MATERIALS 13 



(II) CONDITIONS NECESSARY FOR FOSSILIZATION 



(a) Biological 



Although in exceptional cases impressions, and even 

 petrifactions, of soft tissues have been found, 1 fossiliza- 

 tion is not likely to follow the death of an animal that 

 failed to secrete mineral, or other chemically stable, 

 structures during life. Most purely organic tissues 

 become dissipated by decomposition very soon after 

 death, even if they escape digestion in the alimentary 

 tract of a contemporary. Since a large proportion of 

 Invertebrates now living, particularly of the simpler 

 and smaller types, are without shells or skeletons, the 

 existing fauna cannot be expected to leave an approxi- 

 mately complete representation for study by future 

 palaeontologists. There is no reason to believe that 

 the proportion of normally perishable forms was ever 

 inferior to that at the present day probably it was 

 greater in more remote periods. Palaeontology is thus 

 practically cut off from the study of a very considerable 

 number of groups of animals ; and, as a rule, can deal 

 directly only with the hard parts of such classes as are 

 capable of fossilization. T Even among such structures, 

 much diversity of durability exists. The hard parts 

 of Invertebrates may be roughly classified as coherent 

 or disjunct, and as siliceous, calcareous or chitinous. 

 Coherent shells or skeletons (such as those of typical 

 Gastropoda or Echinoidea) will retain their shape after 

 dissolution of the associated soft tissues, and so may 

 provide satisfactory evidence of the form and characters 



1 The frozen carcasses of animals preserved by " cold-storage " in Arctic 

 regions since the Pleistocene period are, geologically considered, transient, 

 and could not survive any climatic changes that raised the temperature 

 of their surroundings. 



