DYNAMICS OF DISTRIBUTION 355 



continue at its original velocity. Now a vertically migrating organism viewed from out- 

 side the system will follow the continuous curving line in the diagram; in the region 

 AB for each ascent it makes it will be carried for a greater distance than formerly. The 

 number of such animals below each unit of surface area in the region AB will be thus 

 reduced, and the phytoplankton might be allowed to increase in this region. The 

 same effect will be produced by an acceleration of the lower layer in relation to the 

 upper layer. 



In Fig. 192 let a body of water be similarly travelling from left to right, but now let 

 the speed of the lower layer X be reduced to the region AB, perhaps on approaching a 



A B 



Fig. 192. 



continental slope. The continuous curving line will now represent the path of a 

 vertically migrating animal, and it will be seen that the numbers of such an animal below 

 each unit of surface area in the region AB will be increased. A similar effect will be 

 produced by a reduction in speed of the surface layer. If, as is usually so, the animal 

 spends a longer time in the lower than in the surface layer, the retardation of the lower 

 layer will produce a greater concentration of animals than will a retardation of the 

 surface layer. 



In these propositions we have considered animals with an unchanging vertical 

 migration ; if the vertical migration becomes altered at the same time as an acceleration 

 takes place in one of the layers, the effect on distribution will simply be increased or 

 diminished according to whether the change causes the animal to remain for longer or 

 shorter in the accelerated or retarded layers. 



As bodies of water approach an island such as South Georgia — or any land mass — or 

 as currents meet in mid-ocean, such regions of acceleration and retardation may possibly 

 occur and give rise to aggregations of a number of different species which have a similar 

 vertical distributional habit, and to areas of comparative scarcity of animals in which 

 phytoplankton might flourish. But it would seem that the separation and aggregation 

 of animals caused by changes in current velocity may be very local and temporary in 

 character, and whilst contributing to the general effect, it is thought that they would 

 hardly give rise to the correlations of zooplankton with phosphate values, which suggest 

 the gradual adjustment of distribution with the phytoplankton over a considerable 

 period of time. 



The object of submitting the propositions in these two sub-sections is to show what 

 a potent agent vertical migration may be in the determination of plankton distribution, 



