IMPERFECTION OF THE RECORD. 11 



can also be told with any degree of certainty only in the 

 case of species still living, or in the case of assemblages 

 of genera which are now found at some particular depth. 

 But in basing conclusions as to depth on fossils, it must 

 be remembered that the fauna of any locality may 

 include forms which have lived at higher or lower levels 

 and have been transported from their original home 

 after death. Perhaps the safest indication of shallow- 

 water and the proximity of land is given by the occur- 

 rence of land animals and plants in a marine formation. 



If all the plants and animals which have lived in past 

 times had been preserved as fossils, we should have a 

 complete record of the succession of life on the earth, and 

 all questions of phylogeny could be easily answered. But 

 we have already seen several reasons why this record 

 the palffiontological record must be imperfect. Thus, 

 some animals have no skeleton. Others, particularly land 

 animals, do not become entombed. And some rocks which 

 originally contained fossils have had them dissolved by 

 the percolation of water containing carbonic acid. In 

 addition to this, the fossils in some of the older rocks 

 have been obliterated by metarnorphism. The palaeonto- 

 logical record is also incomplete because of the imperfection 

 of the geological record. Between some formations enor- 

 mous masses of strata are missing, having been removed 

 by denudation. And although these gaps may be 

 filled up elsewhere, it does not render the record of life 

 perfect, since in going from one district to another, we 

 almost certainly pass into a different marine province, or 

 into an area where the conditions of temperature, depth, 

 or the sea-bottom have changed, and where as a result we 

 shall meet with another assemblage of organisms. 



