THE DESERT CLIMATE 35 



that have prevented the covering of the ground 

 with vegetation and the peophng of the vegetation 

 with a normal fauna. It is also to be constantly 

 remembered that many different factors go to deter- 

 mine that land shall be desert : here it is drought, 

 there it is wind ; in other places, as we shall see 

 in the next chapter, it may be shifting sand, or 

 saHnity of the soil, or absence of drainage or of 

 humus. 



