THE EUROPEAN SEAS. 



265 



more particularly in the poftion north of latitude 

 50°. 



Like our west European races of men, the mem- 

 bers of the Celtic and Lusitanian marine provinces 

 may be considered to have been derived from other 

 regions ; three- fourths of the whole assemblage 

 may be thus directly accounted for, and this purely 

 derivative character of the fauna belongs to zones 

 which extend through more than thirty degrees of 

 latitude. When southern and northern seas shall 

 have been more diligently dredged, some others of 

 the forms of this intermediate region will, doubt- 

 less, be discovered to be also immigrants. 



The marine fauna of this broad zone of mixed 

 races is as characteristic an assemblage, in the sense 

 of the palaeontologist, as that of the Arctic, or any 

 other parent province ; and it may be worth while 

 to glance at the process by which a derivative fauna 

 acquires a distinctive character. 



There are certain Testacea, such as the common 

 limpet (Patella mdgata), which seem to have their 

 limits, or specific centres, within our European zone 

 of mixed forms. This mollusk has its northern 

 limit on the Norway coast, somewhere south of the 

 Lofoden islands. It is not found within the Arctic 

 province ; it could not exist there now ; and, unless 

 the climatal conditions of that region should have 

 been, at some time, greatly different to what they 

 now are, it may be safely added that it never could 



