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 faunz! 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 faunz 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; Ltheostoma evides Jordan and Copeland; Ltheostoma 
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. 
