352 DISCOVERY REPORTS 



flowing in the direction EF. Let Fig. 187 represent moderate phytoplankton condi- 

 tions, and let us observe at midnight an organism a migrating between X and Y 

 then it will take up positions a ly a 2 , a 3 in the surface layer at the end of one, two and 

 three days respectively. In Fig. 1 88 .a 



let the area of moderate phyto- 

 plankton M be separated from the 

 region of dense phytoplankton D 



M 



•a 2 



■K 



B a s 



* 



F <- 



by the line JK, and let the dense ..--'' a 3 



phytoplankton cause the organisms ..--'si 



to migrate deeper into the water 

 mass F, so that they travel faster 

 in the direction of EF. The or- 

 ganisms a would now take up the 

 positions «j , a 2 , « 3 under the region Fig. 1 88 . 



M, but would move to « 4 and o b 



under the region D. Again organisms would become fewer per unit of surface area under 

 regions of dense phytoplankton than under regions of moderate phytoplankton. Now 

 if we pass from a region of moderate phytoplankton to one in which the phytoplankton 

 is so poor that the zooplankton organisms, according to the hypothesis, can remain in 

 the surface layer, then we shall get a belt of organisms along the line JK between the 

 two regions, just as we did in the example of currents going at different speeds in the 

 same direction. 



Now let us return to the consideration of the more usual conditions of currents 

 flowing in the same direction at different speeds, and look at the matter more closely. 

 So far in our diagrams we have only considered the movement of single organisms or of 

 several organisms which are separated by distances equal to those which they travel in 

 the course of 24 hours. Let us consider what will happen to a number of organisms close 



A B C 



Start of observations .abcdefghijklmnop 



A B 



After 24 hours , ,/,-■,■ — 



^ abcdejglujklmnop 



A B 



After 48 hours ... , 



T ubcdejgnijkl 



mnop 



A B 



After 72 hours 



abcdefgh ijkl mnop 



Fig. 189. 



to one another. In Fig. 189 let the line ABC be the surface of the sea, and the portion 

 AB represent a region of moderately low phytoplankton, and BC a region of com- 

 paratively high phytoplankton. Let abcde...p represent a number of organisms below 

 the surface at midnight when we start our observations. Now suppose the phyto- 



