286 Groups of Allied Species Chap. x. 



main divisions of the animal kingdom suddenly appear in the 

 lowest known fossiliferous rocks. 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 he 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 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 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 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 doubt- 

 ful 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 com- 

 mencement 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 140 million years 

 can hardly be considered as sufficient for the development of the 

 varied forms of life which already existed during the Cambrian 

 period. It is, however, probable, as Sir William Thompson insists, 

 that the world at a very early period was subjected to more rapid 

 and violent changes in its physical conditions than those now 

 occurring ; and such changes would have tended to induce changes 

 at a corresponding rate in the organisms which then existed. 



To the question why we do not find rich fossiliferous deposits 

 belonging to these assumed earliest periods prior to the Cambrian 

 system, I can give no satisfactory answer. Several eminent geo- 

 logists, with Sir R. Murchison at their head, were until recently 

 convinced that we beheld in the organic remains of the lowest 



