6 ANIMAL LIFE IN DESERTS 



of less than 5 inches (127 mm.) a year always pro- 

 duces desert. On the other hand, there are areas 

 between the Pacific Ocean and the Californian 

 ranges of mountains, and in the Jordan Valley, 

 which are desert though they receive 18 inches 

 (460 mm.) of rain. In the first area the causes 

 are the very high winds to which it is exposed, 

 and the fact that most of the rain falls in winter, 

 when it is of little service to the plants. In the 

 second area, the lower end of the Jordan Valley 

 and the land round the Dead Sea, the cause is the 

 salinity of the soil. One concludes that it is un- 

 profitable to devote much attention to the mean 

 annual rainfall because the number of inches re- 

 ceived is of less importance than certain peculiarities 

 in its distribution through the seasons and through 

 successive years, and because such factors as soil 

 and exposure are also of prime importance. 



The rainfall in deserts is always very unevenly 

 distributed throughout the months and the seasons, 

 and unless one or more dry seasons of several 

 months' duration occur, the condition of the land 

 is not desert or even semi-desert. The graph 

 (Fig. 1) shows the seasonal distribution of the 

 rainfall which is characteristic of most places in 

 the Great Palsearctic Desert. It will be seen that 

 there is a dry or nearly dry season in summer, 

 averaging six months, and a relatively wet season 

 in winter. In the deserts of South Australia a 

 similar alternation of dry hot weather and wetter 

 cold weather occurs with regularity. 



The fact that the rainless period and the hot 



