34 ANIMAL LIFE IN DESERTS 



once the great contrast between the summer and 

 the winter. The summer is rainless ; it is very- 

 hot by day but much cooler at night, and the rela- 

 tive humidity of the air varies very greatly in the 

 twenty-four hours ; the daily fluctuation in the 

 temperature of the air is exceeded by that of the 

 temperature of the surface of the soil ; the sky 

 is clear, the day long, the atmosphere dry, and the 

 sun vertical or nearly so at midday, and it foUows 

 that the effects of the sun's heat and chemical 

 rays are very great ; the winds are strong and 

 constant in direction, and occasionally violent and 

 destructive ; evaporation is extremely active and 

 far exceeds the amount of moisture received by 

 the soil. In every respect the climate in winter is 

 different from that in summer. It is information 

 of this sort which we shaU glean from mean 

 statistics, but they only teU us a part, and an 

 unimportant part, of the truth. It is not the 

 contrast between the seasons, immense though it 

 is, which is the essential character of the desert 

 climate. 



Rather it is the extraordinary deviations from 

 the mean which mark out the desert chmate as a 

 thing quite distinct. It is to the maximum and 

 minimum figures, not to the means, which we must 

 look if we are fully to reaHze the difficulties with 

 which desert plants and desert animals have to 

 contend. It is not the regular succession of summer 

 and winter, however dissimilar, but the occurrence 

 of long droughts, torrential rains, unusual frosts, 

 violent whirlwinds, and other irregular phenomena 



