CHAPTER II 

 THE SOIL AND WATERCOURSES 



The physical environment of a creature includes 

 not only the climate of its habitat, but also certain 

 factors which may collectively be termed " geo- 

 logical." Among them are the type of soil on which 

 it lives, the flatness or hilliness of the country, and 

 its exposure to or protection from sun, wind, and 

 rain, and the existence of supplies of water other 

 than rain and dew. 



The influence of the geological environment upon 

 the desert plants and animals is almost as important 

 as the influence of cUmate, though less attention 

 has been given to it. The prevalent notion that 

 deserts are uniform stretches of sand dune is entirely 

 erroneous, for desert soils exhibit a wide range of 

 chemical and of physical characters. The soil of 

 a desert is commonly gravel, or sand, or bare rock, 

 or sunbaked mud ; other types of soil are found, 

 and almost the only soils which never occur are 

 those which result from the presence of plentiful 

 vegetation, for instance loam and peat. The scarcity 

 of humus is probably an important factor tending 

 to prohibit all but a few specialized plants from 

 colonizing deserts; in fact this, and the general 



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