DYNAMICS OF DISTRIBUTION 



349 



in relation to the sea bottom. If we consider its motion in relation to the bottom our 

 task will be more complicated ; we will first of all consider its motion in relation to 

 one of the layers, the surface, and 

 then if we wish to consider it in 

 relation to the bottom we can, know- 

 ing the speed and direction of the 

 surface layer, make a simple adjust- 

 ment. It follows then in Fig. 179 that 

 when we observe from a point S 

 within the surface layer IFthe layers 

 X, Fand Zwill be moving in relation 

 to the surface with increasing speeds 

 of 1, 2 and 3 units per day in the opposite direction to their actual motion, i.e. they are 

 being left behind by the point of observation in the surface layer. Such a system, with 

 four water layers W, X, Y, Z, observed from the point S, is shown in Fig. 180. In this 

 figure let us suppose that the density of the phytoplankton in the surface layer varies. 

 Between A and B let there be a moderate concentration producing a vertical migration 

 in the organism a in the layers W and X only. Starting our observations at midnight 

 a will follow the path a,a u o 2 ,a 3 'm the first three days. Now let us suppose that at B 

 A B C D 



Fig. 179. 



Fig. 180. 



B 



D 



Fig. 181. 



the phytoplankton concentration increases, so that a is caused to migrate during the day 

 into layer Y; it will now follow the path a 3 , # 4 , a 5 , a 6 to C. Now let us suppose there is 

 a further increase in the phytoplankton making the organism a migrate into the layer Z, 

 so that it now follows the path a G , « 7 , a s , a 9 . If now we consider the same set of con- 

 ditions but consider three organisms spaced out at a, b and c in Fig. 181 ; in three days' 

 time they will be at a 3 , b z and c a , and in six days' time they will be at «„, b 6 and c 6 . 

 Now suppose the water mass X is travelling at 2 units of distance a day faster than W, 

 and Y 2 units of distance faster than X, and Z 2 units of distance faster than Y, then 

 between A and B the organisms a, b and c will be one unit of distance apart (because for 



45 



