132 SCIENCE SKETCHES. 



new species or varieties may be formed with time ; 

 and these new forms may again invade the terri- 

 tory of the parent species. Again, colony after 

 colony of species 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 eventful 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 hundred times at- 

 tempted. 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 canni- 

 bal, and to each species each member of every 

 other species is an alien and a savage. 



