II4 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 Amcrican 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 
