86 LECTURES ON EVOLUTION m 



be incomplete, that it can only preserve remains 

 found in certain favourable localities and under 

 particular conditions; that it must be destroyed 

 by processes of denudation, and obliterated by 

 processes of metamorphosis. Beds of rock of any 

 thickness, crammed full of organic remains, may 

 yet, either by the percolation of water through 

 them, or by the influence of subterranean heat, 

 lose all trace of these remains, and present the 

 appearance of beds of rock formed under con- 

 ditions in which living forms were absent. Such 

 metam orphic rocks occur in formations of all ages; 

 and, in various cases, there are very good grounds 

 for the belief that they have contained organic 

 remains, and that those remains have been abso- 

 lutely obliterated. 



I insist upon the defects of the geological re- 

 cord the more because those who have not 

 attended to these matters are apt to say, " It is 

 all very well, but, when you get into a difficulty 

 with your theory of evolution, you appeal to the 

 incompleteness and the imperfection of the geo- 

 logical record ; " and I want to make it perfectly 

 clear to you that this imperfection is a great fact, 

 which must be taken into account in all our 

 speculations, or we shall constantly be going 

 wrong. 



You see the singular series of footmarks, drawn 

 of its natural size in the large diagram hanging 

 up here (Fig. 2), which I owe to the kindness 



