52 PRINCIPLES OF PALAEONTOLOGY. 



tures in many great groups ; of animals inhabiting fresh water 

 our knowledge is rendered still further incomplete by the small 

 proportion that fluviatile and lacustrine deposits bear to marine ; 

 whilst we have only a fragmentary acquaintance with the air- 

 breathing animals which inhabited the earth during past ages. 



Lastly, the imperfection of the palaeontological record, due 

 to the causes above enumerated, is greatly aggravated, espe- 

 cially as regards the earlier portion of the earth's history, by the 

 fact that many rocks which contained fossils when deposited 

 have since been rendered barren of organic remains. The 

 principal cause of this common phenomenon is what is known 

 as " metamorphism " that is, the subjection of the rock to a 

 sufficient amount of heat to cause a rearrangement of its par- 

 ticles. When at all of a pronounced character, the result of 

 metamorphic action is invariably the obliteration of any fossils 

 which might have been originally present in the rock. Meta- 

 morphism may effect rocks of any age, though naturally more 

 prevalent in the older rocks, and to this cause must be set 

 down an irreparable loss of much fossil evidence. The most 

 striking example which is to be found of this is the great Lau- 

 rentian series, which comprises some 30,000 feet of highly- 

 metamorphosed sediments, but which, with one not wholly un- 

 disputed exception, has as yet yielded no remains of living 

 beings, though there is strong evidence of the former existence 

 in it of fossils. 



Upon the whole, then, we cannot doubt that the earth's 

 crust, so far as yet deciphered by us, presents us with but a 

 very imperfect record of the past. Whether the known and 

 admitted imperfections of the geological and palseontological 

 records are sufficiently serious to account satisfactorily for the 

 deficiency of direct evidence recognizable in some modern 

 hypotheses, may be a matter of individual opinion. There can, 

 however, be little doubt that they are sufficiently extensive to 

 throw the balance of evidence decisively in favor of some theory 

 of continuity, as opposed to any theory of intermittent and 

 occasional action. The apparent breaks which divide the great 

 series of the stratified rocks into a number of isolated forma- 

 tions, are not marks of mighty and general convulsions of 

 nature, but are simply indications of the imperfection of our 

 knowledge. Never, in all probability, shall we be able to point 

 to a complete series of deposits, or a complete succession of 

 life linking one great geological period to another. Neverthe- 

 less, we may well feel sure that such deposits and such an 



