22 INVERTEBRATE PALAEONTOLOGY 



(III) IMPERFECTION OF PALAEONTOLOGICAL 

 EVIDENCE 



The account of the conditions necessary for fossiliza- 

 tion given in the preceding paragraphs shows clearly how 

 rare must be fulfilment of those conditions. But the 

 three essentials there indicated (which may be sum- 

 marized as possession of hard-parts, a mode of death not 

 involving destruction of the shell, and a place and 

 medium of burial whence agents of decay are excluded) 

 by no means include all that is required for fossils to 

 become " palaeontological evidence." For such a con- 

 summation, the rock in which the organism has been 

 entombed must remain on, or be elevated into, dry land ; 

 it must be exposed by natural or artificial excavations ; 

 and it must be visited by a " fossil-collector" at the 

 precise time when the stratum has been laid bare. Even 

 then there is always the possibility that the fossil may 

 be overlooked, or broken beyond repair during extrac- 

 tion ; it may collapse during preparation, disintegrate 

 during storage, or, with tragic bathos, be lost. 



It has been shown that the chief fossil-bearing rocks 

 are those formed on the sea-floor. Elevation of the 

 sediments is thus usually inevitable if their fossil-contents 

 are to be displayed. It is possible, perhaps even 

 probable, that isostatic crustal movements will normally 

 raise the detrital zones along the sea-margin, sooner or 

 later. But, whatever views may be held as to perma- 

 nence of ocean basins, it is obvious that deposits formed 

 on the floor of deeper parts of the sea are likely to 

 reappear above its surface only very locally (in volcanic 

 regions) or after long delay. Thus, while the coastal 

 fauna is favourably situated in this respect, the less 

 profuse, but no less interesting, life of the open ocean 

 will rarely be accessible in the fossil state. Deep-sea 



