38 PROGRESS OF PALAEONTOLOGY n 



contains certain fossils which are peculiar to it; 

 and that the order in which the strata, character- 

 ised by these fossils, are super-imposed one upon 

 the other is always the same. This most im- 

 portant generalisation was rapidly verified and 

 extended to all parts of the world accessible to 

 geologists ; and now it rests upon such an immense 

 mass of observations as to be one of the best 

 established truths of natural science. To the 

 geologist the discovery was of infinite importance 

 as it enabled him to identify rocks of the same 

 relative age, however their continuity might be 

 interrupted or their composition altered. But to 

 the biologist it had a still deeper meaning, for it 

 demonstrated that, throughout the prodigious 

 duration of time registered by the fossiliferous 

 rocks, the living population of the earth had 

 undergone continual changes, not merely by the 

 extinction of a certain number of the species 

 which had at first existed, but by the continual 

 generation of new species, and the no less constant 

 extinction of old ones. 



Thus the broad outlines of paleontology, in so 

 far as it is the common property of both the 

 geologist and the biologist, were marked out at 

 the close of the last century. In tracing its sub- 

 sequent progress I must confine myself to the 

 province of biology, and, indeed, to the influence 

 of palaeontology upon zoological morphology. And 

 I accept this limitation the more willingly as the 



