26o A HISTORY OF FISHES 



antarctic region, but nearly always belong to distinct species. 

 The Antarctic Zone may be conveniently divided into a glacial 

 region, including the Antarctic continent and South Georgia, 

 and a periglacial region, outside the limits of the pack-ice, 

 including Kerguelen and Macquarie Islands. 



In the Northern Hemisphere the seasonal variations in the 

 temperature of the sea are very much greater, and the zones 

 of distribution are not so easy to define, although on the whole 

 the same isotherms may be regarded as giving fairly satisfactory 

 boundaries. Whereas, in the south the isotherms are roughly 

 parallel with each other, in the northern seas the spreading of 

 the great ocean currents produces an effect that they are 

 crowded together in the west, but widely separated in the east. 

 This is particularly marked in the North Atlantic, and on the 

 coast of North America the meeting of the cold Labrador 

 Current with the warm water of the Gulf Stream produces a 

 very abrupt change from ice-cold to almost tropical conditions, 

 with a corresponding change in the characteristic fishes. The 

 isotherm of 6° C. runs for a space almost directly north and 

 south oflf the coasts of Labrador and Newfoundland, then 

 turns in a north-easterly direction towards Iceland, curves to 

 the south of that island, runs still further north-eastwards, and 

 finally bends southwards to meet the Norwegian coast: starting 

 from about 45° N. latitude on the coast of America, this isotherm 

 ends at about 68° N. on the coast of Norway, a diflference of 

 latitude of more than one thousand two hundred sea miles 

 (Fig. 98). The reasons for these irregularities of temperature 

 cannot be detailed here, and it must suffice to point out that the 

 principal factors involved are the Gulf Stream or North Atlantic 

 Drift, which carries warm water to the shores of Western 

 Europe, and the Labrador Current, which brings ice-cold 

 water, with icebergs and pack-ice, southwards during the early 

 part of the year. 



A glance at the accompanying map (Fig. 98) shows that the 

 isotherm of 12° C, marking the boundary between the sub- 

 tropical and subarctic regions of the North Temperate Zone, 

 runs roughly to the mouth of the English Channel, and it is in 

 this region that the characteristic fish-fauna shows a definite 

 change. It is, so to speak, the meeting ground of two great 

 areas, and the Pilchard (Sardina), Anchovy (Engraulis), Red 

 Mullet {Mullus), and other lovers of warm water, give place to 

 the typically northern forms like the Herring {Clupea), Cod 

 {Gadus), and Plaice {Pleuronectes) . Many Mediterranean species 



