THE AGE OF FISHES 185 



ever, lies in another direction. The exceedingly small plants 

 and animals that make up this drifting life are usually scat- 

 tered throughout the water so that the task of concentrating 

 them profitably, in large quantities, by filtration or otherwise, 

 has so far resisted the inventiveness of man. The truth is that 

 whales and fishes are so much more efficient than we are in 

 collecting plankton and turning it into edible food that there 

 is little immediate likelihood of man improving on nature. 



There is still a further step in the long food chain that con- 

 nects food fishes with sea water and the Ocean River itself: 

 this lies in the invisible food needed as fertilizer for the small 

 plant cells. Plants on land send down their roots into the soil 

 and thus extract the chemical salts of nitrogen and phos- 

 phorus, the other fertilizer elements, and the minor growth 

 elements needed to build up their tissues; only in the soil do 

 they find such material. In the ocean, though, there is no need 

 for food-seeking roots; the salt water is a gigantic hydroponics 

 system, a bath of liquid fertilizer. 



The chemical salts of sea water vital to the nourishment of 

 plants — particularly the phosphorus and nitrogen compounds 

 — are dissolved in very small concentrations, and are easily 

 used up. The solutions used for hydroponics farming become 

 weakened as the plants extract the chemicals for growth; the 

 goodness of the soil is removed by excessive cropping so that 

 it becomes impoverished unless periodically fertilized; and the 

 ocean waters in like manner lose to the plankton their food 

 chemicals. We have already mentioned the constant rain of 

 dead plankton from the surface waters to the bottom. At in- 

 creasing depths, by the inexorable processes of decay, this con- 

 tinual rain of microscopic bodies is turned back into fertilizer 

 salts, just as a gardener turns kitchen refuse into fertilizer in 

 his compost heap. Thus chemical pabulum is removed from 

 the surface and accumulates in the deep, cold waters — the 

 chemical reservoirs in the oceanic hydroponics system. 



