Barriers to Dispersion of River Fishes 3 1 9 



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 colonization 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 htmdred 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 aHen and a 

 savage. 



