3 I 2 Barriers to Dispersion of River Fishes 



of the lower Roanoke. In this case it is likely that we have 

 to consider the results of local erosion. Probably the divide has 

 been so shifted that some small stream with its fishes has been 

 cut off from the Holston and transferred to the Roanoke. 



The passage of species from stream to stream along the 

 Atlantic slope deserves a moment's notice. It is under present 

 conditions impossible for any mountain or upland fish, as the 

 trout or the miller's thumb,* to cross from the Potomac River 

 to the James, or from the Neuse to the Santee, by descending 

 to the lower courses of the rivers, and thence passing along 

 either through the swamps or by way of the sea. The lower 

 courses of these streams, warm and muddy, are uninhabitable 

 by such fishes. Such transfers are, however, possible farther 

 north. From the rivers of Canada and from many rivers of 

 New England the trout does descend to the sea and into the 

 sea, and farther north the whitefish does this also. Thus these 

 fishes readily pass from one river basin to another. As this is 

 the case now everywhere in the North, it may have been the 

 case farther south in the time of the glacial cold. We may, I 

 think, imagine a condition of things in which the snow-fields 

 of the Alleghany chain might have played some part in aiding 

 the dift'usion of cold-loving fishes. A permanent snow-field on 

 the Blue Ridge in western North Carolina might render almost 

 any stream in the Carolinas suitable for trout, from its source 

 to its mouth. An increased volume of colder water might carry 

 the trout of the head streams of the Catawba and the Savannah 

 as far down as the sea. We can even imagine that the trout 

 reached these streams in the first place through such agencies, 

 though of this there is no positive evidence. For the presence 

 of trout in the upper Chattahoochee we must account in some 

 other way. 



It is noteworthy that the upland fishes are nearly the same 

 in all these streams until we reach the southern limit of possible 

 glacial influence. South of western North Carolina the faunaj 

 of the different river basins appear to be more distinct from 

 one another. Certain ripple-loving types are represented by 

 closely related but unquestionably difi'erent species in each 



* Coitus icialops Rafincsquc. 



