300 Barriers to Dispersion of River Fishes 



of upland fishes; but that this barrier is not impassable is 

 shown by the identity of the fishes in the streams * of the uplands 

 of middle Tennessee with those of the Holston and French 

 Broad. Again, streams of the Ozark Mountains, similar in 

 character to the rivers of East Tennessee, have an essentially 

 similar fish fauna, although between the Ozarks and the Cum- 

 berland range lies an area of lowland bayous, into which such 

 fishes are never known to penetrate. We can, however, imag- 

 ine that these upland fishes may be sometimes swept down 

 from one side or the other into the Mississippi, from which 

 they might ascend on the other side. But such transfers cer- 

 tainly do not often happen. This is apparent from the fact 

 that the two faunas f are not quite identical, and in some cases 

 the same species are represented by perceptibly different varie- 

 ties on one side and the other. The time of the commingling of 

 these faunae is perhaps now past, and it may have occurred 

 only when the climate of the intervening regions was colder 

 than at present. 



The effect of waterfalls and cascades as a barrier to the dif- 

 fusion of most species is self-evident; but the importance of 

 such obstacles is less, in the course of time, than might be ex- 

 pected. In one way or another very many species have passed 

 these barriers. The falls of the Cumberland limit the range of 

 most of the larger fishes of the river, but the streams above it 

 have their quota of darters and minnows. It is evident that 

 the past history of the stream must enter as a factor into this 

 discussion, but this past history it is not always possible to 

 trace. Dams or artificial waterfalls now check the free move- 

 ment of many species, especially those of migratory habits; 

 while conversely, numerous other species have extended their 

 range through the agency of canals. J 



* For example, Elk River, Duck River, etc. 



f There are three species of darters (CoHogaster copelandi Jordan, Hadrop- 

 ierus evides Jordan and Copeland, Hadropterus scierus Swain) which are now 

 known only from the Ozark region or beyond and from the uplands of Indiana, 

 not yet having been found at any* point between Indiana and Missouri. These 

 constitute perhaps isolated colonies, now separated from the parent stock 

 in Arkansas by the prairie districts of Illinois, a region at present uninhabitable 

 for these fishes. But the non-occurrence of these species over the intervening 

 areas needs confirmation, as do most similar cases of anomalous distribution. 



J Thus, Dorosoma cepedianum Le Sueur and Pomo\fbus chrysochloris Rafi- 

 nesque have found their way into Lake Michigan through canals. . 



