GENERAL BIOLOGY 663 



Sailors know well the sky-blue colours peculiar to the tropical 

 surface forms. Herring-fishermen also know that the blackish- 

 brown back of the herring is almost invisible from above, and 

 only when occupying a slanting position or making a sudden 

 turn does the herring become visible, its mirror-like sides 

 emitting a silvery flash. The deep-sea fishermen are equally 

 acquainted with the dark, black, brown, violet, or red colours 

 peculiar to deep-sea animals. No scientist can claim the dis- 

 covery of these phenomena, which are as well known as the 

 colours of the ocean itself. 



When considering the peculiar colours of marine animals, 

 and their variation in different surroundings, many naturalists 

 concluded that the colouring was due to their attempts to adapt 

 themselves to the colours of their surroundings, in order to 

 make themselves invisible or to protect themselves against 

 enemies, just as is supposed to be the case with the land fauna. 



This idea requires confirmation by acquiring more exact 

 knowledge as to the conditions of light and the colours of animals 

 from similar depths. Our knowledge regarding the penetra- 

 tion of light in the ocean has been as deficient as our knowledge 

 of the vertical distribution of the animals, and the whole subject 

 has thus been a matter of suppositions and ideas rather than of 

 actual knowledge. 



During the Atlantic cruise of the "Michael Sars " we 

 investigated the intensity of light at different depths and also the 

 colours of the animals. The results obtained by the photometer 

 at a few stations in the Sargasso Sea are referred to on 

 pp. 251-2. On a sunny day when the water was perfectly clear Penetration 

 and transparent, light-rays of all colours, but very few red rays, °f^'§^'- 

 were observed at a depth of 100 metres. At 500 metres the 

 light acted strongly on the photographic plates, especially the 

 blue rays, but the green rays were absent ; even at 1000 metres 

 the influence of the sunlight could be traced on the plates, but 

 at I 700 metres no influence was noticeable. 



As we have seen in Chapter IX. the different water-layers Animals of 

 in the Sargasso Sea contain animals of very different colouring, t^he^ Sargasso 

 certain general features in the colouring being easily recognisable 

 in certain regions. In the hauls from 500 to 750 metres and 

 deeper we found only black fishes and red crustaceans (prawns). 

 At 300 metres we found the laterally compressed Sterno- 

 ptychidse with silvery sides and brownish backs. In the upper 

 layers we met with transparent young fish, for instance lepto- 

 cephali, or silvery ScopelidiDe and blue flying-fish. 



