DISPERSION OF FRESH-WATER FISHES. 107 
explained, in a word, by the hypothesis of the progressive 
adaptation of the young of certain Percide to a peculiar 
place of refuge and a peculiar food-supply. 
“Perhaps we may, without violence, call these the 
mountaineers among fishes. Forced from the populous 
and fertile valleys of the river beds and lake bottoms, they 
have taken refuge from their enemies in the rocky high- 
lands, where the free waters play in ceaseless torrents, and 
there they have wrested from stubborn Nature a meagre 
living. Although diminished in size by their continual 
struggle with the elements, they have developed an ac- 
tivity and hardihood, a vigor of life, and glow of high 
color almost unknown among the easier livers of the lower 
lands.” 
It is noteworthy that among the European gen- 
era of Percide, one of them, Aspro, has assumed 
a similar habitat, and adapted — apparently as a 
result of its surroundings— characters similar to 
those of Etheostoma. It is not likely that Aspro is 
an ancestor of Etheostoma, still less likely that As- 
pro is descended from the latter genus. The simi- 
lar development of the two seems rather a case of 
analogous variation, the influence of similar condi- 
tions in different places on similar organisms. 
It is remarkable, also, that in mountain regions 
in which no Percide are found, fishes very similar 
to the Darters in appearance and habits, though 
totally different in structure, have by analogous 
agencies been developed. Loaches, Cat-fishes, 
Gobies, Characins, Sculpins, in different parts of 
the world inhabit swift mountain streams, and in 
a similar way become dwarfed and concentrated, 
taking the place in their respective habitats which 
