94 SEA FISHERIES 



Denmark on the right and the Shetlands on the left ; the 

 bottom of the urn is thus on a level with the Dogger. 

 This is the zone of the green algae of the Halosphaera 

 genus. Then inside this urn-shaped area is a smaller 

 area of similar shape, but without a foot; there is the 

 zone of Peridinaceae, of the Coscinodiscus family. Finally, 

 filling up the second urn is a homogeneous mass of 

 Peridinaceae of the Ceratium family. 



It will, of course, be understood that Cleve's charts 

 represent an approximation, an average ; in reality plank- 

 ton is subject to annual variations. These variations 

 are ushered in by a superficial sterility of the waters. 

 Among their causes, in the spring, is the rush of the 

 Atlantic waters, which at this season turn towards the 

 northern hemisphere along the eastern shores of the 

 ocean ; in autumn, the extension of the Gulf Stream 

 towards European waters ; and in winter, the flow of 

 the arctic waters, which make towards the south-east. 



VII 



In the course of these pages I have often had occa- 

 sion to speak of the formation of plankton. I must now 

 indicate the principal conditions of its formation. 



Shallow seas are richer in plankton than deep seas, and 

 the seas of the tropical and the temperate zones are on 

 the whole poorer than the polar seas, although the con- 

 trary might be expected from the analogy of terrestrial 

 vegetation. The cold currents are thus more favourable 

 to the development of plankton than are the warm 

 currents. In general, at the limits of both hot and cold 

 currents, there are considerable agglomerations of 

 plankton. 



The fact is that between the two ascending currents 

 are produced, which are the best agents of pelagic 



