14 ANIMAL LIFE IN DESERTS 



from that average, give little information to the 

 biologist. 



It is probable that dew is of extreme importance 

 to the desert flora and fauna, but as practically no 

 reliable statistics relating to actual dew-fall have 

 been published, I am compelled to leave the matter 

 in a very unsatisfactory state. It would be of great 

 interest to biologists if i^eteorological observatories 

 situated in deserts would accumulate information as 

 to the amount of dew which actually falls under 

 natural conditions, and the seasons and regularity 

 of its fall. 



To sum up, the ordinary meteorological statistics 

 are enough to assure us that the aridity of deserts 

 is great ; they do not, however, tell us the whole 

 truth, for three reasons. The first is that much of 

 the recorded rainfall is not to be reckoned as " avail- 

 able moisture," because a large proportion falls in 

 heavy downpours which run off the surface of the 

 ground into the water channels, and in some places 

 an equally large proportion falls in such small 

 amounts that it fails to penetrate the soil before it is 

 evaporated. The second reason is that rain which 

 faUs in the cold season, when vegetation is inactive, 

 is of little value to the plants ; the third, that from ^ 

 year to year the climate deviates widely from the 

 normal. A certain flora may adapt itself to grow 

 in a given spot, but may be exterminated by several 

 unusually severe summers, after it has been estab- 

 lished for several years. Shortage of water, and 

 particularly shortage at seasons when the plant's 

 requirements are large, is a most important factor 



