324 A. Gray—North American Flora. 
for both. There opportunities may be afforded for a passing 
acquaintance with the botany of the Atlantic border of t 
United States, in company with the botanists of the American 
Association, who are expected to muster in full force. 
be asked of me, then, is to portray certain out- 
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 ascribed. 
For, indeed, however interesting or curious the facts of the 
case may be in themselves, they become far more instructive 
when we attain to some clear conception of the dependent rela- 
tion 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, 
shind. Among the trees the White Birch and the Chestnut 
will be identified, if not as exactly the same, yet with only 
may be said to be no 
of several other trees. Only as you proceed westward an 
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 1s not 
far to seek. This was a region of forest, upon which the aborig- 
ines, 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 tals 
forest could bear exposure to the fervid summer's sun; 2D 
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 Europe, 
yr 
