ZOOGEOGRAPHY 



297 



short of the regional boundaries in certain directions. Many species may 

 even have an extensive range in two or more areas. Thus Rayia pipiens, 

 the common leopard frog, occurs in general over all of North America 

 east of the Sierra Nevada Range; and the raccoon, Procyon lotor, frequents 

 the deciduous forests of eastern United States and the prairie-plains region 

 as well. If all animals showed as little relation to natural vegetation 

 areas as do these two, the major environments mentioned above would 

 probably never have been recognized'. Without pursuing this uncon- 

 formity any further it may be said that there are two classes of animals 

 which frequently ignore the major environments; first, terrestrial forms 



i-^iades^ 



miS^- 



Fig. 208. — Range of Thamnophis sauriius sackeni (Kenn.), the southeastern ribbon snake. 



{After Rulhven.) 



which are tolerant of a wide range of conditions, and, second, more or less 

 aquatic forms. The leopard frog probably owes its general distribution 

 mostly to its semi-aquatic habits, and the raccoon to its tolerance. 



In view of the facts (1) that the apparent major environments are 

 occupied to a different extent by different animals, (2) that many species 

 which are chiefly confined to them transcend their limits, and (3) that 

 many species occur over a large part of two or more regions, it may be 

 concluded that these areas, when regarded as animal regions, are more 

 apparent than real. They are the ranges of conspicuous plants with 

 which are associated in certain parts some animals and plants which find 

 there congenial conditions. These areas are not units in their biotas 



