GEOLOGICAL CONTEMPORANEITY 379 



The subsequent distribution of living beings in zones is 

 the result of a gradual 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 proposi- 

 tions demand our assent ? 



These assumptions are two : the first, that the com- 

 mencement of the geological record is coeval with the 

 commencement of life on the globe ; the second, that 

 geological contemporaneity is the same thing as chrono- 

 logical 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 demon- 

 stration. 



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 

 wishes to prove an alibi, it is of no use for him to get a 

 thousand witnesses simply to swear that they did not 

 see him in such and such a place, unless the witnesses 

 are prepared to prove that they must have seen him had 

 he been there. But the evidence that animal life com- 

 menced with the Lingula-flags, e.g., would seem to be 

 exactly of this unsatisfactory uncorroborated sort. The 

 Cambrian witnesses simply swear they " haven't seen 

 anybody their way " ; upon which the counsel for the 

 other side immediately puts in ten or twelve thousand 

 feet of Devonian sandstones to make oath they never 

 saw a fish or a mollusk, though all the world knows there 

 were plenty in their time. 



But then it is urged that, though the Devonian rocks 

 in one part of the world exhibit no fossils, in another they 

 do. while the lower Cambrian rocks nowhere exhibit 



