144 PLANTS AND MAN 
muddy estuaries. It Jooks as aboriginal as any of the 
plants among which it grows: but the facts that the 
genus to which it belongs is American (with a few 
species in Australia and New Zealand), that it itself is 
found native in the western States and not in the 
eastern, and that it has been long cultivated in 
gardens, furnish convincing proof that it is really an 
alien. But it is seldom that the evidence is so satis- 
factory as in this case. More usually the range of the 
doubtful members of the flora is continuous, extending 
from regions where they are truly native to others 
where they are undoubtedly exotic. For instance, 
many annual plants of the Mediterranean region have 
followed the spread of agriculture across the former 
forest areas of Central and Western Europe into our 
own islands. Plants native in France have been trans- 
ported into England, and English natives into Ireland; 
east Irish plants have spread westward—sixty years 
ago, save for a single record of P. hybridum, Papaver 
dubium was the only Poppy known west of the 
Shannon; now all four British species occur, several 
of them in many places. The flora of Europe, as 
pointed out already, diminishes in variety as we pass 
westward into the outlying areas. Those species 
whose aboriginal distribution stopped short of the 
western limit of the land had no doubt a fluctuating 
western or northern or southern boundary to their 
range, dependent on temporary conditions. Thus, a 
hard winter might kill back a plant already at the 
limit of its natural range, or a warm summer, by 
ripening abundance of its seed, might result in its 
slight advance. The general effect of human opera- 
tions has been to lessen competition and increase 
