GENERAL BIOLOGY 685 



larval stages living at the surface to the adult fishes of the deep 

 sea, presents a vast field for future research and opens up a 

 vista of possibilities, which may explain the adaptation to special 

 surroundings peculiar to each species. 



Investigations in the deep regions below 500 metres 

 should evidently, first of all, attack the questions whether a 

 regular decrease in the size of the eye occurs with increasing 

 depth, and whether the number of blind species and blind 

 individuals is not far greater than is generally supposed. Our 

 pelagic hauls only exceptionally went below 1500 metres, 

 but nevertheless we found in the deepest hauls no less than 

 three species of blind fishes, of which two were new to science, 

 besides one blind squid. In the deep oceans, where the depth 

 exceeds 5000 or 6000 metres, we might perhaps expect inter- 

 esting discoveries if large and efficient appliances were towed 

 after the vessel with 5000 or 6000 metres of wire out. 



But if it be the case that the size of the eyes in pelagic Large eyes in 

 fishes decreases vertically with the decreasing intensity of light, fromTheS-^ 

 how can we explain the fact that the bottom-fishes, like Alacrtci^us bottom. 

 arniatus, living in abyssal depths possess large and apparently 

 well-developed eyes? In order to explain this, the possible 

 existence of a source of light other than sunlight has been 

 sought for, but nothing has so far been discovered beyond the 

 light produced by the organisms themselves. We shall therefore 

 have to consider at the same time the power of emitting and the 

 power of perceiving light possessed by the animals, so that we 

 must take their light-organs as well as their eyes into account. 



From what has been said we see that a remarkable 

 coincidence exists between the development of light - organs 

 and eyes in pelagic fishes. The Scopelidae, Sternoptychidse, 

 and Stomiatidae, which live above 500 metres, possess well- 

 developed light-organs and eyes, while from 500 metres down- 

 wards light-organs and eyes both decrease in size. 



Along the sea-bottom, however, the fishes possess only eyes Abyssal 

 and no special light-organs. We have previously seen that h°"°e'es^b?t 

 the invertebrates are luminous even in abyssal depths, and at no light- 

 present the large eyes of the bottom fishes cannot be explained °''§^"^- 

 otherwise than by supposing that the light emitted by the 

 invertebrate bottom animals is so strong that objects on the 

 bottom may be seen by the eyes of fishes. As regards most of 

 the bathypelagic fishes we may, on the other hand, suppose that 

 they have little use for eyes, because pelagic life in great depths 

 is scanty, and not so definitely localized as on the sea-bottom. 



