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, 



