256 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



lata occur associated with these lower forms. Up to 1868 there were 

 catalogued some one hundred and twenty-six species (Bigsby) from 

 the Cambrian deposits of the world, most of which belonged to 

 Canada, Scandinavia, and the British Isles (twenty species in 1880, 

 Etheridge). The prodigious development of the Brachiopoda in 

 the Silurian period, apart from the mere consideration of numbers, 

 is important, as representing the climax of development in this re- 

 markable group of organisms. Henceforward they show a pretty 

 steady decline, although the rate of decline for the various periods 

 is different for different regions of the earth's surface. The fact 

 that there are some fifteen hundred or more species in the Silurian 

 deposits, of which considerably over five hundred are already rep- 

 resented in the Lower Silurian, and less than two hundred in the 

 Cambrian, is extraordinary, whichever way it be considered, for, 

 w r hether in the Cambrian we were somewhere near the beginning of 

 life, or very distantly remote from it, as is much more likely to 

 have been the case, the difficulty of explanation is in no wise af- 

 fected. In either case the suddenness of the Silurian apparition 

 is the same, and this is the more remarkable, seeing that there is 

 scarcely any advance in the number of species of the Upper as com- 

 pared with the Lower Cambrian. In the Bohemian Silurian basin 

 Barrande enumerates six hundred and forty species, whereas in the 

 Cambrian there are but two ! 



It is usually assumed that the Silurian species of brachiopods 

 are vastly in excess of the Devonian, but the latest revised tables 

 seem to indicate that this is not the case, and, indeed, it is not 

 improbable that the numerical balance will be found to weigh on 

 the other side. The number of genera is, however, greatly re- 

 duced, from about seventy to fifty, and this reduction is further 

 carried into the Carboniferous period, where we have but forty 

 genera, representing some eight hundred to nine hundred species. 

 That there should have been, so soon after the climax had been 

 reached, such a rapid decline is not a little surprising, but yet the 

 suddenness of this decline is in no w r ay comparable with the sud- 

 denness of the apparition already noticed in the Silurian. More 

 surprising is the almost total absence of forms from the Permian 

 deposits, where the deficiency can only partially be explained by 

 the circumstance of the limited extent which these deposits occupy, 

 and the special conditions under which their formation was effected. 



