The Geographical Distribution of Fishes 245 



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 some cases 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 be 

 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 

 dift'erences in distribution are largely vertical, some living at 

 greater depths than others, and they shade off by degrees from 

 bathybial into semi-bat hybial, and finally into ordinary pelagic 

 and ordinary shore types. Apparently all of the bassalian fishes 



