DISTRIBUTION OF ANIMAL LIFE 25 



Like the inhabitants of great cities, the denizens of 

 the deep must have an outside food-supply, and this 

 they must ultimately derive from the surface layer. 



The careful investigation of life in the sea has shown 

 that not only the surface layer, but all the intermediate 

 zones teem with life. Nowhere is there a layer of 

 water in which animals are not found. But, as we have 

 seen, the algce upon which the life of marine animals 

 ultimately depends, live only in the upper waters ; 

 below 100 fathoms they begin to be rare, and below 

 200 fathoms they are absent. Thus it is evident that 

 those animals which live in the surface layers have, 

 like an agricultural population, their food-supply at 

 hand, while those that live in the depths must, like 

 dwellers in towns, obtain it from afar. Many of the 

 inhabitants of what may be termed the middle regions 

 are active swimmers, and these undoubtedly from 

 time to time visit the more densely peopled upper 

 strata. They also visit the depths and afford an in- 

 definite food-supply to the deep-sea dwellers. 



But probably by far the larger part of the food con- 

 sumed by abysmal creatures consists of the dead 

 bodies of animals which sink down like manna from 

 above. The surface layers of the ocean teem with 

 animal and vegetable life. Every yachtsman must at 

 times have noticed that the sea is thick as a puree with 

 jelly-fish, or with those little transparent, torpedo- 

 shaped creatures, the Sagitta. What he will not have 

 noticed, unless he be a microscopist, is that at almost 

 all times the surface is crowded with minute organisms, 

 foraminifera, radiolaria, diatoms. These exist in quite 

 incalculable numbers, and reproduce their kind with 

 astounding rapidity. They are always dying, and 

 their bodies sink downwards like a gentle rain.* In 



* Owing to the comparative absence of bacteria in deep-sea 

 water their bodies undergo little decay. 



