THE SMALL GAME OF THE SEA 



187 



they inhabit. The temperature at 300 fathoms off the west of 

 Ireland is 50° F. — ' the temperature which marks the southern 

 hmits of the northern forms ' in those waters. So ' vertically 

 as well as horizontally the fauna termed by me the southern 

 one appears to exist within the same limits of temperature. 

 The different species appear to be at hberty to move within 



Fig. 20. — The temperature of the North Atlantic at a depth of 55 fathoms. 

 (Drawn by Helland-Hansen and reproduced from The Depths of Ocean by per- 

 mission.) Note : the 50° line in this map is the southern boundary of the 

 northern marine animals on both sides of the Atlantic. (After Hjort.) 



these limits and to be independent of depth. Thus there are 

 many observations showing that the southern species occur in 

 deeper waters on the Atlantic slope than they do in the North 

 Sea. This is easy to understand because in the North Sea 

 only the shallow upper layers are affected by high tempera- 

 tures. . . . On the shallow coast banks and on the Dogger 



