N"o. 647] PROGRESSION OF LIFE IN THE SEA 



501 



enters the water, an amount which varies with the length 

 of the day, the altitude of the sun, and the clearness of 

 the air and of the water; (2) the presence in adequate 

 quantity of mineral food substances, especially nitrates 

 and phosphates; and (3) a temperature favorable to the 

 growth of the species which are present in the water at 

 the time. Experiments with cultures of diatoms have 

 shown clearly that if the food-salts required are present, 

 and the conditions as to light and temperature are satis- 

 factory, other factors, such as the salinity of the water 

 and the proportions of its constituent salts, can be varied 

 within very wide limits without checking growth. The 

 increased abundance of plankton, especially of diatom 

 and peridinian plankton, in coastal waters and in shallow 

 seas largely surrounded by land, such as the North Sea, 

 is due to' the supply of nutrient salts washed directly 

 from the land by rain or brought down by rivers. An 

 exceptional abundance of plankton in particular locali- 

 ties, which produces an exceptional abundance of all ani- 

 mal life, is also often found where there is an upwelling 

 of water from the bottom layers of the sea. These condi- 

 tions are met with where a strong current strikes a sub- 

 merged bank, or where two currents meet. Food-salts 

 which had accumulated in the depths, where thev could 

 not be used owing to lack of light, are brought by the 

 upwelling water to the surface and become available for 

 plant growth. The remarkable richness of fish life in 

 such places as the banks of Newfoundland and the Agul- 

 lias Banks off the South African coast, each of which is 

 the meeting-place of two great currents, is to be explained 

 in this way. 



Our detailed knowledge of the steps in the food-chain 

 from the diatom and peridinian to the fish is increasing 

 rapidly. The Copepod eats the diatom, but not every 

 Copepod eats every diatom ; they make their choice. The 

 young fish eats the Copepod, but again there is selection 

 of kind. Even adult fishes like herring and mackerel, 

 which were formerly sup])osed to swim with o\)on mouth, 

 straining out of the water whatever caiiK^ in xhr'w way, 

 are now thought largely to select their t'ood." 



22 Bullen, Jourii: Mar. Biol. Assoc., 9, 1912, p. 394. 



