different groups of plants. Why have they not, supposing them to have
been ever so distinct originally, become more blended and confounded
together in the lapse of ages?

_Agency of man in the dispersion of plants._--But in addition to all the
agents already enumerated as instrumental in diffusing plants over the
globe, we have still to consider man--one of the most important of all.
He transports with him, into every region, the vegetables which he
cultivates for his wants, and is the involuntary means of spreading a
still greater number which are useless to him, or even noxious. "When
the introduction of cultivated plants," says De Candolle, "is of recent
date, there is no difficulty in tracing their origin; but when it is of
high antiquity, we are often ignorant of the true country of the plants
on which we feed. No one contests the American origin of the maize or
the potatoe; nor the origin, in the Old World, of the coffee-tree, and
of wheat. But there are certain objects of culture, of very ancient
date, between the tropics, such for example as the banana, of which the
origin cannot be verified. Armies, in modern times, have been known to
carry, in all directions, grain and cultivated vegetables from one
extremity of Europe to the other; and thus have shown us how, in more
ancient times, the conquests of Alexander, the distant expeditions of
the Romans, and afterwards the crusades, may have transported many
plants from one part of the world to the other."[866]

But, besides the plants used in agriculture, the numbers which have been
naturalized by accident, or which man has spread unintentionally, is
considerable. One of our old authors, Josselyn, gives a catalogue of
such plants as had, in his time, sprung up in the colony since the
English planted and kept cattle in New England. They were two-and-twenty
in number. The common nettle was the first which the settlers noticed;
and the plantain was called by the Indians "Englishman's foot," as if it
sprung from their footsteps.[867]

"We have introduced every where," observes De Candolle, "some weeds
which grow among our various kinds of wheat, and which have been
received, perhaps, originally from Asia along with them. Thus, together
with the Barbary wheat, the inhabitants of the south of Europe have
sown, for many ages, the plants of Algiers and Tunis. With the wools and
cottons of the East, or of Barbary, there are often brought into France
the grains of exotic