DISTRIBUTION AND MIGRATIONS 261 



have their northern Hmit at about this latitude, which also 

 marks the southerly limit of the Salmon and Trout {Salmo) as 

 marine fishes. The isotherm of 6° C, marking the northern 

 boundary of the Temperate Zone, is not so satisfactory as a 

 limit of distribution, but it is fairly close to the northern hmit 

 of many of our own fishes, and marks the southern limit of the 

 typically Arctic Char [Salvelinus) as a sea fish. 



In the North Pacific the isotherms, although less irregular 

 than in the North Atlantic, have the same general arrangement, 

 being close together in the west and farther apart in the east. 

 Again, there seems to be a fairly close correspondence between 

 the temperature and the range of the fishes, and such regions 

 as the Bering Sea, Okhotsk Sea, and northern Japanese Sea, 

 each have their characteristic faunas. Certain subarctic 

 fishes are common to both Atlantic and Pacific, and in other 

 cases the genera are the same, but there is one species in one 

 ocean and a very closely related species in the other. The 

 species common to both do not generally extend, however, 

 along the northern coasts of Europe and Asia, but it is certain 

 that they had such a continuous range in the Arctic Ocean in 

 fairly remote times, when the climatic conditions are known 

 to have been considerably milder than they are to-day. After 

 the Eocene period there was a land connection between Alaska 

 and Siberia, but in order to explain the similarity between the 

 fish-faunas of the Atlantic and Pacific it is necessary to assume 

 that this was subsequently broken, and there is evidence to 

 suggest that this happened more than once. The Pacific 

 Herring {Clupea pallasii) provides an interesting example of 

 what is known as discontinuous distribution. This fish is 

 related to our own Herring (C harengus), and both species 

 extend into the Arctic Zone. The Pacific species has an isolated 

 colony in the White Sea, but outside on the coasts of Northern 

 Europe and Asia, both to the east and to the west, the Atlantic 

 species occurs. 



The last of the zones, the Arctic, has a very poor fish-fauna. 

 A certain number of Bull-heads (Cottidae) are common here, 

 and some of the Cods (Gadidae) and Flat-fishes {Pleuronectidae) , 

 a few of which seem to have a very wide range right round the 

 North Pole, are found only within the limits of this zone. One 

 family {^oarcidae) found in this zone also occurs in the Antarctic, 

 but is represented by quite distinct genera in the two regions, 

 and there are no species of fish common to the North and 

 South Polar seas as was formerly supposed. 



