SUDDEN APPEARANCE OF GROUPS 359 



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 

 Cambrian and Silurian trilobites are descended from some 

 one crustacean, which must have lived long before the Cam- 

 brian 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 spe- 

 cies 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 peri- 

 ods 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 habi- 

 tation 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 formation ; and the 

 previous 140 million years can hardly be considered as suffi- 



