The Geographical Distribution of Fishes 103 
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 ananimal. 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. Gunther: * 
Northern Zone.—With Dr. Gtinther 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.” 
