ANIMALS— PHYSICAL ENVIRONMENT 105 



and C. affinis), and of the Rock Dove {Columba 

 livia). 



None of these sub-species are very highly modified 

 for hfe in the desert, and the species from which 

 they are derived are distinguished for their abiUty 

 to Hve in many types of cUmate and country ; 

 in fact, the two owls, the dove, and the Common 

 Swift all breed as far north as Northern Europe. 

 It seems that the presence of caves, with their 

 relatively unvarying temperature and humidity, 

 may have enabled these vigorous and adaptable 

 species to penetrate a short distance into the desert. 

 Certain of them now breed and roost in houses 

 and ruins, but this is clearly only a slight extension 

 of the cave-haunting habit. 



It is probable that a closer study of the cave fauna 

 in such countries as Egypt and South Palestine 

 would reveal most interesting forms of Hfe. The 

 contrast in cKmate between open desert and cave 

 is so great that the colonization of the cave by 

 animals which are specialized to hve in the sur- 

 rounding desert would present great difficulties. 



The eggs of birds which breed upon the surface 

 of the desert and are exposed to its summer climate 

 are in a position of great danger, because if they 

 are left uncovered by the parent birds they are 

 rapidly killed by the heat during the daytime. 

 It should be remembered that, at any rate in the 

 Great Palsearctic Desert, most of the birds breed 

 during the hot weather, e.g. Sandgrouse (page 85), 

 Common Pratincole, and others. It is only the 

 largest of eggs, those of the ostrich, which can 



