176 PAST AND PRESENT 
we seek in this direction an explanation? The evi- 
dence is entirely against such a solution. These 
plants (and animals) are found chiefly—many of them 
entirely—in the wildest parts of the country, and bear 
fully the stamp of natives of old standing. Human 
foreign intercourse is not so old but that the intro- 
ductions which it effected are still easily discernible 
to the student: the plants which have come to us thus 
bear the imprint of their origin; they spread outwards 
from centres of human activity, and are absent from 
undisturbed areas; they cannot in most cases com- 
pete with the indigenous vegetation, and only exist by 
confining their attempts at colonization to places 
where man has ousted the native flora—such as tilled 
land, roadsides, railway tracks. Even those aliens 
which have succeeded in winning a place among the 
native plants, such as the Monkey Flower (Mimulus 
Langsdorfi) or Michaelmas Daisies (Aster spp.) of 
North America, which are found sometimes in quite 
wild situations, the experienced field botanist detects 
readily enough. The introduction of the plants in 
question by man has never been advocated by a 
responsible biologist. 
Assuming, then, that these groups owe their pres- 
ence to natural agencies, the next question that arises 
is, Could they have come to our shores across the 
existing seas, or must we relegate their arrival to 
periods when different distribution of sea and land 
would aid their migration by allowing them to travel 
across a land surface, or at least to cross sea-barriers 
less wide than the present? This leads us to con- 
sider the means of dispersal possessed by the species 
in question, and to measure these against the nature 
