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Art. V. — On the Distribution of Fresh-water Fishes in the Allegheny Region of South- 



western Virginia. 



By E. D. Cope, A. M. 



Sect. I. — Preliminary. 



The distribution of fresh-water fishes is of especial importance to the questions of 

 the origin and existence of species in connection with the physical conditions of the 

 waters and of the land. Tliis is of course owing to the restricted nature of their 

 habitat, and the impossibility of their making extended migrations. With the 

 submergence of land beneath the sea, fresh-water fish are destroyed in proportion to 

 the extent of tlie invasion of salt water, while terrestrial vertebrata can retreat 

 before it. Hence every inland fish fauna dates from the last total submergence of 

 the country. 



Prior to the elevation of a given mountain chain, the courses of the rivers may 

 generally have been entirely different from their later ones. Subsequent to such a 

 period, thej^ can only have undergone partial modifications. As subsequent sub- 

 mergences can rarely have extended to the highlands where such streams originate, 

 the fishes of such rivers have only been destroyed so far as they were unable to reach 

 those elevated regions, and preserve themselves from destruction from salt water by 

 sheltering themselves in the mountain streams. On the other hand, the occurrence 

 of a period of elevation of the land, and of consequent greater cold, would congeal 

 the waters, and cover their courses with glaciers. The fishes would be driven to the 

 neighborhood of the coast, though no doubt in more southern latitudes a sutficient 

 extent of uncongealed fresh waters would flow by a short course into the ocean, to 

 preserve from destruction many forms of fresh-water fishes. 



Thus, through many vicissitudes, the fauna of a given system of rivers has had 

 opportunity of uninterrupted descent from the time of elevation of the mountain 

 range in which it has its sources. 



In order to prove an earlier origin for such fauna, it is necessary to assume that 

 fresh-water fishes can gradually accustom themselves to respiration in salt water, and 

 a change of food of the same degree, and vice versa ; phenomena which have not 

 been observed in any types now characteristic of either habitat. It will not do to 



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