PHVSICAL GEOGRAPHY OF THE PLEISTOCENE 595 
4. It should be noted further that the changes in the processes of 
erosion and sedimentation—changes in kind as well as in rate—were 
not limited to the areas actually covered by the ice, or even to the 
areas affected by drainage from it, or by icebergs which floated out 
beyond its edge. Modifications of erosion and sedimentation were felt 
in all areas affected, directly or indirectly, by the change of climate. 
The great ice-sheets, with the recurrent disturbance which they 
probably occasioned in the crust of the earth and the lesser changes 
in the surface of the ocean; with their recurrent inhibition of the usual 
processes of erosion and sedimentation over great areas; with their 
recurrent modification of these processes over other great areas 
beyond the ice-sheets themselves; and with their recurrent inaugura- 
tion on a large scale of processes of erosion and sedimentation which 
were unusual, might, without consideration of further changes of an 
indirect character, furnish adequate bases for important time divisions. 
Especially is this the case since the influence of the ice-sheets must 
have been felt in a physical way, throughout most if not all the earth. 
IV. CHANGES IN LIFE 
The great changes in the physical processes, which this on-com- 
ing of the ice-sheets brought into operation, effected corresponding 
changes in life, and in the processes which depend on life. In the 
first place, the total-amount of land life must have been greatly 
reduced. If account be taken of mountain glaciation in both hemi- 
spheres as well as of the ice-sheets, it is probably within the limits of 
truth to say that conditions became so far inhospitable as nearly to 
eliminate land life from about one-seventh of the land of the globe, and 
to have rendered conditions relatively inhospitable over a still larger 
area. The effect upon the life of the sea is less easily stated, but it 
also must have been great, for the average reduction of the tempera- 
ture of the sea must have been considerable. 
The crowding of land life off 8,000,000 square miles, more or less, 
must have tended to concentrate it upon the land which still remained 
hospitable, and to decimate or exterminate those forms which could — 
not migrate readily. Migration must have been forced upon the 
sea life as well as upon that of the land, and the shifting of the zones 
of both must have resulted in a shifting of the sites of organic deposi- 
