338 GROUPS OF ALLIED 8PECIE8 



belonging to several of the main divisions of the animal 

 kingdom suddenly appear in the lowest known fossiliferous 

 rooks. Most of the arguments which have convinced me 

 that all the existing species of the same group are 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 difEered 

 greatly from any known animal. Some of the most ancient 

 animals, as the Nautilus, Lingula, etc., do not differ much 

 from living species; and it cannot on our theory be sup- 

 posed, that these old species were the progenitors of all the 

 species belonging to the same groups which have subse- 

 quently appeared, for they are not in any degree inter- 

 mediate 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 occurred less 

 than twenty or more than four hundred million years ago, 

 but probably not less than niuety-eight or more than two 

 hundred million years. These very wide limits show how 

 doubtful the data are; and other elements may have here- 

 after to be introduced into the problem. Mr. Croll esti- 

 mates that about sixty 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 formation; and the previous one hundred and 

 forty million years can hardly be considered as suflBcient 

 for the development of the varied forms of life which 

 already existed during the Cambrian period. It is, how- 

 ever, probable, as Sir William Thompson insists, that the 

 world at a very early period was subjected to more rapid 



