124 DOTTINGS ON THE ROADSIDE. [Chap. VIII.— B. S. 
plants is most advantageously replaced by another; 
and here, at last, we get at a chemical explanation of 
the advantages enjoyed by new-comers, and why, in 
a struggle for existence between them and the natives 
of the soil, they must ever come off victorious. 
A weed, then, in our language, signifies a na¬ 
turalized herb, which has a soft and membranaceous 
look, grows fast, propagates its kind with rapidity, 
and spreads, to the prejudice of endemic or cultivated 
plants, in places in some way or other disturbed by 
the agency of man. 
Whence do different countries derive their weeds ? 
is a question that naturally suggests itself. Off-hand, 
one would be inclined to answer that all countries 
indiscriminately, having a climate similar to that of 
Europe, would be the sources whence Europe derived 
its weeds. And to a great measure this is true. 
Many European weeds have an undoubted Asiatic and 
African origin; but if any part of the world might 
be expected to have supplied its due share, it would 
be the temperate parts of the North American con¬ 
tinent, where many European plants, such as thistles, 
have become naturalized to such an extent as to 
have become a perfect pest. From the constant in¬ 
tercourse between Europe and North America, and 
the number of North American plants cultivated in 
European gardens, one would have expected a great 
many North American plants to be naturalized in 
Europe; but this is by no means the case. North 
American plants, however easily grown in European 
gardens, do not show any great disposition to escape 
