stricted to particular places, nearly in the proportion in which
they appear to be thus diffusible .or otherwise. The diffusible
nature of seeds belonging to th e ' cryptogamous species has
been often adverted to, and by many has been thought to
afford a sufficient explanation of the almost universal dis-
persion of the cellular tribes of. plants. M. de Humboldt
seems, indeed; to admit the original diffusion of the fungi
and other agamous plants, but M. De Candolle with greater
reason rejects this opinion. He remarks, that the determi-*-
nation of species is very difficult in these- tribes 'of simple
structure* and that the minuteness of seeds explains the
wide extension of such tribes. Among the more perfect
plants, it seems that the species which vegetate on the
sea-coast and at the mouths of rivers, are the most widely
spread. -
Thus the phenomena to be explained coincide with the sup-»
position, th a t plants have been accidentally spread, when we
advert to the facilities afforded by their nature and organization.
They equally agree with this hypothesis when we
direct our attention to the situation of countries. For;
L The phenomena related to the dispersion of plants to
islands and sea-coasts, are, as we have seen, in many in*
stances such as to force upon us the conviction, that particular
species have spread themselves from one p la c e . to
another. I now refer to some of the facta connected
with the dispersion of plants over different islands in
the Pacific, and with the corresponding vegetation off opposite
shores. But generally considered, the diffusion of tribes
over islands coincides with what we should expect to find on
the supposition now contemplated* and. give the negative to
any other.
II. The different proportions in which the same species occur
in the flora of continents and great regions tend to establish
the same belief. I allude, to the fact, that so many
tijbes are common to the northern continents where they
approach and afford facilities for migration; and.that the
number diminishes as the great lands become separated by
wider oceans towards the equator, till in the south we find
regions so entirely diverse in their botanical productions, as
are those of South Africa, South America, and Austral
Asia.
On the whole/, we . may conclude, with a great degree of probability,
that each;tribe of plants, and especially of the more
perfect plants, had on the earth oiie original habitation, from
which it has been dispersed according to the capabilities
afforded by its structure, and the aid of external agencies.