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 all life within a given area, except such 
animals as can find a refuge underground. As such 
storms are frequent, great natural fertility 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 fertility 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, asa 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 
