PHYSICAL GEOGRAPHY OF THE PLEISTOCENE 271 



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 m 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 hfe 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 hmits 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 Hfe 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- 

 tion, perhaps especially of the sites where limestone was made. 

 At the same time, the rate at which it was formed, the whole earth 

 considered, was probably much reduced. 



It would seem, from the series of physical changes sketched, that 

 very profound changes in life should have followed, but it must be 

 confessed that, in spite of the conditions which it would seem must 

 have been favorable for great destruction of life, and for imposing 

 great modifications upon that which survived, statistical evidences of 



