842 THE AMERICAN NATURALIST.  [Vou. XXXV. 
Later on, with the wearing away of the inter-stream uplands, 
which at first were flat-topped, but which become more and 
more reduced by the dendritic etchings of the smaller streams, 
the divides gradually become more sharp, and the country 
eae -— n - 
*3 d « é 
"1 A» P Sar tus. 
Fic. 2. — Valleys of later stage. (After Salisbury.) 
reaches its maximum degree of roughness and cliff habitat, 
and consequently the greatest isolation of its streams and their 
faunas. By degrees these sharp divides are lowered, and head- 
water streams favorably located begin to capture fellow streams 
with their faunas, and thus there is a mixing of rapid-water 
faunas. 
The time of maximum roughness in the topography coin- 
cides with that of maximum isolation, but with the fusion and 
concentration of drainage lines a period begins which favors 
wide distribution by the removal of barriers ; thus in a country 
approaching baselevel a wide distribution of the fauna will be 
greatly facilitated with a premium placed upon the lowland 
fauna. This stage of wide distribution favors interbreeding, 
