444 LAND ANIMALS 



become very hot during the summer, and such regions absorb moist 

 air currents without the cooling necessary for precipitation. Such 

 plains, at either low or high altitude, usually do not reach the moisture 

 content required for forest growth. 



Although these water relations hold true in general they cannot be 

 applied unchanged as an adequate explanation of the existence of all 

 grasslands. Despite much study, it is impossible as yet to account for 

 the extension of the tall grass prairie as a great peninsula through 

 Illinois and northwestern Indiana. This region has many climatic 

 characteristics similar to those in the woodlands into which it pene- 

 trates. The ratio of rainfall to evaporation is a factor in prairie forma- 

 tion ; so also is the tendency for precipitation to be greater in the grow- 

 ing season and to be more irregular than in the forests to the north 

 and east. It is no more irregular, however, than in the bordering forests 

 on the south and east. At one time, apparently, the water table stood 

 higher than in the upland forests near by. The type of soil and the 

 prevalence of prairie fires are factors in the maintenance of the prairie, 

 but there is evidence that they are not primary causes of its formation. 1 



The concentration of the small amount of rain within limited and 

 often short periods affords temporarily favorable conditions for plant 

 and animal life. But there is great danger even for well-adapted 

 animals in the irregular recurrence of rains and in unusually prolonged 

 droughts. During the dry winter of 1863, whole herds of wild animals 

 were destroyed in the South African steppes ; 2 during the great droughts 

 in the Pampas along the La Plata, both domestic animals and the 

 native species die, thousands of pampas deer {Odocoileus bezoarticus) 

 among others; in certain parts of India, ten years or more are required 

 after a drought to restore the full number of native species. 3 Parts of 

 northeastern Brazil are conspicuously subject to disastrous drought of 

 this kind. 4 



In general, only animals that can tolerate dry air are fully equal 

 to the conditions of the open country. Animals that require moist air, 

 such as snails, isopods, and amphibians, are not entirely absent; but 

 they do not attain a great development in number of species and in- 

 dividuals and their occurrence may depend on special adaptations or 

 on local conditions. On the other hand, a large number of reptiles and 

 insects thrive in the open country, especially insects with incomplete 

 metamorphosis such as termites and grasshoppers, which are easily 

 able to survive on account of their independence of rain and their 



