CHAPTER VII 
THE GEOGRAPHICAL DISTRIBUTION OF FISHES 
1 omy £7, |OOGEOGRAPHY.—Under the head of distribution we 
c. (4 =) consider the facts of the actual location of species 
asmees] of organisms on the surface of the earth and the 
laws by which their location is governed. This constitutes 
the subject-matter of the science of zoogeography. In physical 
geography we may prepare maps of the earth or of any part of 
it, these bringing to prominence the physical features of its 
surface. Such maps show here a sea, there a plateau, here a 
mountain chain, there a desert, a prairie, a peninsula, or an 
island. In political geography the maps show their physical 
features of the earth as related to the people who inhabit 
them and the states or powers which receive or claim their 
allegiance. In zoogeography the realms of the earth are con- 
sidered in relation to the species or tribes of animals which 
inhabit them. Thus series of maps could be drawn representing 
those parts of North America in which catfishes or trout or 
sunfishes are found in the streams: In like manner the distri- 
bution of any particular fish as the muskallonge or the yellow 
perch could be shown on the map. The details of such a map 
are very instructive, and their consideration at once raises a 
series of questions as to the cause behind each fact. In science 
it must be supposed that no fact is arbitrary or meaningless. 
In the case of fishes the details of the method of diffusion of 
species afford matters of deep interest. These are considered 
in a subsequent chapter. 
The dispersion of animals may be described as a matter of 
space and time, the movement being continuous but modified 
by barriers and other codnitions of environment. The ten- 
dency of recent studies in zoogeography has been to consider 
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