56 



CHAPTER IV. 



THE IMPERFECTION OF THE PAL^ONTOLOGICAL 

 RECORD. 



As has been already pointed out, the series of the stratified 

 formations is an imperfect one, and is likely ever to remain 

 so. The causes of this " imperfection of the geological record," 

 as it has been termed by Darwin, are various ; but it is 

 chiefly to be ascribed to our as yet incomplete knowledge of 

 the geology of vast areas of the earth's surface, to denuda- 

 tion, and to the fact that many of the missing groups are 

 buried beneath other deposits, whilst more than half of the 

 superficies of the globe is hidden from us by the waters of 

 the sea. The imperfection of the geological record neces- 

 sarily implies an equal imperfection of the " palaeontological 

 record ; " but, in truth, the record of life is far more imperfect 

 than the mere physical series of deposits. As we are here 

 chiefly concerned with the biological aspect of the question, 

 we may advantageously consider some of the main causes of 

 the numerous breaks and gaps in the palaeontological record 

 at some length. 



I. CAUSES OF THE ABSENCE OF CERTAIN ANIMALS IN Fos- 

 SILIFEROUS DEPOSITS. In the first place, even if the series 

 of the stratified deposits had been preserved to us in its 

 entirety, and we could point to the sedimentary accumula- 

 tions belonging to every period of the earth's history, there 

 would still be enormous deficiencies in the palseontological 

 record, owing to the differences in the facility with which 

 different animals may be preserved as fossils. This subject 



