no 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 hmits 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 faunae 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-faunae 

 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 



