152 



DEVELOPMENT OF ORGANIC LIFE 



[Ch. IX. 



succession in the ancient seas in the same order of time a 

 they would stand in an ascending series in a zoological classi- 



fication. 



endeavour 

 from 



or those older than the Lower Silurian (Nos. 36 and 37 f 

 the Table, p. 62) with which we are so imperfectly acquainted 

 that it is dangerous to derive from them any conclusions 

 founded simply on negative evidence. Additions made from 

 year to year may change the whole aspect of this primordial 

 fauna of Barrande. Already indeed the notion of its extreme 

 poverty and inferiority of grade has to some extent had to 

 be abandoned by the detection in it of an Orthoceras in 

 Sweden. 



To begin with the Lower Silurian (No 35 of the same 

 table), we find that it contains representatives of all the 

 groups to which we have alluded, and the Cephalopoda 

 alone have already furnished the conchologist with several 

 hundred species and a long list of genera. Many of these 

 chambered shells, especially the Ammonites and Orthocerata, 

 were of large size, and they may possibly have swarmed the 



) in the ancient ocean because there were no fishes to 



mor 



mar 



compete with them. It has been n 

 of < progressive evolution,' that all the cephalopods of this era 

 are referable to the tetrabranchiata, a family which is not so 

 highly organised as the dibranchiata, to which the belemnites, 

 so abundant in the Lias, Oolite, and Chalk, as well as not a 



few of the living cuttlefish belong. 



Doubtless the absence of 

 all genera of this highest order from the Silurian, Devonian, 



may 



obtained 



a grade as it afterwards reached. But the cogency of such 

 reasoning is somewhat weakened by the fact, that several 

 genera of Octopods now exist in our seas, which are without 

 internal bones, like those possessed by the Sepia, or ex- 

 ternal shells, like those of the Nautilus. Such soft-bodied 

 cephalopods, therefore, could not be expected to leave 

 behind them any lasting memorials of their existence. It is 

 only by assuming that there were no such genera in the 

 paleozoic seas, that we can confidently infer a comparative 



