318 



Comparative Evolution of Biomes 



Fig. 141. Distribution of the widespread mouse Peromysciis maniculatiis and 

 its Arctic flea MonopsijUus thamhus. (From Holland.) 



into South America without evidence of grassland extension from 

 North to South America. It is possible that horses were successful 

 in subclimax communities having abiuidant grasses, and that the 

 horses dispersed into South America in company with the many 

 subclimax grass genera now common to both Americas. 



