IX GEOLOGICAL CONTEMPORANEITY 279 



lowering of the general temperature, which first 

 began to be felt at the poles. 



It is not now proposed to inquire whether these 

 doctrines are true or false; but to direct your 

 attention to a much simpler though very essential 

 preliminary question What is their logical basis ? 

 what are the fundamental assumptions upon which 

 they all logically depend ? and what is the evidence 

 on which those fundamental propositions demand 

 our assent ? 



These assumptions are two : the first, that the 

 commencement of the geological record is coeval 

 with the commencement of life on the globe ; the 

 second, that geological contemporaneity is the 

 same thing as chronological synchrony. Without 

 the first of these assumptions there would of 

 course be no ground for any statement respecting 

 the commencement of life ; without the second, all 

 the other statements cited, every one of which 

 implies a knowledge of the state of different parts 

 of the earth at one and the same time, will be no 

 less devoid of demonstration. 



The first assumption obviously rests entirely on 

 negative evidence. This is, of course, the only 

 evidence that ever can be available to prove the 

 commencement of any series of phenomena ; but, 

 at the same time, it must be recollected that the 

 value of negative evidence depends entirely on the 

 amount of positive corroboration it receives. If A.B. 



