GEOLOGY. 



LIFE CHANGES ON THE GLOBE. 



It has generally been assumed by geologists that, through the 

 similarity of fossil remains existing in analogous formations in differ- 

 ent countries, we were put in possession of the means of establishing 

 a comparative chronology universally applicable in our mundane 

 world. The age, for example, giving geological latitude to the 

 word, of Silurian slates and sandstones, coal measures, or chalk, 

 was supposed to be the same all over the globe. If the characteristic 

 fossils were found, they were presumed to decide the epoch to which 

 the strata belonged, and few geologists, even in the recesses of their 

 own minds, ventured to question the dogmas of a scientific orthodoxy 

 which were not without convenience in application, and which were 

 widely received. In a recent communication to the London Geologi- 

 cal Society, Prof. Huxley assails the grounds of this belief, and enters 

 into an inquiry as to the nature of the evidence on which assump- 

 tions in geology, generally considered as fundamental, are based. 

 The first of these assumptions is, that the commencement of [that 

 part of] the geological record [which has hitherto been deciphered] 

 is coeval with the commencement of life on the globe ; and the 

 second, that geological contemporaneity is the same thing as chrono- 

 logical synchrony. Without the first of these assumptions, says Prof. 

 H., there would of course be no ground for any statement respecting 

 the commencement of life ; and, without the second, all statements 

 implying 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 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 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 



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