68o DEPTHS OF THE OCEAN 



that only becomes luminous on the surface of the animal, the 

 phosphorescence seems to present an analogy or likeness to 

 certain chemical reactions, for instance, the slow oxidising of 

 organic compounds (grape sugar, etheric oils), which are accom- 

 panied by a feeble emission of light. In higher specialized 

 organs chemical processes of a more complex nature probably 

 take place. From the structure of the organs we may be 

 induced to believe that the development of the organ must 

 have depended on the fact that its function was intended to be 

 seen by an eye. The light emission must evidently be of 

 vital importance to the life of the animal and to the maintenance 

 of the species. The discussion of these questions must there- 

 fore be postponed until we have mentioned the eyes of the 

 different animals. 



Eyes 



Nothing has appeared more hopeless in biological oceano- 

 graphy than the attempt to explain the connection between the 

 development of the eyes and the intensity of light at different 

 depths in the ocean. In a trawling from abyssal depths in the 

 ocean we may find fishes with large eyes along with others with 

 very small eyes or totally blind. Nowhere would a more perfect 

 uniformity be expected than in the dark and quiet depths of the 

 ocean, Brauer, who has given a valuable contribution to our 

 knowledge of the eyes of deep-sea fishes, remarks in his treatise 

 on the fish collections of the " Valdivia " Expedition: "If the 

 surroundings really acted directly on the organisms, and were 

 the only agents which could produce alterations, their influence 

 would be much more uniform and general. Instead of this we 

 find the greatest variation. Thus we find the eyes now 

 altered or permutated, now highly differentiated even in closely 

 related forms," 



The conditions, however, where these different forms live, 

 are not so uniform as was supposed, or rather, these forms do 

 not really live under the same conditions. First of all it made 

 a great difference when we learnt that certain fishes were bottom 

 dwellers and others pelagic in their habits. 



Most, if not all, bottom dwellers from abyssal depths have 

 large eyes, very often larger than those of bottom fish living 

 in the strong light of the coast banks. Perhaps there is a 

 maximum in the development of eyes in bottom fish at a certain 

 depth followed by a decrease in size as we proceed still deeper. 

 But even the deepest living forms, which must be supposed to 



