islands,
and during the agitations of the atmosphere which sometimes accompany
earthquakes and volcanic eruptions.

_Comparative number of living and fossilized species of plants._--It
will appear from these observations that, although the remains of
terrestrial vegetation, borne down by aqueous causes from the land, are
chiefly deposited at the bottom of lakes or at the mouths of rivers, yet
a considerable quantity is drifted about in all directions by currents,
and may become imbedded in any _marine_ formation, or may sink down,
when water-logged, to the bottom of unfathomable abysses, and there
accumulate without intermixture with other substances.

It may be asked whether we have any data for inferring that the remains
of a considerable proportion of the existing species of plants will be
permanently preserved, so as to be hereafter recognizable, supposing the
strata now in progress to be at some future period upraised? To this
inquiry it may be answered, that there are no reasons for expecting that
more than a small number of the plants now flourishing in the globe will
become fossilized; since the entire habitations of a great number of
them are remote from lakes and seas, and even where they grow near to
large bodies of water, the circumstances are quite accidental and
partial which favor the imbedding and conservation of vegetable
remains. Suppose, for example, that the species of plants inhabiting the
hydrographical basin of the Rhine, or that region, extending from the
Alps to the sea, which is watered by the Rhine and its numerous
tributaries, to be about 2500 in number, exclusive of the cryptogamic
class. This estimate is by no means exaggerated; yet if a geologist
could explore the deposits which have resulted from the sediment of the
Rhine in the Lake of Constance, and off the coast of Holland, he could
scarcely expect to obtain from the recent strata the leaves, wood, and
seeds of _fifty_ species in such a state of preservation as to enable a
botanist to determine their specific characters with certainty.

Those naturalists, therefore, who infer that the ancient flora of the
globe was, at certain periods, less varied than now, merely because they
have as yet discovered only a few hundred fossil species of a particular
epoch, while they can enumerate more than one hundred thousand living
ones, are reasoning on a false basis, and their standard of comparison
is not the same in the two cases.

_Submarine