112 SCIENCE SKETCHES. 



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, 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 trans- 

 fers certainly do not often happen. This is appar- 

 ent from the fact that the two faunae l 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 inter- 

 vening regions was colder than at present 



The effect of waterfalls and cascades as a barrier 

 to the diffusion 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 passed 

 these barriers. The falls of the Cumberland limit 



1 There are three species of Darters (Etheostoma copelandi 

 Jordan ; Etheostoma evides Jordan and Copeland ; Etheostoma 

 scierum 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 consti- 

 tute 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. 



