The Geographical Distribution of Fishes 249 



other causes, if other causes exist, take place most rapidly there 

 and produce most far-reaching modification. As elsewhere 

 stated, the coral reefs of the tropics are the centers of fish-life, 

 the cities in fish-economy. The fresh waters, the arctic waters, 

 the deep sea and the open sea represent forms of ichthyic back- 

 woods, regions where change goes on more slowly, and in them 

 we find survivals of archaic or generalized types. For this rea- 

 son the study in detail of the distribution of marine fishes .of 

 equatorial regions is in the highest degree instructive. 



Realms of Distribution of Fresh-water Fishes. — If we consider 

 the fresh-water fishes alone we may divide the land areas of 

 the earth into districts and zones not differing fundamentally 

 with those marked out for mammals and birds. The river 

 basin, bounded by its shores and the sea at its mouth, shows 

 many resemblances, from the point of view of a fish, to an 

 island considered as the home of an animal. It is evident that 

 with fishes the differences in latitude outweigh those of con- 

 tinental areas, and a primary division into Old World and New 

 World would not be tenable. 



The chief areas of distribution of fresh-water fishes we may 

 indicate as follows, following essentially the grouping proposed 

 by Dr. Giinther : * 



Northern Zone. — With Dr. Giinther we may recognize first 

 the Northern Zone, characterized familiarly by the presence of 

 sturgeon, salmon, trout, white-fish, pike, lamprey, stickleback, 

 and other species of which the genera and often the species are 

 identical in Europe, Siberia, Canada, Alaska, and most of the 

 United States, Japan, and China. This is subject to cross- 

 division into two great districts, the first Europe-Asiatic, the 

 second North American. These two agree very closely to the 

 northward, but diverge widely to the southward, developing a 

 variety of specialized genera and species, and both of them pass- 

 ing finally by degrees into the Equatorial Zone. 



Still another line of division is made by the Ural Mountains 

 in the Old World and by the Rocky Mountains in the New. In 

 both cases the Eastern region is vastly richer in genera and 

 species, as well as in autochthonous forms, than the Western. 

 The reason for this lies in the vastly greater extent of the river 

 * " Introduction to the Study of Fishes." 



