54 STEPPE FAUNAS AND TEMPERATE 



deposed by a younger male so soon as his powers begin 

 to fail. 



Again, the social instinct is an aid in another way. 

 The more helpless forms, such as the rodents, usually 

 appoint sentinels when they feed, these sentinels giving 

 warning of the approach of danger in time for the 

 party to seek safety in flight or underground. In the 

 forest, where food is more uniformly distributed and 

 the natural shelter greater, social animals are less 

 frequent. 



Another marked feature of steppe animals is their 

 great fertility, seen alike in the steppe rodents and in 

 insects like the locusts. This is associated with the 

 risks of the natural habitat — risks of drought and con- 

 sequent lack of food, risks of storm, risks associated 

 with winter cold and summer heat. In Central Asia 

 the dreaded ' buran ' or hurricane may practically 

 exterminate aU life within a given area, except such 

 animals as can find a refuge underground. As such 

 storms are frequent, great natural fertUity is necessary 

 to repeople the devastated regions. Similarly, a season 

 of deficient rainfall, or a series of such seasons, must 

 kill out large numbers of animals. 



We have already seen that this fertihty of steppe 

 animals is associated with strong migratory instincts. 

 Now no natural barriers separate steppe regions from 

 the neighbouring areas of forest, of desert, of semi- 

 desert, and so forth. We find then, as a special character 

 of steppe animals, the fact that they tend periodically 

 to overrun the means of subsistence within their own 

 region, and therefore to flow out into the neighbouring 

 regions. The direction of the winds and the position 

 of Europe on the western border of a great continent 

 give it a moist climate, and led to its being forest-clad 



