FIG, 20-2 Biotic provinces of North America 

 (Blair 1950, Dice 1943, Miller 1951, Smith 1949). 



the tundra (arctic realm) and coniferous forests 

 (cold temperate sub-region), but greater and greater 

 intercontinental divergences become apparent as ex- 

 amination progresses southward (Udvardy 1958). 

 Allen considered humidity to become more important 

 than temperature southward in North America, be- 

 cause it determined the prevalent type of vegetation 

 with which animal distribution was correlated. So he 



separated the warm, temperate sub-region into an 

 eastern humid province that was heavily forested and 

 a western arid province containing grassland and 

 desert. These provinces were characterized by the 

 presence or absence of prominent generic types of 

 animals. Allen's minutest subdivision was the faunal 

 area, characterized by a combination of species not 

 found elsewhere. 



Distributional units 273 



