208 LAY SERMONS, ADDRESSES, AND REVIEWS. [x. 



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. 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 commenced 

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

 of this unsatisfactory uncorroborated sort. The Cam- 

 brian witnesses simply swear they "haven't seen any- 

 body 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 no- 

 where exhibit fossils, and hence no living being could 

 have existed in their epoch. 



To this there are two replies : the first, that the 

 observational basis of the assertion that the lowest 

 rocks are nowhere fossiliferous is an amazingly small 

 one, seeing how very small an area, in comparison to 

 that of the whole world, has yet been fully searched ; 

 the second, that the argument is good for nothing unless 

 the unfossiliferous rocks in question were not only 

 contemporaneous in the geological sense, but synchronous 



