12 THE VOYAGE OF H.M.S. CHALLENGER. 



is in perfect harmony, for these minute organisms multiply with extraordinary rapidity 

 in positions illuminated by and exposed to the sun, but abound less in shady places and are 

 never found to vegetate in darkness. Hence in the great depths of the sea, to -which light, 

 because of absorption, cannot penetrate, only dead valves of Diatoms will be found. 



I have mven at length in another communication ' arguments based on the existence 

 of Diatoms in the stomachs of abyssal Echini, which seem to me to show that these 

 organisms might possibly live at depths of 1340 fathoms, and that sunlight might 

 penetrate to greater depths than at present supposed. Mr. Murray, however, informs me 

 that it was observed during the Expedition that the great majority of deep-sea animals 

 lived by eating the surface layers of mud or ooze forming the bed of the ocean, which 

 usually contained a large number of the remains of surface animals and Diatoms ; his 

 opinion is that a small amount of organic matter is carried down with the dead shells and 

 frustules from the surface waters where the organisms lived, and that the deep-sea animals 

 obtain their nutriment from it. Had Diatoms been observed in the alimentary canal of 

 fishes, they might have been obtained at less depths. As, however, Echinodermata live 

 only on the bed of the ocean itself, it must be regarded as very probable that these 

 frustules formed a portion of the food of these animals. 



The significance of such observations in connection with deep-sea life is of the highest 

 value, as tending to explain the existence of animals in a normal state at the greatest 

 depths — a fact that has already been recognised by many marine scientific expeditions 

 organised by Great Britain, the United States, and other nations. The existence of life 

 in darkness must be regarded as exceptional, although there are numerous and well-known 

 instances of rare and blind animals (such as Proteus) which inhabit caves into which no 

 light can penetrate. 



On the other hand, the facts that the eyes are fully developed in some of the animals 

 that inhabit the deep sea, and that they possess beautiful and brilliant colours, seem 

 necessarily to imply the presence of light in these abysses, since, in its absence, the organs 

 of vision should become atrophied, and the colouration be more or less dark. 



These conclusions have been universally accepted, but the belief that light could not 

 penetrate to such depths on account of absorption in its passage through the water afforded 

 ground for advancing hypotheses to explain the facts, the reality of darkness at the 

 bottom not being doubted. 



Among these hypotheses it has been held that the phosphorescent light given off by the 

 bodies of abyssal animals is sufficient to make up for the absence of sunlight. Although 

 the weight of this suggestion is very great, it may be asked — Can marine animals 

 emit light when alive and in their normal state ? Fishes, Crustacea, and other marine 

 creatures are phosphorescent when they are dead or at least out of their proper medium. 



' Nuove Osservazione sulla Profundita cui giunge la Vegetazione delle Diatomee ncl Mare. — Mem. Pont. 

 Accad. d. Nuov. Lined, voL i. 1885. ; 



