222 CONTINUITY. 



donts, as instances of this when compared with their more 

 recent representatives. 



The records of life on the globe may have been destroyed 

 by the fusion of the rocks, which would otherwise have pre- 

 served them, or by crystallisation after hydrothermal action. 

 The earlier forms may have existed at a period when this 

 planet was in course of formation, or being segregated or 

 detached from other worlds or systems. We have hardly evi- 

 dence enough to speculate on the subject, but by time and 

 patience we may acquire it. 



Were all the forms which have existed embalmed in rock 

 the question would be solved ; but what a small proportion 

 of extinct forms is so preserved, and must be, if we consider 

 the circumstances necessary to fossilise organic remains ! On 

 the dry land, unwashed by rivers and seas, when an animal 

 or plant dies, it undergoes chemical decomposition, which 

 changes its form ; it is consumed by insects, its skeleton is 

 oxidised and crumbles into dust. Of the myriads of animals 

 and vegetables which annually perish we find hardly an 

 instance of a relic so preserved as to be likely to become a 

 permanent fossil. So again in the deeper parts of the ocean, 

 or of the larger lakes, the few fish there are perish and their 

 remains sink to the bottom, and are there frequently consumed 

 by other marine or lacustrine organisms or are chemically 

 decomposed. As a general rule, it is only when the remains 

 are silted up by marine, fiuviatile or lacustrine sediments that 

 the remains are preserved. Geology, therefore, might be ex- 

 pected to keep for us mainly those organic remains which 

 inhabited deltas or the margins of seas, lakes, or rivers ; here 

 and there an exception may occur, but the mass of preserved 

 relics would be those of creatures so situated ; and so we find 

 it, the bulk of fossil remains consists of fish and amphibia ; 

 shell-fish form the major part of the geological museum, 

 limestone and chalk rocks frequently consisting of little else 

 than a congeries of fossil shells ; plants of reed or rush-like 

 character, fish which are capable of inhabiting shallow waters, 

 and saurian animals, form another large portion of geological 

 remains. 



Compare the shell-fish and amphibia of existing organisms 



