326 A. Gray—North American Flora. 
cious idea of increased strength gained by competition. Oppor- 
tunity may count for more than exceptional vigor; and the 
7 
by races unfit for emigration. ; 
Singularly enougb, this deficiency of herbaceous plants '8 
being supplied from Europe, and the incomers are spreading 
with great rapidity; for lack of other. forest material eve? 
apple-trees are running wild and forming extensive groves 
Men and cattle are, as usual, the agents of dissemination. But 
colonizing plants are filling, in this instance, a vacancy which 
was left by nature, while ours was made by man. We may 
agree with Mr. Ball in the opinion that the rapidity with which 
the intrusive plants have spread in this part of South Americ 
“is to be accounted for, less by any special fitness of the mmr 
grant species, than by the fact that the ground is to a ‘ 
tent unoccupied.” 
