464 LAND ANIMALS 



— are attracted in turn by these large herbivores ; they follow the herds 

 and flocks, congregate like them at the water places during the dry 

 season, migrate with them, and disperse again with them over the 

 grassy plains during the moist season. 



The abundance of life in open tracts of land is variable. Many of 

 them represent areas where the maximum amount of life is crowded 

 together. This is naturally dependent upon the amount and condition 

 of the grazing afforded. The stiff, hard grass of the Cameroon savanna, 

 which is as high as a man and higher, is less favorable than the low, 

 fine grass of the "Siissveld" of South Africa. Opportunity to evade the 

 effects of drought by migration is also important. In tropical South 

 Africa, where the total rainfall is small in amount, the rain is dis- 

 tributed over the entire year in such a way that there is a zone of 

 winter rain in the west, a zone of summer rain in the east, and a tran- 

 sitional zone in the middle. 68 



The amount of mammalian life that filled many stretches of the 

 South African plains in earlier times was almost beyond belief; and 

 even yet there are regions that abound in animal life. Livingstone and 

 other travelers of the beginning and middle of the nineteenth century 

 tell of wonderful sights, and in the present century Berger 69 estimated 

 the number of zebras seen daily by him in British East Africa as 1000, 

 the number of hartebeeste as 3000 to 5000, with other large mammals 

 similarly abundant. Many parts of north Tibet are likewise rich in 

 animal life, but do not by any means equal the African steppes. 

 Prschewalski 70 describes a rich fauna in the pasture land along the 

 Schugo River; "Only by moving about from one place to another could 

 they find the necessary food on the wretched meadows." More recently 

 Andrews 71 writes of seeing thousands upon thousands of Mongolian 

 antelopes that poured in a yellow flood over a mountain rim and spread 

 out into the fertile plain. Sometimes a thousand or so would dash away 

 from the main herd, only to stop abruptly and feed. At one time on the 

 Great Plains of North America the bison were so numerous that the 

 herds extended as far as the eye could see and colored the prairie 

 black. In a similar manner, but in smaller herds, the guanacos charac- 

 terized the Patagonian plains over which they were scattered by the 

 thousands. 72 In Manitoba the insects may number 9,500,000 per acre 

 in the spring when the larger animals are hibernating and be reduced 

 to only a million in late June. 19 Many of these animals, feeding pri- 

 marily on the grassland plants, have played an important role in 

 preventing the invasion of trees into parts of these plains whose soil 

 and rainfall would otherwise have supported a forest growth. 



