NORTH AMERICAN FLORA. 261 
lines of the vegetation of the United States and the Canadian 
Dominion, as contrasted with that of Europe ; perhaps also to 
touch upon the causes or anterior conditions to which much 
of the actual differences between the two floras may be as- 
eribed. For, indeed, however interesting or curious the facts 
of the case may be in themselves, they become far more in- 
structive when we attain to some clear conception of the de- 
pendent relation of the present vegetation to a preceding state 
of things, out of which it has come. 
As to the Atlantic border on which we stand, probably the 
first impression made upon the botanist or other observer com- 
ing from Great Britain to New England or Canadian shores, 
will be the similarity of what he here finds with what he left 
behind. Among the trees the White Birch and the Chestnut 
will be identified, if not as exactly the same, yet with only 
slight differences — differences which may be said to be no 
more essential or profound than those in accent and intona- 
tion between the British speech and that of the “‘ Americans.” 
The differences between the Beeches and Larches of the two 
countries are a little more accentuated ; and still more those 
of the Hornbeams, Elms, and the nearest resembling Oaks. 
And so of several other trees. Only as you proceed westward 
and southward will the differences overpower the similarities, 
which still are met with. 
In the fields and along open roadsides the likeness seems 
to be greater. But much of this likeness is the unconscious 
work of man, rather than of Nature, the reason of which is 
not far to seek. This was a region of forest, upon which the 
aborigines, although they here and there opened patches of 
land for cultivation, had made no permanent encroachment. 
Not very much of the herbaceous or other low undergrowth 
of this forest could bear exposure to the fervid summer’s sun ; 
and the change was too abrupt for adaptive modification. The 
plains and prairies of the great Mississippi Valley were then 
too remote for their vegetation to compete for the vacancy 
which was made here when forest was changed to grain-fields 
and then to meadow and pasture. And so the vacancy came 
to be filled in a notable measure by agrestial plants from Eu- 
