82 LIFE IN THE SEA [CH. 



of the Norwegian Sea. At the bottom of the latter 

 is a stratum of cold Arctic water and overlying 

 this is the warmer and salter water from the Gulf 

 Stream, while between these strata is a layer inter- 

 mediate between the two in character, and this is 

 the region of optimal conditions for the fish. Each 

 of the principal strata of water shifts periodically with 

 the rhythmic movements of the Gulf Stream and 

 Arctic current, and the fish migrate so as always to 

 remain in the mixture of water from the two sources. 

 A bottom-living, sessile animal is unable to respond 

 to changes in the physical conditions by migrations ; 

 it can only do so by changes in its own mode of 

 metabolism. It can, however, distribute itself over a 

 wide area by reason of the fact that it produces free- 

 swimming larvae which are carried in the currents of 

 the sea. But a nektic organism which usually also 

 produces free-swimming larvae must generally have 

 a wider range of distribution than a bottom-living 

 fixed one, since it can migrate so that it will always 

 remain within the area of optimal conditions. It is 

 usual nowadays to describe the movement of an animal 

 as a 'tropism' or ' taxis/ The tropistic theory is the 

 result of the modern tendency to look upon all the 

 activities of an organism as the results of known 

 chemical or physical laws. A taxis is some movement 

 on the part of an organism which follows inevitably 

 on some external stimulus, unless it is prevented by 



