GEOGRAPHIC DISTRIBUTION 



315 



volans is in general limited to the deciduous forests, while the range of 

 G. sabrinus approximates that of the coniferous forests. Limitations of 

 these species to forests is mostly caused by their feeding on nuts and 

 seeds, to a lesser extent by their habit of ''flight." Two animals that 

 do not pay much attention to vegetation areas are Rana pipiens, the 

 common leopard frog, which occurs all over North America east of the 

 Sierra Nevada range; and the raccoon, Procyon lotor, which lives in 

 deciduous forest areas and the prairie-plains region as well. The leopard 



Fig. 270. — Proximity of ranges of three varieties of one species of garter snake. West 

 of the Mississippi and in Mexico, Thamnophis sauritus proximus; in Florida, Thamnophis 

 sauritus sackeni; north of Florida, Thamnophis sauritus sauritus. {Modified from Ruthven.) 



frog ignores forests because of its semiaquatic habits, the raccoon because 

 of its tolerance of various conditions. 



Besides vegetation, important physical conditions bearing on the dis- 

 tribution of animals are temperature and rainfall. Even in a limited 

 area the amount of rainfall differs greatly, as shown in the map of 

 Michigan in Fig. 269. * 



These several factors are sufficient to illustrate that geographic dis- 

 tribution is not wholly a historical development. Ecology and the time 

 and place of origin of species have worked together. 



