NORTH AMERICAN FLORA. 263 



It has been maintained that the ruderal and agrestial Old 

 World plants and weeds of cultivation displace the indigenous 

 ones of newly settled countries in virtue of a strength which 

 they have developed through survival in the struggle of ages, 

 under the severe competition incident to their former migra- 

 tions. And it does seem that most of the pertinacious weeds 

 of the Old World which have been given to us may not be 

 indigenous even to Europe, at least to western Europe, but 

 belong to campestrine or unwooded regions farther east ; and 

 that, following the movements of pastoral and agricultural 

 people, they may have played somewhat the same part in the 

 once forest-clad western Europe that they have been playing 

 here. But it is unnecessary to build much upon the possibly 

 fallacious idea of increased strength gained by competition. 

 Opportunity may count for more than exceptional vigor ; and 

 the cases in which foreign plants have shown such superiority 

 are mainly those in which a forest-destroying people have 

 brought upon newly-bared soil the seeds of an open-ground 

 vegetation. 



The one marked exception that I know of, the case of recent 

 and abundant influx of this class of Old World plants into a 

 naturally treeless region, supports the same conclusion. Our 

 associate, Mr. John Ball, has recently called attention to it. 

 The pampas of southeastern South America beyond the Rio 

 Colorado, lying between the same parallels of latitude in the 

 south as Montreal and Philadelphia in the north, and with 

 climate and probably soils fit to sustain a varied vegetation, 

 and even a fair proportion of forest, are not only treeless, but 

 excessively poor in their herbaceous flora. The district has 

 had no trees since its comparatively recent elevation from the 

 sea. As Mr. Darwin long ago intimated : " Trees are absent 

 not because they cannot grow and thrive, but because the only 

 country from which they could have been derived — tropical 

 and sub-tropical South America — could not supply species to 

 suit the soil and climate." And as to the herbaceous and 

 frutescent species, to continue the extract from Mr. Ball's in- 

 structive paper recently published in the Linnaean Society's 

 Journal, " in a district raised from the sea during the latest 



