82 EVIDENCES AND LIMITATIONS 



its life is extinct, it should be buried in time and in a 

 manner to escape entire distintegration either by the 

 action of the atmosphere or by other destructive causes. 

 Only a small fraction of the organisms of by-gone ages 

 can have left their traces in the deposits that have since 

 accumulated. Finally, only a very limited portion of 

 the earth's crust has been sufficiently examined to lay 

 bare the secrets which it contains.^ 



Yet, in spite of these limitations, the science of 

 paleontology, which is concerned with the study of 

 fossil remains whether animal or vegetable, has thrown 

 much light upon the past history of organic life on this 

 planet. The time measures which here have to be 

 employed are purely geological, and cannot be trans- 

 lated accurately into such terms as years and centuries. 

 But the strata of the earth's crust have been distributed 

 with approximate accuracy into a series of successive 

 periods; and paleontologists by means of this distri- 

 bution have been enabled to ascertain the relative 

 antiquity of a large number of existing species as well 

 as of species now extinct, the fossil remains of which 

 have been discovered in various geological strata. The 

 results have been in accord with the requirements 

 of the evolutionary theory. The lower and simpler 

 species are most ancient, and in certain instances long 

 series of ancestral forms have been discovered which 

 seem to indicate the evolution of living species, through 

 many intervening stages, from remote and widely dif- 

 ferent forms of life. 



» Darwin discusses this incompleteness in op. cit., ch. x. 



