ANIMALS THAT LIVE WITHOUT WATER I49 



are the desert areas that are actually uninhabited by 

 both), in which normal agriculture is impossible, and 

 in which nearly all indigenous animals and plants are 

 specialized to endure continuous aridity. It is estimated 

 that by this definition as much as 20 per cent of the 

 earth's surface may be called desert. With slight excep- 

 tions along the coasts and river valleys, a desert climate 

 prevails over the whole of northern Africa and east- 

 ward to northwestern India and the heart of China, 

 reaching maximal aridity in Egypt, Saudi Arabia, Syria, 

 Mesopotamia, Persia, Afghanistan, Baluchistan, Turki- 

 stan, and the Gobi Desert (collectively called the Great 

 Palearctic deserts). The desert of Australia is second in 

 point of size, and deserts occupy large areas in the 

 western part of North America and in Mexico, eastern 

 Patagonia, western Argentina, and southwestern Africa. 



The desert is characterized by extreme aridity and 

 sometimes by high mean temperatures, and for a good 

 part of the year the prevailing winds may be dry and 

 desiccating. A rainfall of less than 5 inches per year is 

 typical, but a greater rainfall may be oflFset by persistent 

 hot, dry winds. The rainfall is invariably distributed un- 

 evenly throughout the year, the rainy season varying 

 with the latitude, prevailing winds, and the disposition 

 of the neighboring mountains. In some areas rain falls 

 on only a few days in the year and is so variable that a 

 single day's downpour may exceed the total rainfall of 

 an unusually dry year. Because the ground lacks cover, 

 a great proportion of this water runs off immediately and 

 is of little use to living organisms. But all soil— whether 

 gravel, sand, sun-baked mud, loam, or peat— has a re- 

 markable capacity to hold water and to raise it from 

 lower levels by capillary action, and consequently the 

 subsurface water is generally a more important factor 

 in sustaining plants than is the rare rainfall. 



In the absence of clouds and moisture, the daytime 

 temperatures in the summer may rise well above 125° F. 

 in the shade, while at night the temperature falls rapidly 



