phe daies ec 
162 A. Gray—Pertinacity and Predominance of Weeds. 
directly or indirectly by man. Some—such as Dandelion, 
Yarrow, and probably the common Plantain and the common 
Purslane—are importations as weeds, although the species nat- 
urally occupy some part of the country. 
y weeds are so pertinacious and aggressive, is too large 
and loose a question: for any herb whatever when successfully 
aggressive becomes a weed; and the reasons of predominance 
may be almost as diverse as the weeds themselves. But we 
may enquire, whether weeds have any common characteristic 
which may give them advantage, and why the greater part of 
the weeds of the United States, and probably of similar tem- 
perate countries, should be foreigners. 
As to the second question, this is strikingly the case through- 
out the Atlantic side of temperate North America, in which 
the weeds have mainly come from Europe; but it is not so, 
or hardly so, west of the Mississippi in the region of prairies 
and plains. So that the answer we are accustomed to give 
Europe; and in the next place, we suppose that most of the 
herbs in question never were indigenous to the originally forest- 
covered regions of the Old World; but rather, as western and 
northern Europe became agricultural and pastoral, these plants 
came with the husbandmen and the flocks, or followed them, 
from the woodless or sparsely wooded regions farther east where 
they originated. This, however, will not hold for some of them, 
such as Dandelion, Yarrow, and Ox-eye Daisy. It may be said 
that our weeds might have come to a considerable extent 
from the bordering more open districts on the west and south. 
But there was little opportunity until recently, as the settle- 
ment of the country began on the eastern border; yet a certalp 
number of our weeds appear to have been thus derived: for 
instance, Mollugo verticillata, Erigeron Canadense, Xanthium, 
Ambrosia artemisvefolia, Verbena hastata, V. urticifolia, etc. 
ica peregrina, Solanum Carolinense, various species of 
a 
region and beyond—especially by rail-roads—other plants are 
1e Eastern States as weeds, step by step, by 
a 
