THE EUKOPEAN SEAS. 



269 



upon this fauna new forms have come in, mainly of 

 North Atlantic origin ; the whole assemblage is 

 seen changing its facies progressively from southern 

 to northern, till coming down to more recent times 

 the accession of immigrants is again West African. 

 What is of interest here, is, that the relation of the 

 existing fauna can be traced back stage by stage as 

 it underwent change in the course of the tertiary 

 period. The whole series, from the present back to 

 the earlier fauna of the Turin beds, or of Mont- 

 pellier, is the most complete and consecutive of 

 any that we are yet acquainted with. 



Palaeontologists have taken marine faunas as the 

 measures of the duration of geological divisions of 

 time, and in this way the Mediterranean basin be- 

 comes the type of the true tertiary period. What 

 the precise relations of the existing fauna may be 

 to any earlier portion, are points which for the 

 present can be considered as having received only 

 a general answer ; the tendency of recent inves- 

 tigation has been to increase the amount of agree- 

 ment between the present and the earlier stages of 

 the tertiary period. Mr. Jeffreys goes so far as to 

 state that "it is most probable that every species 

 which Philippi has described as inhabiting the 

 coasts of lower Italy will eventually be discovered 

 to have also had its existence in the tertiary epoch, 

 and perhaps vice versa." 



The disappearance of so many forms, and ap- 

 pearance of so many others, and that over so wide 



