438 DEPTHS OF THE OCEAN 



of the Canaries, a stretch of more than 40 degrees, or 2400 miles, 

 A survey of the animal life on this long stretch of sea-floor is 

 very interesting. As the temperature gradually falls toward the 

 north the fauna changes. Some species are hardy, and are dis- 

 tributed over a greater part of the area ; others can only live 

 under more uniform conditions, and therefore have a more 

 limited area of distribution. 



Zoological oceanography has long recognised this, and 

 zoological literature contains much information regarding the 

 distribution of animals within our area of investigation. I will 

 mention only one example, for which purpose I choose the 

 excellent survey of the mollusca of Arctic Norway by G. O. 

 Sars, recording the geographical distribution of 1 74 species of 

 lamellibranchs and 366 species of gasteropods. 



Of the 174 lamellibranchs no less than 128 or 74 per cent 

 were known also from Great Britain ; 119 or 70 per cent from 

 the Mediterranean, and 56 or 32 per cent from boreal North 

 America. 



Of 366 gasteropods found in Norway, 225 or 62 per cent 

 were also known from Great Britain ; 133 or 36 per cent from 

 the Mediterranean, and ']^ or 23 per cent from the coasts of 

 boreal North America. A great many species of molluscs have 

 been taken in the Mediterranean as well as in Norway, and 

 quite a number of forms are common to the faunae of Norway 

 and of North America. 



Examining the conditions in various parts of the coast of 

 Norway, we see that the Mediterranean species rapidly decrease 

 in number as we go north from western Norway, for instance, 

 from the latitude of Bergen towards the North Cape. While 

 119 lamellibranchs and 133 gasteropods are common to the 

 Mediterranean and Southern Norway, Northern Norway and the 

 Mediterranean have only 49 lamellibranchs (28 per cent) and 35 

 gasteropods {10 per cent) in common. Also south of the 

 Mediterranean we find a similar decrease in the number of 

 species common to both areas ; thus only 5 species of lamelli- 

 branchs and 4 species of gasteropods are common to Madeira 

 and Northern Norway. 



1^ A thorough understanding of the distribution of different 

 animals, or of the different animal-communities, is, however, not 

 obtainable by m.eans of records of this kind, for it makes a 

 world of difference whether a few specimens of a species have 

 been found in a certain locality or whether it lives there in 

 great quantities. A complete knowledge of the distribution of 



