248 THE CONDITIONS OF LIFE IN THE SEA [PART III 



(6) " North " and " South " fishes. Long ago Heincke and 

 Mobius^ divided the fishes of the Baltic into " north " and " south " 

 fishes. More lately Henking made a somewhat similar classifica- 

 tion. The terms are relative ones, and apply only to the general 

 habits of the fishes. South-fishes, or summer-fishes, are those 

 which inhabit the water of the European Stream and its offshoots, 

 or live in proximity to this water mass. The best examples of 

 these fishes are the garfish (Belone), the hake, the sole, the turbot, 

 and the summer herring. North-fishes, or winter-fishes, inhabit 

 the mixture of the Atlantic and Arctic waters. The best examples 

 are the cod, the ling, the halibut, and the winter herring. Fishery 

 statistics shew that the largest catches of these fishes are made in 

 the North Sea, at the times and places when the water of each 

 type predominates^. 



Sunlight. I have already indicated (in Chap. VII.) the manner 

 in which various marine organisms react towards changes in the 

 intensity of the light falling upon them. Such reactions may be 

 called " heliotropic," or "phototropic," and are really reflexes towards 

 changes in external stimuli, and may be compared with the 

 nervous process which impels a moth to dash into a naked flame, 

 or a sea-bird to fly against the windows of a lighthouse. The 

 crowding of the trochospheres of Phyllodoce against the best 

 illuminated side of the vessel of water which contains them ; 

 the similar behaviour of copepods, which has been observed both 

 in aquaria and in the sea ; and the distribution of Sagitta with 

 respect to the depth of water, or the amount of sunlight, are 

 probably instances of nervous reflexes. They are due to the 

 stimulus of the peripheral nervous system, and the conversion, in 

 the central nervous system, of the impulses thus generated into 

 motor impulses, which cause the organism to move towards, or 

 away fi:-om the source of light. 



It is quite different with regard to the behaviour of the marine 

 protophyta towards changes in the intensity of the incident light. 

 Perhaps one ought not to say that these organisms are not affected 

 by light in the same manner as that in which a copepod is affected, 



1 Bericht Untersuchung. deutschen Meeres, ii. p. 278, Kiel, 1877-81. 



2 Henking, Rappts. Proc.-Verb. vol. in. 1905. 



