264 ESSAYS. 
geological period, and bounded on the west by a great moun- 
tain range mainly clothed with an alpine flora requiring the 
protection of snow in winter, and on the north by a warm- 
temperate region whose flora is mainly of modified sub-tropical 
origin — the only plants that could occupy the newly-formed 
region were the comparatively few which, though developed 
under very different conditions, were sufficiently tolerant of 
change to adapt themselves to the new environment. The 
flora is poor, not because the land cannot support a richer one, 
but because the only regions from which a large population 
could be derived are inhabited by races unfit for emigration.” 
Singularly enough, this deficiency of herbaceous plants is 
being supplied from Europe, and the incomers are spreading 
with great rapidity; for lack of other forest material even 
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 
America “is to be accounted for, less by any special fitness 
of the immigrant species, than by the fact that the ground is 
to a great extent unoccupied.” 
The principle applies here also; and in general, that it is 
opportunity rather than specially acquired vigor that has 
given Old World weeds an advantage may be inferred from 
the behavior of our weeds indigenous to the country, the 
plants of the unwooded districts — prairies or savannas west 
and south, — which, now that the way is open, are coming in 
one by one into these eastern parts, extending their area con- 
tinually, and holding their ground quite as pertinaciously as 
the immigrant denizens. Almost every year gives new exam- 
ples of the immigration of campestrine western plants into 
the eastern States. They are well up to the spirit of the 
age; they travel by railway. The seeds are transported, 
some in the coats of cattle and sheep on the way to market, 
others in the food which supports them on the journey, and 
many in a way which you might not suspect, until you 
