. Knight, based on mounted skeleton in 
American Museum 
animal being compelled to devote nearly all its time to getting 
enough to eat. 
CAUSE OF THE EVOLUTION. 
The evolution of the Horse, adapting it to live on the dry 
plains, probably went hand in hand with the evolution of the 
plains themselves. At the commencement of the Age of Mam- 
