THE SOIL AND WATERCOURSES 37 



badness of the drainage, are common characteristics 

 of desert soil. 



It is to be remembered that various types of desert 

 pass gradually into one another. Clay desert passes 

 by insensible steps through clay desert containing 

 a few stones to stony desert ; at the other end of 

 the scale stony desert may contain so many large 

 stones and so little fine soil that it is not far removed 

 from rock desert. Further, an area which is uniform 

 so far as soil is concerned may be bare desert in 

 one ' place, semi-desert or steppe in another, by 

 reason of the presence of windbreaks, or of different 

 exposure, or of other factors. One cannot draw 

 sharp lines of delimitation between the different 

 types of desert, nor between areas which are desert 

 and others which are semi-desert. The Hmits of a 

 desert are indefinite not only in point of space ; they 

 vary also with time. This is so characteristic a 

 feature, and so striking when one has actually seen 

 it, that I shall return to it again and merely state 

 in passing that bare earth tenanted by a few special- 

 ized plants and animals and exposed to the full rigour 

 of a desert summer may enjoy a delightful spring 

 during which it is carpeted with lush, unspecialized 

 flowering plants and teeming with bees and butter- 

 flies. For two or three months one may truly say 

 that the desert has vanished (Figs. 24, 25, facing 

 p. 58). 



It is impossible to understand the relationships 

 between the soil of a desert and its flora and fauna 

 without understanding the powers which are 

 possessed by all soils, in varying degrees, of holding 



