RECENT LAND MAMMALS OF WESTERN NORTH AMERICA 



151 



They tolerate temperatures from those found In Death Valley in 

 summer (about 120° F) to those of the northern Great Plains in 

 winter (about — 40° F). I cannot see that temperature played a 



Fig. 4. General distribution of the genera Dipodomys (broken line 

 boundary) and Perognathus (soHd line boundary). All evidence indicates 

 that these mammals had their evolutionary history approximately within 

 their present range. There is no indication that they ever approached 

 the Bering land bridge. 



major role in keeping these mammals on the North American 

 continent. I suspect it was soil type with the accompanying vege- 

 tation that prevented these rodents from approaching the land 

 connection. 



Porcupine (Family Erethizontidae) . The porcupine did not 



