THE OCEAN FEEDING-GROUNDS 95 



pullulation; and this takes us back to the vertical 

 currents of Nathansohn. Wherever these currents exist, 

 there plankton abounds. The Straits of Messina are 

 celebrated in this respect. The Irminger Sea is the 

 richest in plankton of all the waters visited by the 

 " Plankton Expedition." In the Atlantic, almost under 

 the equator, although the temperature of the water is 

 over 77, there are enormous accumulations of plasmon. 

 The Challenger, between the Admiralty Isles and the 

 Carolines, and Hawaii and Tahiti, gathered enormous 

 quantities of diatoms in the warm superficial layers of 

 the sea. The waters to the north of Ascension exhibit 

 the same wealth of organisms ; and they, like all the 

 regions mentioned, excepting the last, occupy the centre 

 of a cold ascending current. I repeat that cold water 

 is necessary to the formation of plankton, but it is not 

 absolutely indispensable, and the action of a single 

 vertical current often suffices. The explanation of 

 this phenomenon is simple. The struggle for life is 

 more intense at the surface ; hence a dearth of 

 alimentary material. But let us take the case of a 

 vertical current : we find that alimentary matter which 

 has sunk elsewhere is thrown up to the surface once 

 more, and thus makes the surface habitable. I must 

 draw attention to two important conditions relating to 

 the lines of encounter and separation of tidal currents 

 in the open sea, and to suspended alluvial matter. It 

 may be laid down as a general thing that wherever two 

 ocean currents come together or separate there will be 

 abundance of plankton. I have verified this law in the 

 Channel, and in this connection I have carefully located 

 the lines of encounter and separation of the Channel 

 currents (p. 80). We see, then, that plankton does not 

 abound in extremely rapid currents, and are able to 



