484 GEOLOGY. 



a comparison between the pre-glacial life-record and the post-glacial. 

 But the pre-glacial record is wholly a fossil one, subject to the well- 

 known defects of such a record, and subject also to the special forma 

 of destruction that attended the ice invasions. The existing record, 

 on the other hand, is one of immediate and unobstructed observation, 

 and is therefore immeasurably more complete. It follows that many 

 pre-glacial species are found in this very full record that would not 

 appear in a fossil record comparable with that of the pre-glacial time, 

 and hence the number of apparent extinctions of Pliocene species is 

 very much less than would appear if the comparison were made with 

 a post-glacial fossil record — such a record of present life, for example, 

 as would be found by geologists some millions of years hence, if it 

 had in the meanwhile been subjected to the usual geological agencies 

 of burial and destruction. Without doubt, multitudes of pre-glacial 

 species yet live that are imminently moribund, and many of these 

 would not be found in a fossil list of the distant future, under usual 

 geologic conditions. It is very difficult to make adequate allowances 

 for this inequality in the records when comparing pre-glacial and post- 

 glacial life, and hence it is difficult to measure, by such a comparison, 

 the destructive effects of the intervening ice invasion. 



It is to be noted further that the resilience of life is very rapid, 

 when measured in geologic terms. The excessive possibilities of mul- 

 tiplication of most living creatures give great capacity for recovery 

 from depletions, and as our present census is taken some thousands 

 of years at least after the last notable ice invasion, there has been, 

 without question, great increase of life, especially in the higher lati- 

 tudes most affected by the glaciation. 



An added source of embarrassment in the comparison is the espe- 

 cially disturbing influence of man. This is indeed to be regarded as 

 a geological influence, and to be put in the same category as the influ- 

 ence of other races, as they have risen to dominance; but none the 

 less it qualifies the comparison of life before and after the glacial period, 

 so far as it concerns the destructive effects of the ice invasions. 



Of the marine Pliocene invertebrates, more than half the known 

 species are now living, whereas, in the transition between several of 

 the more ancient periods, nearly all species disappeared. Of the Plio- 

 cene plant species, a very considerable percentage are still living. On 

 the other hand, the land vertebrates were very generally rep'aced by 



