IMPERFECTION OF GEOLOGICAL RECORD 89 



descended from a single progenitor apply with equal 

 force to the earliest known species. For instance, it 

 cannot be doubted that all the Cambrian and Silurian 

 trilobites are descended from some one crustacean, which 

 must have lived long before the Cambrian age, and 

 which probably differed greatly from any known ani- 

 mal. Some of the most ancient animals, as the Nau- 

 tilus, Lingula, etc., 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 subsequently 

 appeared, for they are not in any degree intermediate 

 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 encounter 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 oc- 

 cuiTcd 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 bow doubtful the 

 data are; and other elements may have hereafter to be in- 

 troduced into the problem. Mr. CroU 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 mu- 



