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. 
