114 DENIZENS OF THE DESERT 



tain borders at the northwestern end of the 

 sink, there came to exist there a large, flat, iso- 

 lated area separated from the other adjacent 

 regions by high mountains and the remaining 

 waters of the great Blake Sea. The animals and 

 plants which came to occupy this region, being 

 cut off from others of their kind by natural 

 barriers, in many cases finally developed char- 

 acters peculiar to themselves, and in some in- 

 stances these characters became sufficiently 

 marked to form new species. The Palm Springs 

 ground squirrel was one of these animals that 

 have shown in marked manner the effects of 

 this isolation. Although the ancient Blake Sea 

 has dried up and the recently formed Salton 

 Sea is the only physical feature restraining the 

 general distribution of the ground squirrel over 

 the sands of the entire Salton Sink, yet he clings 

 to his ancient home and maintains his identity 

 as a distinct species of the northwestern arm of 

 the Colorado Desert. 



