COMMUNITIES IN DRY, OPEN LANDS 451 



uals. 18 Although certain survival values are evident, there is still no 

 completely satisfactory explanation for such gregariousness. 



Animal communities of the steppes. — The amount of moisture 

 present and the duration of the dry season, together with the condition 

 of the soil, and the number of grazing animals, are decisive for the 

 development of vegetation in the open country. Subxeric and xeric 

 areas may accordingly be distinguished, the former by more abundant 

 moisture. 



The subxeric regions are composed principally of the tropical sa- 

 vannas. A belt of land thickly overgrown with high grass extends 

 through the equatorial lands of the Old and New World, where the 

 forest ends: the savannas of tropical Africa, the grass wildernesses of 

 the Indo-Malayan archipelago, the llanos of the Orinoco region, and 

 the Amazonian campos. There are numerous modifications of tropical 

 savanna, with transitions to more xeric formations, notably in the 

 African Sudan with its complete gradation from the northern border of 

 the Congo rain-forest to the Sahara. 



The xeric areas may be included in general under the heading of 

 steppes, in the sense that the geographer uses this very inclusive word. 

 They are extensive dry areas with sparse vegetation, in which a longer 

 or shorter period of rain annually ensues. Where this rainy period is 

 irregular and the rain may fail to appear at all, we have deserts, 

 which are closely related to steppes. Through the subtropic and tem- 

 perate zones there extends an extensive, more or less compact belt of 

 steppes around the earth in the northern and southern hemispheres. 

 The northern steppe belt includes the high steppes of central Asia and 

 the steppes of southwest Asia, on the one hand continuing into southern 

 Russia and running into Hungary, and on the other hand connecting 

 with the steppes of North Africa through Asia Minor and Syria; in 

 North America this belt includes the enormous area to which the 

 prairies, the great plains, and the Great Basin belong. In the southern 

 hemisphere, South Africa from the Cape to the Congo savanna, from 

 the east coast to the west, is steppe, just as are the western two-thirds 

 of Australia, and in South America the pampas of Argentina and the 

 Patagonian plains. Interspersed in this steppe zone lie deserts as ex- 

 treme formations with a minimum of precipitation and of plant growth. 

 The south Mediterranean steppes gradually pass southward through 

 the semi-desert into the Sahara, which is continued eastward into the 

 Libyan and Arabian deserts; to these are connected the Persian desert 

 areas, and in central Asia the Gobi follows; in North America, desert 

 formations occur in California and in the Great Basin. South of the 

 equator, the desert type is represented by the Kalahari in South Africa 



