families and genera resembling each other, and even allots
species of the same genera to different countries ; the species
of plants are yet for the most part distinct in regions
thus cut off from each other. We have likewise observed,
that although the general law of Nature is what we
have stated it to be, yet many exceptions to it may be found.
We have in the next place remarked, that Nature has provided
means for the dispersion of the vegetable tribes, which
have been, to a certain extent, effectual in giving origin to
colonies or migrations of plants, and that many plants have
actually been conveyed by "such means to» distant shores
where they have vegetated. The most striking of t^ese instances
were, as it was to bfe presumed, examples in which
species had been conveyed to climates unfavourable to their
multiplication. Had the climate of the Orkneys been congenial
to the plants brought by the Gulph-stream, an abundant
vegetation from the seeds thus conveyed would long ago have
been-formed, and we should not be able to ascertain the fact
of any recent transportation of plants from tropical climates
by the great oceanic currents. But since we know that plants
have actually migrated from distant coasts', the fa c t: being
here shown by circumstances unfavourable to their multiplication,
and since it is highly improbable that these are the
only instances of the same kind which have taken place, have
we not reason to advert to the same fact in other examples,'
when identical species are found vegetating in distant places ?
A survey of the vegetation of islands and Opposite sfibrés
will throw light upon this subject.
We may observe in the first place, that in small islands;
very remote from continents, the species of plants are very
few, and sometimes quite peculiar. Thus in Kerguelen’s Land,
or the Island of Desolation, when visited by Captain Cook,
although there was soil sufficient to afford a considerable verdure,
it was found that this appearance entirely depended on
orte small plant, and . the whole flora of the land contained
only sixteen or. eighteen species, including some sorts of
mosses, and a kind ..of lichen. There was n o t'; th e least, appearance
of a shrub in the whole Country, nor were there any
animals except seals and maiine birds.; These animals were
only visitants ; but all the plants were thought by Mr. Anderson,
from whom we have the. account, .to. be peculiar to
that island.
We have • one instance of an island at no great distance
from a continent having a peculiar vegetation. Mr. R. Brown
has remarked, that there is not even a single indigenous species,
characterising the vegetation of St. Helena, that has
been found either on the banks of the Congo, or on any other
part of the western coast of Africa. Does, the diversity of
marine and atmospheric currents moré < completely separate
this island from the continent than its situation would imply,
or are the nature of soil; and other local circumstances', the
cause, of so marked a diversity •?, The last supposition seems
the .most probable; because* not only the species of plants,
but likewise, the genera in St. Helena., are different from those
of the African cqast. -
But, generally, the; flora of islands near tp continents consists
in part ofcspeciéa found also on the nearest main-lands.
In .spine, examples of this kind, circumstances confirm and
seem fully to,; establish, a conditio n which .présents itself
as a probablesjponjecture, namely, that in suck cases the
plants have .been conveyed, from{one,.shore to the other.
The Araucanja Exeelsa . of »„New : iGaledonia has been
found in New Holland. If j a doubt is raised whether it
migratèd:,from one place ,to the other,, we have to observe,
that fhe .same,, tribe inhabits- also the intermediate ? Norfolk
island, and that the part of Néw Holland where it exists is
the coast , opposite to . the islands beforejj mentioned. The
Gpodenia Littoralis whichi grows on the western shores > of
Terra Australis, h a s . been found on thé -opposite- coast-of
South America: >dt,iis; afsqs found in New Zealand, which is
situated between those shores.
M. De Candolle has remarked, that the British, isles contain
1485 vascular plants, of which there are only forty-three
that have not been discovered in France.
Malta and Sicily have some plants which belong, tp Europe,
and others of an African stock.
The opposite shores .of the Mediterranean present a remarkable
sameness in their yegetation. Out of *4577 species