GENERAL BIOLOGY 701 



organisms may be supposed to have been adapted to certain 

 external conditions prevailing in the water-layers which surround 

 them. But these water-layers are not stationary, and the con- 

 ditions in a certain water-layer may change in many different 

 ways from time to time. These changes alter the habitat of 

 the animals and cause active or passive migrations. The 

 study of these migrations is specially interesting as showing 

 the influence of physical conditions acting upon the animals. 



From time immemorial it has been known that many Daily vertical 

 animals ascend at night to the surface of the ocean. Fisher- '"^srations. 

 men have during ages turned this knowledge to advantage in 

 setting their drift-nets at night at the surface of the sea to 

 capture the herring. Recently it has proved possible to trawl 

 successfully for herring along the sea-bottom, but only during 

 the daytime. All sailors can tell us that at night great 

 numbers of animals gather in the surface waters, which are 

 never seen there in the daytime. An interesting instance of 

 this was mentioned in Chapter IX. While fishing with long-lines 

 on the Faroe banks our lines were set for cod along the bottom 

 in about 200 fathoms ; the lines were hauled at night, and the 

 stomachs of the cod contained squids, which had been eaten 

 during the day, while at night numerous squids were seen at 

 the surface darting into the glare of our electric lamp hanging 

 over the side. Most fishermen have had similar experiences. 



A certain amount of information has also been gathered as 

 to the vertical migrations of minute pelagic organisms moving 

 towards the surface at night. Chun especially has investigated 

 the extent of these migrations, and found that the majority 

 of small pelagic organisms migrate generally within a vertical 

 range of 30 to 50 metres. Steuer draws attention to the fact 

 that vertical migrations very rarely involve all the pelagic forms 

 of a locality ; at all events they do not migrate in the same 

 manner, for there are many transitions between forms which 

 only retreat vertically during a few hours in the daytime, and 

 forms which rise only during the darkest nocturnal hours. If 

 the forms were large enough to be seen in the water, we should 

 "by day as well as by night be able to observe a continuous 

 rise and fall of organisms. Only during the day we should 

 see a larger congregation in deeper water, and at night at 

 the surface." ^ 



Some instances of the difference plainly observable in our 

 catches by day and by night have already been mentioned 



1 Steuer, op. cit. 



