32 



ANIMAL LIFE IN DESERTS 



between rainfall received and possible evaporation 

 is immensely and irregularly increased in the summer 

 and autumn months, the very period when the other 



Table showing the ratio between evaporation and rainfall month 

 by month in 1908, at two stations in the Algerian coastal zone 

 and two in the Algerian Sahara. Evaporation has been 

 calculated as from a free water surface. Readings from a 

 Piche evaporimeter have been corrected. 



forces of nature aU combine to make life impossible 

 for any plant or animal except those specialized for 

 life in a desert. The ratio is lower and less fluctuat- 

 ing in the coastal non-desert stations (Algiers and 

 Oran). Cannon states that if the annual evaporation- 

 rainfall ratio of the littoral zone be taken as unity, 

 that of the high plateau is 3-7 and that of the desert 

 (average of Laghwat, Ghardaia, and El Wad) is 18-6. 

 It will also be appreciated that in an unusually 

 dry year the evaporation-rainfaU ratio will be vastly 

 increased, so that, at a time when the mere drought 

 is pressing hardly on the flora and fauna, an increased 

 discrepancy between rainfaU and evaporation is also 

 thrown into the scale against them. If, for instance, 

 the normal evaporation-rainfall ratio for Mohave, 

 with a mean rainfall of 4-97 inches (125 mm.) is 

 19-1, then in the dry year with a rainfall of only 

 2-20 inches (55 mm.) the ratio would be 43-2, if we 



