44 



THE AMERICAN BOTANIST 



measure by their surroundings from the fires that often sweep 

 the prairie and make it treeless, they continued to grow, illus- 

 trating very nicely the considerable changes that may come to 

 a region in response to some slight advantage afforded to 

 certain plants. As the trees grew large enough to afford rest, 

 food, or shelter to the birds crossing the plain, the planting 

 must have proceeded with vigor. Many of the species now in 

 the island are extremel}^ ag'gressive when once established and 

 we may expect to see our island assume much larger propor- 

 tions in the future if man does not interpose his authority 

 enforced by the axe and the brush-hook. 



