220 DISCOVERY REPORTS 



perennially more fertile than others. In this event, the poverty of phytoplankton at such 

 localities as Callao, Arica and Pichidanque Bay may be interpreted, not only as an 

 indication of conditions in the immediate past, but also perhaps as evidence of the 

 inflow of barren oceanic water in regions of convergence. 



If vertical currents are important as a source of production, horizontal currents may 

 be no less important in collecting plankton together in patches and thus in providing 

 the larger animals with a feeding ground. With reference to the vertical migration of 

 plankton animals, Hardy (1935) has shown the possibilities in navigation open to an 

 organism migrating regularly between two currents. By this mechanism he has shown 

 how animals might be collected together in patches, disperse and perhaps migrate 

 towards food or away from an uncongenial environment. 



Fig. 67. Hypothetical diagram of a surface eddy. (After Hardy.) 



In this connection the collection of animals in the eddy off the Lobos Islands becomes 

 of great interest. Hardy gives two hypothetical cases of currents which would collect 

 organisms together without their having to alter their migrational rhythm. The first is 

 of an eddy situated over another current. We are indebted to him for permission to 

 reproduce his illustration in Fig. 67 ; the lower current is represented by fine broken 

 lines and the paths of two hypothetically migrating organisms A and B which are carried 

 to the positions A' and B' are shown by heavy lines. Inside the Lobos Islands, a 

 surface layer of warm saline water is strongly suggestive of an eddy lying above the 

 main current: and here the collection of thick zooplankton, of anchovy, bonito, birds, 

 seals and whales, is probably to be identified in principle with the theoretical con- 

 siderations Hardy has sketched. 



