DYNAMICS OF DISTRIBUTION 



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animals, the unbroken parts of the lines being their paths in the surface currents, and 

 the broken parts their paths when below. If we start to observe an animal at A it will 

 follow more or less the path to A' and B to B'. We see that at some points in their 

 journev they may be carried farther apart than at others. The distance travelled in the 

 surface current is shown as approximately equal to that travelled in the lower current. 

 The surface current will likely be travelling faster than the lower one ; but we have 

 seen that the animals usually 

 spend a shorter time in the 

 surface layer and a longer time 

 below. C will take the path 

 to C" and E to E' . So we could 

 pick out numerous examples of 

 the possible movements of mi- 

 grating animals from different 

 parts of the region ; it must be 

 remembered that these con- 

 siderations are theoretical and 

 based on the assumption that 

 the range of migration remains 

 more or less constant. 



We will consider two rather 

 more special examples. Fig. 173 

 shows in a somewhat idealized 

 form for the purpose of the 

 diagram a surface swirl such 

 as would seem to exist at the 

 time of our survey to the west of the northern end of our island. It is not necessarily 

 a vortex descending in the centre, but perhaps rather a disc of surface water rotated 

 by the moving water masses at its periphery. The fine broken lines show the likely 

 movement at 150 m. Now our vertically migrating animals will follow paths from A to 

 A' and from B to B'. 



If a current in the southern hemisphere meets an obstruction it will tend to be curved 

 round to the left ; thus when two currents meet they may be deflected round in opposite 

 directions. Now if the water of one current is perhaps warmer or less saline than the 

 other, and so slightly less dense, it may, as it curves round, partly overflow the other, 

 giving rise to a condition as shown in Fig. 174. This diagram, for demonstration, is again 

 ideal rather than usual. Such completely symmetrical conditions may rarely be met 

 with, but frequently they may approach it. In this ideal diagram an animal starting at 

 A would pass to A' and back to A again, and one at B to B' and back to B again. It 

 will be noted that the animals are shown as travelling longer or shorter distances for 

 each sojourn in the upper or lower layers as they pass out towards or inwards from the 



Fig. 173. 



