272 



LOCAL DURATION OF SPECIES. 



the Faluns of Bordeaux and Touraine, suggest the 

 probability that in these last we have an earlier 

 stage still in the history of our fauna, referable 

 to the time when the Mediterranean depression had 

 not yet been opened to the Atlantic waters. 



The precise relation of our existing fauna to any 

 earlier stage, in respect of common species, is a large 

 question, involving many subordinate points upon 

 which naturalists and palaeontologists have much to 

 learn before they will be agreed ; the tendency of 

 more recent investigation makes the amount of 

 agreement between the present and the past to be 

 greater than was once supposed. Leaving aside 

 the question, how new forms are introduced into a 

 fauna, the history of our European seas teaches us 

 thus much that is certain — that it is possible to 

 point to contingencies under which the component 

 members of a fauna may seem to migrate, disap- 

 pear, and die out ; that certain conditions of ex- 

 istence are so intimately connected with the con- 

 tinuance of the separate members of a fauna, that, 

 unless these are maintained, their duration there 

 becomes impossible. A small amount of change 

 may cause such species to disappear, and, in all 

 cases, the duration of any species over a given area 

 will depend on its power of adapting itself to 

 change. Hence the unequal terms of the existence 

 of species in our present tertiary, as in all ante- 

 cedent bygone faunas. 



Another inquiry still suggests itself — To what 



