128 Barriers to Dispersion of River Fishes 
after species may be destroyed by other species or by uncongenial 
surroundings. 
The ultimate result of centuries on centuries of the restlessness 
of individuals is seen in the facts of geographical distribution. 
Only in the most general way can the history of any species be 
traced; but could we know it all, it would be as long and as event- 
ful a story as the history of the colonizaticn and settlement of 
North America by immigrants from Europe. But by the fishes 
each river in America has been a hundred times discovered, its 
colonization a hundred times attempted. In these efforts there 
is no co-operation. Every individual is for himself, every struggle 
a struggle of life and death; for each fish is a cannibal, and to each 
species each member of every other species is an alien and a 
savage. 
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