STEPPES OF ASIA AND AMERICA 57 
It is the steppes of the more remote parts of the world 
which now support the vast herds of sheep, horses, and 
cattle necessary for the wants of civilized communities. 
Further, as steppe regions do not require to be cleared 
of trees, and, where the rainfall is sufficient, often pro- 
duce abundant crops, we find that it is in the steppe 
regions that the cultivation of cereals is being most 
vigorously pushed at the present time. The regions of 
dense population are mostly regions of abundant rain, 
and therefore were once forest regions. Formerly these 
watered lands were liable to be periodically flooded by 
the overflow of the fertility of the steppes. This great 
tide is now being gradually regulated, and the steady 
stream of wheat, of wool, of meat, and so forth, which 
pours into the civilized world, is the biological equiva- 
lent of the flocks of rodents and the flights of locusts 
which the steppe sent out in earlier days, and of the 
hordes of wandering nomads which came later. 
To this general account we may add a few words on 
the special conditions, climatic and other, found in the 
steppes of temperate Asia, the region to which the 
following description specially refers. Steppe condi- 
tions reign over that great area of Central Asia which 
is practically ringed by mountain chains, as well as to 
the west of it, but the fauna throughout this area is 
not uniform. The mountain chains themselves have 
their own characteristic animals, and the plateau of 
Tibet, with special conditions of climate, has a peculiar 
fauna of its own. Excluding Tibet and the mountain 
chains then, we have a great steppe and desert area 
which extends from the eastern base of the Pamirs to 
the Khingan Mountains, and is separated from the 
forest region to the north by the great area of elevated 
ground in which the Siberian rivers arise, and from the 
