The Geographical Distribution of Fishes 99 
found their origin. This is true especially of the New England 
region, the North Sea, the Gulf of Guinea, and the coast of 
Argentina. The fish fauna of the North Atlantic is derived 
mainly from the North Pacific, the differences lying mainly 
in the relative paucity of the North Atlantic. But in certain 
groups common to the two regions the migration must have 
been in the opposite direction, exceptions that prove the rule. 
Distribution of Marine Fishes.—The distribution of marine 
fishes must be indicated in a different way from that of the 
fresh-water forms. The barriers which limit their range fur- 
nish also their means of dispersion. In somecases proximity 
overbalances the influence of temperature; with most forms 
questions of temperature are all-important. 
Pelagic Fishes.—Before consideration of the coast-lines we 
may glance at the differences in vertical distribution. Many 
species, especially those in groups allied to the mackerel family, 
are pelagic—that is, inhabiting the open sea and ranging 
widely within limits of temperature. In this series some species 
are practically cosmopolitan. In other cases the genera are 
so. Each school or group of individuals has its breeding place, 
and from the isolation of breeding districts new species may he 
conceived to arise. The pelagic types have reached a species 
of equilibrium in distribution. Each type may be found where 
suitable conditions exist, and the distribution of species throws 
little light on questions of distribution of shore fishes. Yet 
among these species are all degrees of localization. The pelagic 
fishes shade into the shore fishes on the one hand and into the 
deep-sea fishes on the other. 
Bassalian Fishes.—The vast group of bassalian or deep-sea 
fishes includes those forms which live below the line of ade- 
quate light. These too are localized in their distribution, and 
to a much greater extent than was formerly supposed. Yet as 
they dwell below the influence of the sun’s rays, zones and 
surface temperatures are nearly alike to them, and the same 
forms may be found in the Arctic or under the equator. Their 
differences in distribution are largely vertical, some living at 
greater depths than others, and they shade off by degrees from 
bathybial into semi-bathybial, and finally into ordinary pelagic 
and ordinary shore types. Apparently all of the bassalian fishes 
