DISTRIBUTION OF FEESHWATEK FISHES. 



209 



As in every other class of animals, these freshwater 

 genera and families vary greatly with regard to the extent 

 of their geographical range; some extend over the greater 

 half of the continental areas, whilst others are limited to 

 one continent only, or even to a very small portion of it. 

 As a general rule, a genus or family of freshwater fishes 

 is regularly dispersed and most developed within a certain 

 district, the species and individuals becoming scarcer towards 

 the periphery as the type recedes more from its central 

 home, some outposts being frequently pushed far beyond the 

 outskirts of the area occupied by it. But there are not want- 

 ing those remarkable instances of closely allied forms occur- 

 ring, almost isolated, at most distant points, without being 

 connected by allied species in the intervening space ; or of 

 members of the same family, genus, or species inhabiting the 

 opposite shores of an ocean, and separated by many degrees 

 of abyssal depths. We mention of a multitude of such 

 instances the following only : — 



A. Species identical in distant continents — 



1. A number of species inhabiting Europe and the tem- 

 perate parts of eastern North America, as Perca fluviatilis, 

 Gastrosteus pwngitius, Lota vulgaris, Salmo salar, Esox lucius, 

 Acipenser sturio, Acipenser mamlosus, and several Petromy- 

 zonts. 



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