THE USES OF FOSSILS 391 
to remove the fossils and obliterate all signs of their existence. 
Other things being equal, the older the strata, the more liable are 
the organic remains to such annihilation, for they have been ex- 
posed the longer to these agents of destruction. Therefore, in 
many of the older strata, fossils are scarce and hard to find. 
Imperfections of the Life Record. — From the above, 
it is readily seen that one of the most important rea- 
sons for the imperfection of the known record of the 
life of the earth, is the fact that of all the countless 
millions of animals and plants that have lived and 
died, only a very small fraction have found preservation 
in the rocks, in any permanent and recognizable condi- 
tion. Among land animals, this failure to be preserved 
has been so marked that our knowledge of the land life 
of the past is unfortunately very slight. 
In addition to these difficulties, knowledge is hindered 
by the fact, that of those creatures which have been 
preserved, many have been destroyed by metamor- 
phism or solution. This has robbed us of much of the 
earlier record, and indeed, so far as exploration has yet 
gone, has left us without any account of the life that 
at the outset peopled the earth. Then too, denudation 
has been so constantly and effectively at work, that but 
a small part of the sedimentary rocks originally depos- 
ited, now remain.! To be sure, we have rocks of all 
1 This of course applies only to the land rocks. What exists in the 
ocean we have no means of learning. 
