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CHAPTER IV. 



THE IMPERFECTION OF THE PAL^E ONTO LOGICAL 



RECORD. 



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

 tions 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 denudation, 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 necessarily 

 implies an equal imperfection of the " palseontological record " ; but, 

 in truth, the record of life is far more imperfect than the mere phys- 

 ical 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 

 palseontological record at some length. 



I. Causes of the Absence of Certain Animals in Fossil- 

 iferous 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 accumulations belonging to every 

 period of the earth's history, there would still be enormous deficien- 

 cies in the palaeontological record, owing to the differences in the 

 facility with which different animals may be preserved as fossils. 

 This subject is sufficiently important to render it advisable to con- 

 sider each of the primary groups of the animal kingdom separately 

 from this point of view : — 



a. Protozoa. — As regards the sub-kingdom of the Protozoa, the 

 Gregarines are destitute of hard parts, and have therefore left no 

 traces of their past existence. The great majority of the Infusorian 

 Animalcules are similarly destitute of hard parts, and are also un- 

 represented as fossils ; though a few forms (e.g., Peridiniiim) possess 



