26o Plant Life and Evolution 



and presumably the region immediately to the 

 east of the mountains then, as now, was one of 

 slight rainfall. In a recent, very interesting study 

 of the prairie flora, Harvey has given a very 

 plausible explanation of the divergencies of the 

 western and eastern floras within the United States. 

 He holds that even during the Tertiary the great 

 plains region was too dry for the growth of forests. 

 With the retreat of the northern forests before the 

 advancing glaciers, these prairie regions, unfitted for 

 forest growth, acted as a wedge, one company of 

 migrants working to the westward, and character- 

 ized by the predominance of coniferous trees; the 

 other flowing eastward, and typically deciduous, fol- 

 lowed the Mississippi and its tributaries and became 

 settled in the Appalachian region of North Carolina 

 and Tennessee, where to-day it forms the finest de- 

 ciduous forests in our country. 



ALPINE PLANTS 



Very interesting is the survival of many northern 

 plants on high mountains, often very remote from 

 each other. It is supposed that these northern 

 plants, driven southward by the increasing cold, re- 

 treated up to the cooler regions of the higher alti- 

 tudes as the climate became warmer, after the re- 

 treat of the glaciers. Even in the tropics, close to 

 the equator, one meets on the tops of high moun- 

 tains a real northern flora, including such familiar 



