THE EUROPEAN SEAS. 



239 



Spain to the south of that Cape, 140 species have 

 not been met with so far north as Vigo ; if the ten- 

 dency to diffusion was equal, the number here not 

 passing north should be about 50, — or along the 

 European Atlantic border the northern elements of 

 the molluscous fauna have a greater southern dis- 

 tribution than the Lusitanian, or southern forms 

 have northwards. 



The naturalist who hopes that the day may come 

 when some of the evidence as to the past conditions 

 of the earth's surface may be interpreted, by the 

 combined aid of the laws of geographical distribu- 

 tion, of the bathymetrical arrangement of marine 

 animals, and of sedimentary matter, must make 

 these, and all allied considerations, his special 

 study. 



A marine fauna is not a constant assemblage. 

 In every latitude along the western shores of Europe, 

 it has long been undergoing a slow rate of change ; 

 southern forms have been extending themselves 

 northward : the testaceous fauna of our western 

 counties is far richer and warmer in its aspect than 

 that indicated by those raised deposits, of compara- 

 tively recent origin, which fringe those coasts. Con- 

 versely, that wide extension of northern forms into 

 southern latitudes which has been referred to, must 

 not be taken as wholly referable to the present — 

 antecedently to the present the tendency of north- 

 ern forms was southerly, and some remain there 

 now, the residual members of that migration. 



