82 LECTURES ON EVOLUTION ni 



The same truth is exemplified if we turn to 

 certain great periods of the earth's history as, 

 for example, the Mesozoic epoch. There are 

 groups of reptiles, such as the Ichthyosauria and 

 the Plesiosauria, which appear shortly after the 

 commencement of this epoch, and they occur in 

 vast numbers. They disappear with the chalk 

 and, throughout the whole of the great series of 

 Mesozoic rocks, they present no such modifications 

 as can safely be considered evidence of progressive 

 modification. 



Facts of this kind are undoubtedly fatal to any 

 form of the doctrine of evolution which postulates 

 the supposition that there is an intrinsic necessity, 

 on the part of animal forms which have once 

 come into existence, to undergo continual modifi- 

 cation ; and they are as distinctly opposed to any 

 view which involves the belief, that such modifi- 

 cation as may occur, must take place, at the saint 

 rate, in all the different types of animal <>r 

 vegetable life. The facts, as I have placed them 

 before you, obviously directly contradict any form 

 of the hypothesis of evolution which stands in need 

 of these two postulates. 



Hut, one great service that has been rendered 

 by Mr. Darwin to the doctrine of evolution in 

 general is this: he has shown that there are two 



f factors in the process of evolution: 01;- 

 them is the tendency to vary, the existence <>t 

 wliidi in .-ill living forms may be proved by 



