TIO SCIENCE SKETCHES. 
species implies, therefore, the existence of barriers 
to their diffusion. We may now consider these 
barriers, and, in the same connection, the degree 
to which they may be overcome. 
Least important of these are the barriers which 
may exist within the limits of any single basin, 
and which tend to prevent a free diffusion through 
its waters of species inhabiting any portion of it. 
In streams flowing southward, or across different 
parallels of latitude, the difference in climate be- 
comes a matter of importance. The distribution 
of species is governed very largely by the tempera- 
ture of the water. Each species has its range in 
this respect,—the free-swimming fishes, notably 
the Trout, being most affected by it; the mud- 
loving or bottom fishes, like the Cat-fishes, least. 
The latter can reach the cool bottoms in hot 
weather, or the warm bottoms in cold weather, 
thus keeping their own temperature more even 
than that of the surface of the water. Although 
water-communication is perfectly free for most of 
the length of the Mississippi, there is a material 
difference between the faunze of the stream in 
Minnesota and in Louisiana. This difference is 
caused chiefly by the difference in temperature oc- 
cupying the difference in latitude. ‘That a similar 
difference in longitude, with free water communi- 
cation, has no appreciable importance, is shown 
by the almost absolute identity of the fish-faune 
of Lake Winnebago and Lake Champlain. While 
many large fishes range freely up and down the 
Mississippi, a majority of the species do not do so, 
and the fauna of the upper Mississippi has more in 
