THE EUROPEAN SEAS. 



21 



seek for an explanation of its inequalities in the 

 same gradual changes and sudden cataclysms to 

 which their undulations, and levels, and ravines 

 have owed their origin. These variations in the 

 form of the surface, whether of dry, or of submerged 

 land, importantly affect the distribution of living 

 creatures, now furthering their progress beyond the 

 regions to which they strictly appertain, now arrest- 

 ing their diffusion, and restricting them within 

 limited areas far more circumscribed than the ex- 

 tent of climatal conditions, for which, were there 

 fitting ground to favour their range, they are adapt- 

 ed by their organization. 



The composition of the waters in which aquatic 

 animals live, is a most important influence in its 

 effect on their distribution. The degree of salt- 

 ness or freshness determines the presence or absence 

 of numerous forms of both fishes and invertebrate 

 animals. Within the European area unusual con- 

 ditions of this influence are manifest in the most 

 northern, and a part of the most southern provinces. 

 In the Arctic region, where unquestionably the 

 small number of testacea in the shallows is in great 

 part due to the comparative freshness of the upper 

 layer of waters ; in the Baltic Sea, where the waters 

 are entirely modified ; in the Black Sea, where the 

 phenomena of the limited and peculiar fauna are in 

 part determined by the peculiar character of this 

 portion of the Mediterranean basin, modified as it 

 is by its nearly complete isolation, and by the great 



