ANIMALS— PHYSICAL ENVIRONMENT 115 



fauna of sandy desert. To that, therefore, I propose 

 to confine our attention entirely. 



It has been shown (page 40) that sand probably 

 presents more difficulties to the desert flora and 

 fauna which attempt to colonize it than does any 

 other type of desert. It is not, therefore, to be 

 wondered at that the fauna of sandy areas, and 

 particularly of areas in which the sand is loose, is a 

 small one, and that it consists largely of animals 

 which in overcoming the difficulties of life in sand 

 have become unfitted for life in any other environ- 

 ment. 



Grinnell has described the conditions of life 

 of certain species of sand-living rodents in the 

 North American deserts. The Round-tailed Ground 

 Squirrel (Citellus t, tereticaudus) requires large areas 

 of sand, and also bushes, under which it can burrow : 

 it is frequently abroad in the daytime, but it avoids 

 direct sunshine by passing rapidly from the shade 

 of one bush to that of another. It cannot endure 

 the fierce heat of midday on bare sand, and if an 

 individual is caught in a trap uninjured but exposed 

 to the sun's rays it dies quickly. This rodent is 

 therefore an animal which requires the co-existence 

 of a geological and floristic factor, in order to enable 

 it to live in a particular place. The Big Desert 

 Kangaroo Rat {Dipodomys deserti, Fig. 38) lives in 

 the same sandy places as the Round-tailed Ground 

 Squirrel, but its requirements are quite different. 

 It is nocturnal, and is sheltered in its burrow from 

 the conditions which prevail during the day; it 

 is therefore indifferent to the presence of bushes. 



