298 THE EVOLUTIONARY CAREER 



the original substance becomes replaced by enduring lime, silica, 

 etc. In this way the fine structure of the body, or bodily part, 

 may be more or less exactly represented. 



Fossils may be impressions, or casts. Thus the marks made 

 by the feet of Reptiles, Amphibians, Mammals, etc., when walking 

 on soft sands, or muds, may become filled up by silt before the 

 marks became obliterated. 



So far as observations of the conditions of the present time show, 

 the process of fossilization, so that definite artifacts may be 

 formed, is exceptional. Putrefaction, crushing, etc., must, in 

 the great majority of cases, lead to the destruction of animal and 

 plant remains before the processes of replacement and mineraliza- 

 tion have gone far enough to lead to preservation of the form 

 of the body, or part. As a rule, the materials of organisms are 

 almost completely dispersed after death. Proteins, fats and 

 carbohydrates are, in time, resolved into CO 2, OH 2 and traces 

 of mineral salts. Even the relatively resistant substances of 

 teeth and hard bone become decomposed : so these substances — 

 though they do occur are rare in the deep-sea clays, which are 

 residues incapable of further change. 



But sediments may contain organic remains, or may be such 

 as they are because of organic activity, even although no definitely 

 formed fossils can be recognized. Carbonaceous materials in 

 rocks are held to be the evidences of life contemporaneous with the 

 deposition of those rocks inasmuch as we do not know how 

 otherwise these materials could be formed. Limestones that have 

 no fossils may have been formed by precipitation of CaCOg 

 from solution in sea-water and this precipitation has been the 

 result of a change in hydrogen-ion-concentration brought about 

 by the photosynthetic processes of planktonic plants. Flints and 

 cherts appear to be formed by deposition of silica after some 

 organic processes. Petroleum may be the result of decomposi- 

 tion, in special circumstances, of organic materials. 



99. ON THE NATURE AND LIMITATIONS OF PALEONTO- 

 LOGICAL RECORDS OF PAST FORMS OF LIFE 



Stratigraphical information enables us to reconstruct the past 

 so that we can infer these things : 



Past geographical conditions ; heights of the land, depths 

 of the seas ; relative extents of land and water, etc. 



