148 ELEMENTS OF BIOLOGY. 



reason to conclude that the sun's rays can at most but 

 penetrate to a depth of a few hundred feet below the sur- 

 face of the sea. In the absence, therefore, of any positive 

 knowledge, it was a justifiable conclusion that animal life 

 could not extend to very great depths in the ocean, since 

 vegetable life would of necessity be absent. In the deep 

 sea, however, we find an assemblage of animals, not essen- 

 tially different from those of shallow seas, living without, or 

 almost without, vegetable life of any kind. A few micro- 

 scopical plants there may possibly be ; but unquestionably 

 there is nothing that could for one moment be regarded as 

 supplying vegetable food to any considerable number of 

 animals. The question then arises, How do these animals 

 support existence ? Some, of course, feed upon the others ; 

 but, equally of course, this must have a limit ; and there 

 must be some which have the means of obtaining food in 

 some other manner. Two explanations have been put 

 forward to account for this singular fact. On the one hand, 

 it has been thought that some of the- deep-sea animals 

 might perhaps have the power possessed by plants, of 

 taking inorganic substances from the surrounding medium, 

 and building up these into the matter of life. This theory, 

 if provable, would be all that is needed ; because then, in 

 point of fact, some of the animals of the deep sea, as 

 regards their mode of feeding, would be really plants, and 

 thus the balance of organic nature would be maintained in 

 equilibrium. This, however, is a mere hypothesis ; and it 

 has been shown, on the other hand, that the sea-water at 

 great depths holds in solution a very much larger propor- 

 tion of organic matter than is normally present ; so that it 

 may practically be regarded as a very dilute soup. It has 

 therefore been suggested, with great probability, that the 

 lower forms of life in these abysses can support life solely 

 upon this dissolved and soluble organic matter. 



Thirdly, it might have been reasonably anticipated that 

 the water at great depths in the ocean would have been 



