DISTRIBUTION AND MIGRATIONS 259 



illustrate the distribution of the genus of Pilchards or Sardines 

 [Sardina) , and it will be observed how closely their geographical 

 range corresponds to the subtropical regions of the Temperate 

 Zones. The Common Pilchard {S. pilchardus) occurs in the seas 

 of Western Europe and in the Mediterranean; a second species 

 {S. sagax) is found on the coasts of Chile and Peru, on the Pacific 

 coast of the United States and Lower California, as well as in 

 South Africa and Japan ; and a third species {S. neopilchardus) in 

 the southern half of Australia and in New Zealand. The 

 absence of Pilchards on the Atlantic coast of America is difficult 

 to understand, but this may be due to the sudden transition 

 from subarctic to almost tropical conditions in the western 

 North Atlantic, where the cold Labrador Current meets the 

 warm Gulf Stream. It would be comparatively easy for a 

 South Amercian species to reach South Africa or New Zealand, 

 since the eggs of these fishes are pelagic and me young have 

 been observed swimming at the surface as far as fifty miles 

 from the shore, but it w^ould be quite another matter to cross 

 the Tropical Zone. The Hakes (Merluccius) represent another 

 genus largely confined to these subtropical regions, but also 

 extending into subarctic and subantarctic waters. 



A remarkable illustration of the part played by temperature 

 and currents in determining the distribution of fishes is provided 

 by certain forms found round the small islands of Tristan 

 d'Acunha and St. Paul, both of which lie in the subtropical 

 region of the South Temperate Zone. Although about four 

 thousand miles apart, the two islands lie roughly on the same 

 isotherm, and the current known as the Antarctic Drift runs 

 direct from one to the other. As a result of these physical 

 factors, there are species of fish common to both islands, but 

 found nowhere else in the w^orld. A similar case of wide 

 geographical range occurs in the subantarctic half of the same 

 zone, three species being common to southern New Zealand and 

 the Magellan-Falkland Islands area, but found nowhere else. 



In the true Antarctic Zone, bounded on the north by the 

 isotherm of 6° C, there is a peculiar and diverse fauna which 

 must have taken a considerable period of time to evolve. The 

 importance of temperature as a factor in distribution is again 

 illustrated by the great similarity between the fishes of South 

 Georgia and Graham Land, which are, however, quite unlike 

 those of the Magellan-Falkland plateau. The great bulk of the 

 Antarctic fauna is made up of fishes known as Nototheniids 

 {Nototheniidae, etc.) : some of these forms occur in the sub- 



