270 



MEDITERRANEAN FAUNA. 



an area, are changes akin to those which the palae- 

 ontologist traces amidst older sea-beds : com- 

 mencing with the early history of onr Atlantic 

 marine fauna, we can follow the incoming and out- 

 going of a long succession of species, sufficiently 

 distinct in themselves to admit of the recognition 

 of the progress of change, yet connected throughout 

 by such - a number of common forms as to make 

 the fauna indivisible as a whole. The duration of 

 this fauna constitutes the true tertiary period of 

 the earth's history, and the kind of change which 

 it presents is precisely that, as will be seen, of 

 every other great period, whether Cretaceous, 

 Oolitic, or Palaeozoic, when estimated, as geological 

 periods ever have been, by the succession of local 

 assemblages. 



In the changes which our own European ma- 

 rine fauna presents, we are in some cases enabled to 

 trace component parts to the localities or regions 

 whence they came, or whither they have gone, 

 and can, moreover, see the dependency of the 

 zoological change on some definite physical dis- 

 turbance. Our imperfect knowledge of the na- 

 ture of the physical changes of the remote past 

 does not as yet enable us to trace such a con- 

 nection; as yet the palaeontologist has hardly 

 done more than note the local rate and order of 

 zoological change ; but all the considerations to be 

 derived from the history of our European marine 

 fauna tend to impress this, that, in all times, the 



