communication, the diversity in the specific character of the
respective vegetations is almost as striking. Thus there is found one
assemblage of species in China, another in the countries bordering the
Black Sea and the Caspian, a third in those surrounding the
Mediterranean, a fourth in the great platforms of Siberia and Tartary,
and so forth.

The distinctness of the groups of indigenous plants, in the same
parallel of latitude, is greatest where continents are disjoined by a
wide expanse of ocean. In the northern hemisphere, near the pole, where
the extremities of Europe, Asia, and America unite or approach near to
one another, a considerable number of the same species of plants are
found, common to the three continents. But it has been remarked, that
these plants, which are thus so widely diffused in the arctic regions,
are also found in the chain of the Aleutian islands, which stretch
almost across from America to Asia, and which may probably have served
as the channel of communication for the partial blending of the Floras
of the adjoining regions. It has, indeed, been observed to be a general
rule, that plants found at two points very remote from each other occur
also in places intermediate.

Dr. J. Hooker informs me that in high latitudes in the southern ocean,
in spite of the great extent of the sea, Floras of widely disconnected
islands contain many species in common. Perhaps icebergs, transporting
to vast distances not only stones, but soil with the seeds of plants,
may explain this unusually wide diffusion of insular plants.

In islands very distant from continents the total number of plants is
comparatively small; but a large proportion of the species are such as
occur nowhere else. In so far as the Flora of such islands is not
peculiar to them, it contains, in general, species common to the nearest
main lands.[845] The islands of the great southern ocean exemplify these
rules; the easternmost containing more American, and the western more
Indian plants.[846] Madeira and Teneriffe contain many species, and even
entire genera, peculiar to them; but they have also plants in common
with Portugal, Spain, the Azores, and the north-west coast of
Africa.[847]

In the Canaries, out of 533 species of phaenogamous plants, it is said
that 310 are peculiar to these islands, and the rest identical with
those of the African continent; but in the Flora of St. Helena, which is
so far distant even from the western shor