same net but forty-five species in the course of a week's work. 

 Thus it comes that the angler, contented with many fishes of 

 few kinds, goes to Northern streams to fish, while the naturalist 

 goes to the South. 



But in most streams the difference in latitude is insignifi- 

 cant, and the chief differences in temperature come from differ- 

 ences in elevation, or from the distance of the waters from the 

 colder source. Often the lowland waters are so different in 

 character as to produce a marked change in the quality of their 

 fauna. These lowland waters may form a barrier to the free 

 movements of upland fishes ; but that this barrier is not 

 impassable is shown by the identity of the fishes in the 

 streams (for example, Elk river, Duck river, etc.) of the 

 uplands of middle Tennessee with those of the Holston and 

 French Broad. Again, streams of the Ozark Mountains, sim- 

 ilar in character to the rivers of East Tennessee, have an essen- 

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

 Cumberland range lies an area of lowland bayous, into which 

 such fishes are never known to penetrate. We can, however, 

 imagine 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 faunae* are not quite identical, and in some cases the 

 same species are represented by perceptibly different varieties 

 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 

 expected. In one way or another very many species have 



* There are three species of darters [Etheostoma copelandi Jordan; Etheostoma evidis 

 Jordan and Copeland : Ethtostoma scurunt Swa n) which are now known only from the Orarlc 

 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 the- e 

 fishes. But the nonoccurrence of these species over the intervening areas needs confirmation, as 

 do most similar cases of anomalous distribution. 



