THE EUROPEAN SEAS. 



17 



The distribution of marine animals is primarily- 

 determined by the influences of climate or tempera- 

 ture, sea-composition and depth, in which pressure, 

 and the diminution of light are doubtless important 

 elements. All these may be combined so as to 

 complicate the character of the fauna of a particular 

 province. This appears to be especially the case in 

 the Arctic seas, as I shall have hereafter to insist 

 upon with much stress. The secondary influences 

 modifying the action of the primary ones are many. 

 Thus the structure of the coast, so far as the mineral 

 character of its rocks is concerned may seriously 

 affect the distribution of particular tribes. Whole 

 families of marine animals depend for their subsist- 

 ence on the presence of sea-weeds, and of the crea- 

 tures that feed upon them. Yet all kinds of rocks 

 are not favourable to the growth of weed, and tracts 

 of sand may be wholly free from marine vegeta- 

 tion, or when giving support to sea-plants, cherish 

 forms, adapted for the subsistence of peculiar ani- 

 mals only. Consequently whole tribes of beings 

 may be present on, or absent from, a range of coast, 

 according to its geological, or rather mineral struc- 

 ture, although every other condition be perfectly 

 favourable to their propagation. And, what is 

 more important, the course and diffusion of whole 

 tribes may be restricted within areas far more 

 limited than their capabilities for enduring ele- 

 mental or bathymetrical conditions warrant, in con- 

 sequence of the barrier interposed to their spreading 



c 



