lar floras, he has
found that no one of the large natural orders is so rich in species
common to other countries, as the Leguminosae. The seeds in this order,
which comprises the largest proportion of widely diffused littoral
species, are better adapted than those of any other plants for
water-carriage.

_The migration of plants aided by islands._--Islands, moreover, and even
the smallest rocks, play an important part in aiding such migrations;
for when seeds alight upon them from the atmosphere, or are thrown up by
the surf, they often vegetate, and supply the winds and waves with a
repetition of new and uninjured crops of fruit and seeds. These may
afterwards pursue their course through the atmosphere, or along the
surface of the sea, in the same direction. The number of plants found at
any given time on an islet affords us no test whatever of the extent to
which it may have co-operated towards this end, since a variety of
species may first thrive there and then perish, and be followed by other
chance-comers like themselves. If neither St. Helena nor Ascension have
promoted the botanical intercourse between the Old and New Worlds, we
may easily account for the fact by remembering that they are not only
extremely minute and isolated spots, but are also bounded by lofty and
precipitous shores without beaches, where the seeds of foreign species
could readily establish themselves.

Currents and winds in the arctic regions drift along icebergs covered
with an alluvial soil, on which herbs and pine-saplings are seen
growing, which may often continue to vegetate on some distant shore
where the ice-island is stranded.

_Dispersion of marine plants._--With respect to marine vegetation, the
seeds, being in their native element, may remain immersed in water
without injury for indefinite periods, so that there is no difficulty in
conceiving the diffusion of species wherever uncongenial climates,
contrary currents, and other causes do not interfere. All are familiar
with the sight of the floating sea-weed,

"Flung from the rock on ocean's foam to sail,
Where'er the surge may sweep, the tempest's breath prevail."

Remarkable accumulations of that species of sea-weed generally known as
gulf-weed, or sargasso, occur on each side of the equator in the
Atlantic, Pacific, and Indian Oceans. Columbus and other navigators, who
first encountered these banks of algae in the Northern Atlantic, compared
them to vast inundated me