GROUPS OP ALLIED SPECIES. 317 



ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES 

 IN THE LOWEST KNOWN FOSSILIFEROUS STRATA. 



There is another and allied difficulty, which is much more 

 serious. I allude to the manner in which species belonging 

 to several of the main divisions of the animal kingdom sud- 

 denly appear in the lowest known fossiliferous rocks. Most 

 of the arguments which have convinced me that all the ex- 

 isting 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 Cam- 

 brian 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, 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 occurred less than twenty or more than four 

 hundred million years ago, but probably not less than ninety- 

 eight or more than two hundred million years. These very 

 wide limits show how doubtful the data are ; and other ele- 

 ments may have hereafter to be introduced into the problem. 

 Mr. Croll estimates 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 sufficient 

 for the development of the varied forms of life which already 

 existed during the Cambrian period. It is however probable, 



