268 GROUPS OF ALLIED SPECIES [CHAP. X. 



to be about as rash to dogmatize on the succession of organic 

 forms throughout the world, as it would be for a naturalist to land 

 for five minutes on a barren point in Australia, and then to discuss 

 the number and range of its productions. 



On the sudden Appearance of Groups of allied Species in the lowest 

 known Fossil if erous 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 suddenly 

 appear in the lowest known fossiliferous rocks. Most of the argu- 

 ments 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 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 

 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 TV. Thompson concludes that the con- 

 solidation 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 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 



