A. Gray—Pertinacity and Predominance of Weeds. 163 
ricaria discoidea, Artemisia biennis. Fifty years ago Rudbeckia 
hirta, which flourished from the Alleghanies westward, was 
unknown farther east. Now, since twent ars, it i 
changed conditions. These are some of the instances which 
may show that predominance is not in consequence of change 
0 
ently 
contributed to the Third Report of the Montreal Horcen tee 
a 
SB 
=~ 
— 
ow 
~I 
- 
(es) 
 _~ 
2 
B 
os’ 
| eg 
= 
lox | 
7 
er 
mee 
ar 
0g 
fy 
~n 
i) 
i} 
= 
2) 
i=] 
oe 
o 
3g 
° 
o 
° 
American weeds to invade Europe. e have offered a fairly 
good explanation.of the first. And Professor Claypole goes 
ie oa 
(or formerly was) main! brought from the Old World to the 
New, and the same may be said of cattle and other emigration ; 
that the cooler and shorter summer of the north of Europe 
renders the ripening of some seed recarious, etc. He does not 
mention the fact that American plants by chance reaching 
