114 SCIENCE SKETCHES. 



Spring, and which dwindle to mere rivulets in the 

 autumn droughts. 



In general, those streams which have conditions 

 most favorable to fish-life will be found to contain 

 the greatest number of species. Such streams in- 

 vite immigration ; and in them the struggle for 

 existence is individual against individual, species 

 against species, and not a mere struggle with hard 

 conditions of life. Some of the conditions most 

 favorable to the existence in any stream of a large 

 number of species of fishes are the following, the 

 most important of which is the one mentioned 

 first: connection with a large hydrographic basin; 

 a warm climate; clear water; a moderate current; 

 a bottom of gravel (preferably covered by a growth 

 of weeds); little fluctuation during the year in the 

 volume of the stream or in the character of the 

 water. 



Limestone streams usually yield more species 

 than streams flowing over sandstone, and either 

 more than the streams of regions having metamor- 

 phic rocks. Sandy bottoms usually are not favor- 

 able to fishes. In general, glacial drift makes a 

 suitable river bottom, but the higher temperature 

 usual in regions beyond the limits of the drift gives 

 to certain Southern streams conditions still more fa- 

 vorable. These conditions are all well realized in 

 the Washita River in Arkansas, and in various trib- 

 utaries of the Tennessee, Cumberland, and Ohio ; 

 and in these, among American streams, the great- 

 est number of species has been recorded. 



The isolation and the low temperature of the 

 rivers of New England have given to them a very 



