THE SOIL AND WATERCOURSES 51 



Mesopotamia and the Nile Valley, and on the great 

 plain through which pass the Amu Dana (Oxus) 

 and Syr Daria (Jaxartes), plant and animal life 

 depend less on local rainfall than on the annual 

 flooding of the ground at the time when the river 

 is at its highest. The height to which the river 

 rises varies greatly from year to year, and depends 

 on precipitation and the melting of snow in far- 

 distant regions. The Nile is under partial control 

 and the height of its annual rise is more constant 

 than it would otherwise be : the Tigris is not under 

 control, and the Euphrates only to a very sHght 

 extent. It follows that in a year of exceptionally 

 high water great areas of Mesopotamia which would 

 otherwise be unflooded are inundated in March and 

 April and remain covered with water for several 

 months. This not only keeps the soil of the plain 

 moister than it would otherwise be in early summer, 

 but materially modifies the summer heats, and 

 appears to double or treble the number of insects 

 which are on the wing throughout the whole sum- 

 mer. On the other hand, this annual flooding of 

 a flat land is very unfavourable to the burrowing 

 animals and appears to limit the species of lizards 

 and small mammals very materially. 



It is noteworthy that the majority of closed 

 drainage basins, from which there is no outlet to 

 the sea, are desert. This is probably due in the 

 main to the fact that in them salts accumulate, 

 until the increasing salinity of the area causes it 

 to become desert. Examples are the great basins 

 of Central China, Eastern Turkestan, and the Sahara, 



