84 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



ill spots where full daylight could penetrate. Thus we find 

 ourselves driven, by the facts here adduced and numberless 

 others, to this question : What are the various causes which can, 

 or must, first occasion eyes to be developed, or conduce to. their 

 preservation or destruction? A precise answer to this ques- 

 tion, unfortunately, is impossible in the absence of all experi- 

 mental data ; but we, as zoologists, may allege the difficulty — 

 indeed, the impossibility — of such experiments, as a sufficient 

 excuse for their never having been hitherto carried out. 



In cases like this, where we are not in a position to treat a 

 physiological question experimentally, we must be allowed to 

 construct a hypothetical explanation of the observed phenomena. 

 I therefore consider myself justified in mentioning a very preg- 

 nant hypothesis, which was put forward some little time since 

 to account for the presence of animals that can see in the deepest 

 parts of the ocean, where positively not a ray of light can pene- 

 trate from above. 



It is not very long since it was universally believed, in 

 accordance with the too rapidly drawn inferences of Edward 

 Forbes, that all animal life ceased on the floor of the ocean at 

 the level where rays of light cease to penetrate (at a few hundred 

 metres). But it is now well known that even highly developed 

 animals live at the enormous depth of from two to three thou- 

 sand fixthoms in both the Pacific and Atlantic Oceans. We 

 have become acquainted, principally through the incessant 

 labours of English, American, and Norwegian naturaUsts, with 

 a wonderful deep-sea fauna, showing the same striking mixture 

 of blind and seeing animals as the fauna of the caverns.^' This 

 case is all the more puzzling, because the chief part of such 

 deep-sea animals as can see are extraordinarily unlike their 

 nearest congeners living at the surface and in the light, so that 

 we are forbidden to suppose that they may be species that have 

 only lately migrated from the surface to great depths ; indeed, 

 it admits of scarcely a doubt that the deep-sea animals that can 

 see are very ancient forms, the survivors of past geological 

 periods. MacCulloch and Dr. Coldstream suggested a pleasing 

 hypothesis in explanation of these striking facts, which was 

 afterwards taken \\p and extended by the naturalists of the 



