Barriers to Dispersion of River Fishes 109 
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 + 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 faunz 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.{ 
* For example, Elk River, Duck River, etc. 
} There are three species of darters (Cottogaster copelandt Jordan, Hadrop- 
terus 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. 
t Thus, Dorosoma cepedianum Le Sueur and Pomolobus chrysochloris Rafi- 
nesque have found their way into Lake Michigan through canals. 
