242 FROM NORTH POLE TO EQUATOR. 



and over mountains. We learn from eye-witnesses that a stream a 

 mile wide is to them no barrier, scarcely even a hindrance, for they 

 throw themselves into it like an irresistible avalanche, so that the 

 water is covered with the dark, moving throng; that the animals 

 associate and separate again, the herds increase and diminish; that 

 old, surly, tyrannous, malevolent bulls avoid the other bisons, 

 having perhaps been expelled from the herd, and compelled, probably 

 only after protracted struggles, to live in hermit-fashion until the 

 following summer; and that, during heavy snowfalls, the herds take 

 shelter in the forests or on the slopes of the mountains. From July 

 onwards they begin their migrations from the north towards the 

 south. Small companies, which, till then, have been leading a com- 

 fortable summer life, combine with others and set out on the journey 

 with them; other troops join the band, which grows as it presses on, 

 until there is, at length, formed one of those extraordinary herds 

 which, united till the next spring, moves and acts as if animated by 

 one soul. When the winter is safely past, the army gradually breaks 

 up, probably in exactly reverse order, into herds, and these divide 

 more and more until at length only small companies are left. This 

 breaking-up takes place during the course of the return journey. 

 Both in going and returning, one herd follows another at some dis- 

 tance, but more or less along the same paths. Specially favourable 

 places, such as low grounds covered with rich grass, cause a tem- 

 porary damming up of the living stream. In such places incalculable 

 herds assemble together, spend days in the same spot, and break 

 up only again when all the grass has been eaten, and hunger urges 

 them to continue their journey. As they march the wolves and 

 bears follow their track, while ea'gles and vultures, birds of ill omen, 

 circle over their heads. 



Scarcity of water, as well as of food, is often a cause of regular 

 migrations. When winter approaches in the south-east of Siberia, 

 more particularly in the high Gobi steppe, all the non-hibernating 

 mammals are compelled, by the peculiar circumstances of these high- 

 lands, to seek refuge in lower-lying regions. The winter in these 

 high grounds of Central Asia is not more severe than in districts 

 lying further to the north or north-east, but it is usually almost 



