Chap. X.] IN LOWEST POSSILIPEEOUS STRATA. 83 



Silurian trilobites are descended from some one crusta- 

 cean, which must have lived long before the Cambrian 

 age, and which probably differed greatly from any 

 known animal. Some of the most ancient animals, as 

 the Nautilus, Lingula, &c., do not differ much from 

 living species; and it cannot on our theory be supposed, 

 that these old species were the progenitors of all the 

 species belonging to the same groups which have sub- 

 sequently appeared, for they are not in any degree in- 

 termediate in character. 



Consequently, if the theory be true, it is indisputable 

 that before the lowest Cambrian stratum was deposited 

 long periods elapsed, as long as, or probably far longer 

 than, the whole interval from the Cambrian age to the 

 present day; and that during these vast periods the 

 world swarmed with living creatures. Here we en- 

 counter a formidable objection; for it seems doubtful 

 whether the earth, in a fit state for the habitation of 

 living creatures, has lasted long enough. Sir W. 

 Thompson concludes that the consolidation of the crust 

 can hardly have occurred less than 20 or more than 

 400 million years ago, but probably not less than 98 or 

 more than 200 million years. These very wide limits 

 show how doubtful the data are; and other elements 

 may have hereafter to be introduced into the problem. 

 Mr. Croll estimates that about 60 million years have 

 elapsed since the Cambrian period, but this, judging 

 from the small amount of organic change since the 

 commencement of the Glacial epoch, appears a very 

 short time for the many and great mutations of life, 

 which have certainly occurred since the Cambrian for- 

 mation; and the previous 140 million years can hardly 

 be considered as sufficient for the development of the 

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