PELAGIC ANIMAL LIFE 617 



are very strictly limited to an intermediate layer situated at 

 a depth of about 300 metres. A closer investigation showed 

 that the individuals captured at a depth of 150 metres 

 were all caught at night. This may be due either to an upward 

 nocturnal wandering or to chance, though on this question the 

 small amount of our material makes it unsafe to hazard an 

 opinion ; in subsequent investigations, however, it will be 

 worth while taking this fact into consideration. Among the 

 individuals captured at 500 metres there must, at any rate, be 

 a few that were taken in the process of hauling in the young- 

 fish trawl through the intermediate layer above, though the 

 majority probably lived at that depth — a deduction supported by 

 the fact that far fewer specimens were found in the young-fish 

 trawl towed at 1000 metres, which may have been captured 

 while hauling in. 



This instance is a good illustration ot our method with its 

 advantages and deficiencies. Clearly the method is trustworthy 

 only in cases where many specimens have been caught. At 

 the same time, it is the only effective method of capture known 

 at present, and it is therefore interesting to inspect the results 

 obtained. 



The distribution of different animal - communities in the 

 ocean rarely coincides with what seem to be natural distribu- 

 tional areas. The fact is that the occurrence of animals is 

 largely influenced by such conditions as depth and temperature. 

 In Chapter VII. we have seen that the limit between the 

 southern and the northern bottom-fishes did not coincide with 

 the border-line between the Atlantic Ocean and the Norwegian 

 Sea, but ran from Ireland or the Channel to Iceland, and 

 thence to the coast of the United States. In the case of 

 pelagic animals we may also distinguish between southern or 

 Atlantic communities and northern communities, the border- 

 line between these two communities very nearly coinciding 

 with the line separating the corresponding communities ol 

 bottom-fish. 



A. The Atlantic Pelagic Communities 



There is a striking difference between the pelagic faunas of 

 the open ocean and of the coast banks. In the open sea we 

 find different pelagic communities according to the different 

 conditions presented at various depths, and by way of introduc- 

 tion it may be useful to inspect the aggregate catches of a 



