24 THE DEPTHS OF THE SEA 



our purpose. This limit separates the surface waters, 

 which are permeable by the light of the sun and 

 in which owing to this life-giving light, algce and 

 vegetable organisms can live, from the deeper waters 

 which the sun's rays cannot reach, and in which no 

 plant can live. The regions pass imperceptibly into 

 one another ; there is no sudden transition. The con- 

 ditions of life gradually change, and the precise level 

 at which vegetable life becomes impossible varies 

 with differing conditions. With strong sunlight and 

 a smooth sea, the rays penetrate further than if the 

 light be weak and the waters troubled. 



Speaking generally, we may place the dividing-line 

 between the surface layer and the deep sea at 300 

 fathoms. Below this no light or heat from the sun 

 penetrates ; and it is the absence of these factors that 

 gives rise to most of the peculiarities of the deep sea. 

 It is a commonplace, which every schoolboy now 

 knows, that all animal life is ultimately dependent on 

 the food-stuffs stored up by green plants ; and that 

 the power which such plants possess of fixing the 

 carbonic acid of the surrounding medium, and building 

 it up into more complex food-stuffs, depends upon the 

 presence of their green colouring matter (chlorophyll), 

 and is exercised only in the presence of sunlight. 

 But, as we have pointed out, ' the sun's perpendicular 

 rays' do not 'illumine the depths of the sea'; they 

 hardly penetrate 300 fathoms. This absence of sun- 

 light below a certain limit, and the consequent failure 

 of vegetable life, gave rise at one time to the belief 

 that the abysses of the ocean were uninhabited and 

 uninhabitable ; but, as we have already seen, this view 

 has long been given up. 



The inhabitants of the deep sea cannot, any more 

 than other creatures, be self-supporting. They prey 

 on one another, it is true ; but this must have a limit, 

 or very soon there would be nothing left to prey upon. 



