INTRODUCTIOX 373 



isolating closely related stocks of land vertebrates from each other 

 has seldom been recognized by zoogeographers and certainly has 

 not been carefully assessed. 



In illustration of the third point I again draw on the Mammalia. 

 Shifts of geographic range now in progress are evident in the Nine- 

 banded Armadillo, in the Hispid Cotton Rat, and seemingly in the 

 Southern Bog Lemming. The northern margin of the geographic 

 range of the Bog Lemming seems to have shifted northward in the 

 past 30 years in the eastern part of the Great Plains. In the same 

 region in the past 40 years the Armadillo has extended the northern 

 margin of its range 75 miles and in the same region and period of time 

 the Cotton Rat has done the same thing by moving approximately 

 200 miles. 



