THE SUCCESSION OF THE VERTEBRATE FAUNAS. 



THE general results of this brief sketch of the Palaeontology 

 of the Vertebrata, may now be summarized from the geological 

 point of view. Two considerations only need be borne in mind. 

 Firstly, owing to the imperfection of the geological record and 

 the incomplete exploration of most formations, any statement 

 now formulated may eventually prove to be quite a partial 

 account of the facts, and every conclusion must be more or less 

 provisional and tentative. Secondly, there are no means of 

 determining the contemporaneity or otherwise of rocks con- 

 taining similar fossils in different widely-separated regions of 

 the globe; and therefore in recording the geographical range 

 of the various faunas it is not possible to discover their limits 

 at any given epoch, but merely to ascertain the maximum 

 extent of their spreading and migrations during the period of 

 their existence. The palaeontologist at present can only deal 

 with the succession of faunas as revealed in some such small 

 area as western Europe, for example, where the series of 

 geological formations is tolerably complete and in recognizable 

 order. The known facts of geology are still too few to restore 

 the life-provinces of the globe at the various stages of its past 

 history. 



UPPER SILURIAN. 



The earliest vertebrate fauna hitherto discovered is con- 

 tained in shallow-water deposits towards the summit of the 

 Silurian system. It is distributed over the greater part of the 

 northern hemisphere, having been already found in England, 

 Scotland, Galicia, the Baltic region, Pennsylvania, New 



