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 Linnzan Society’s 
Journal, ‘in a district raised from the sea during the latest 
