It has been observed by M. De Candolle, that the vegetation
of these several botanical provinces is so distinct, that
when we read in the works of scientific travellers accounts
which state the plants of one region to have been discovered
in another, we ought not to admit the fact as perfectly established,
without a careful inspection of particular specimens.
I shall now proceed to survey a, different series of facts
related to the same inquiry, and point out their bearing on
the conclusions which I have already obtained.
Section III.—O f the means provided by Nature fo r the dispersion
o f Plants— Facts relating to the migrations and
Colonies o f Plants.
I t seems to have been a part of the economy of Nature, if
such an expression may be allowed, to provide for the diffusion
of the vegetable tribes. This may be inferred from the
extreme minuteness of many seeds, admitting their transportation
by winds through spaces of indefinite distance, and
from the wings and feathery appendages with which other
seeds are provided. In order to form, a correct .estimate of
the efficacy of this and other causes, which contribute to the
same result, we must survey facts connected with the actual
dispersion of plants. The means by which the transportation
of seeds have been effected are of various kinds. One of
the most obvious is human agency.
P aragraph 1. — Of the dispersion of Plants by human
agency.
I t has been observed, that mankind have almost everywhere
surrounded their abodes with artificial plantations;
they have changed the most desert spots, and the most
dreary morasses, into corn-fields and gardens; and there is,
even in the northern extremity of Europe, scarcely a poor
hamlet where some exotic tree, or at least some vegetable of
foreign extraction, is, not to be found.* Together with these
• Rudolphi über die Verbreitung oder die angeblichen Wanderungen der Pflanzen.
plants designedly introduced, others are conveyed accidentally.
Thus, with rice and other kinds of grain many plants have
come in which are still found among them, as the knap-weed;
and others which have become very widely spread, as the
erigeron canadense. I t was thus that hemp, as well as the
thorn-apple, was first introduced, neither of which are natives
of these countries. De Candolle observes, that the inhabitants
of the south of Europe so# -together with the wheat
of Barbary, the wild plants’ of Algiers and of Tunis. He
reports an instance of the accidental diffusion of seeds from
other importations. Near the g4te-vpf Montpellier, there is
a field appropriated to the drying of foreign wools after they
have been washed.'1 Scarcely a year'passes, in wlfich some
foreign plant does not make its appearance in this figfdi " He
mentions psoraiea’ palseStina, hypericum crispum, CehtauVea
parviflora.* Many- plants have been ’ dfspersed, in the "first
instance', from gardens among surrounding countries, and" have
become naturalized'. Linnaeus ,remarked;^Chat the erigeron
canaderise was introduced into gardens near Paris from North
America". The seeds had been scattered" by^the wind, afid
this plant wa§-, in the course of a 'rceiMry,^|)ffea.d Pver all
France, Italy,'-Sicily, Belgium, and Germany, f A great
number of TShlopeian plants have been introduced' by colo-
nists iri^fh’e Cape - of Good Hope. Thunberg has Carefully
enumerated these; but he has reckoned among them several
specie's, the introduction of which,5in manner,*Mas been
thought, by some writers, scarcely probaine^' Such are
lemna gibba, typha latifolia, corrigiola littora’lis, alsine media,
rumex aquaticus, bambusa arundinacea.J
P aragraph 2.—By means of animals.
I t has often been said, that animals^contribute to thedisr,
persion of plants. Light seeds may adhere to the fleeces of
* De Oandolle Dict. des Sie. Nat.
+ Linnaeus in Anisen. Acad. de Coloniis Plantar.
$ Thunberg Flora Capens, tom. i. fase. 1... Rudolphi über die Verbreitung der
Pflanzen, 114.
D 2