314 



PRINCIPLES OF ANIMAL BIOLOGY 



ance of mixture was occasioned by the fact that a series of somewhat 

 parallel streams with trees along their courses were separated by strips 

 of grassland. The forest animals were along the streams, the prairie 



35" 



35 to 40 Inches. 

 More than 40 



Fk;. 2G9. — Annual rtiinfall in the lower part of the state of Mieliigan. This illustrates 

 the differences in physical conditions which may prevail even in relatively small areas. 

 (After C. F. Schneider, Publication 9, Michigan Geological (iml Biological Survey.) 



animals between them; but on a map of moderate scale they appeared 

 to be together. 



An actual example of species definitely related to I'orests is the genus 

 of North American flying sciuirrels. As shown in Fig. 2(58, Glaucomys 



