PERTINACITY AND PREDOMINANCE OF WEEDS. 235 
Why weeds are so pertinaceous and aggressive, is too large 
and loose a question: for any herb whatever when success- 
fully aggressive becomes a weed ; and the reasons of predom- 
inance may be almost as diverse as the weeds themselves. 
But we may inquire, whether weeds have any common char- 
acteristic which may give them advantage, and why the 
greater part of the weeds of the United States, and probably 
of similar temperate countries, should be foreigners. 
As to the second question, this is strikingly the case 
throughout 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 must be to a great extent the true one, namely, that, as 
the district here in which weeds from the Old World prevail 
was naturally forest-clad, there were few of its native herbs 
which, if they could bear the exposure at all, were capable of 
competition on cleared land with emigrants from the Old 
World. It may be said that these same European weeds, 
here prepotent, had survived and adapted themselves to the 
change from forest to cleared land in Europe, and therefore 
our forest-bred herbs might have done the same thing here. 
But in the first place the change must have been far more 
sudden here than in Europe; and in the next place, we sup- 
pose 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 agricul- 
tural 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 Dan- 
delion, 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 settlement 
of the country began on the eastern border; yet a certain 
number of -our weeds appear to have been thus derived : 
for instance, Mollugo verticillata, Erigeron Canadense, 
