298 



PRINCIPLES OF ANIMAL BIOLOGY 



(animals and plants), since they mark the limits only for some of the in- 

 cluded species, not for others. A line so drawn as to separate evergreen 

 from deciduous trees, or two species of mammals from one another, may 

 not separate any other two groups of organisms. However, it is often 

 convenient to recognize these major environments, because a genus may 

 be divided into its component species by the regional boundaries, or the 

 range of a species may stop on or near their limits. The range of the 



Fig. 209. — Range of Thamnophis sauritus (Linnseus), the eastern ribbon snake. (After 



Ruthven.) 



North American flying squirrels (Fig. 206) illustrates this correspond- 

 ence with the vegetation areas. Thus the range of Glaucomys volans is in 

 general the deciduous forest region and is limited to the west by the 

 great plains, and to the north by the coniferous forests, while the range 

 of Glaucomys sabrinus closely approximates that of the coniferous forests 

 of the north and the mountainous regions of the west. 



