hical range of any one
species must derange the numbers and distribution of others, let us now
direct our attention to the influence which the inorganic causes
described in the second book are continually exerting on the habitations
of species.

So great is the instability of the earth's surface, that if nature were
not continually engaged in the task of sowing seeds and colonizing
animals, the depopulation of a certain portion of the habitable sea and
land would in a few years be considerable. Whenever a river transports
sediment into a lake or sea, so as materially to diminish its depth, the
aquatic animals and plants which delight in deep water are expelled: the
tract, however, is not allowed to remain useless; but is soon peopled by
species which require more light and heat, and thrive where the water is
shallow. Every addition made to the land by the encroachment of the
delta of a river banishes many subaqueous species from their native
abodes; but the new-formed plain is not permitted to lie unoccupied,
being instantly covered with terrestrial vegetation. The ocean devours
continuous lines of sea-coasts, and precipitates forests or rich pasture
land into the waves: but this space is not lost to the animate creation;
for shells and sea-weeds soon adhere to the new-made cliffs, and
numerous fish people the channel which the current has scooped out for
itself. No sooner has a volcanic island been thrown up than some lichens
begin to grow upon it, and it is sometimes clothed with verdure while
smoke and ashes are still occasionally thrown from the crater. The
cocoa, pandanus, and mangrove take root upon the coral reef before it
has fairly risen above the waves. The burning stream of lava that
descends from Etna rolls through the stately forest, and converts to
ashes every tree and herb which stands in its way; but the black strip
of land thus desolated is covered again in the course of time, with
oaks, pines, and chestnuts, as luxuriant as those which the fiery
torrent swept away.

Every flood and landslip, every wave which a hurricane or earthquake
throws upon the shore, every shower of volcanic dust and ashes which
buries a country far and wide to the depth of many feet, every advance
of the sand-flood, every conversion of salt water into fresh when rivers
alter their main channel of discharge, every permanent variation in the
rise or fall of tides in an estuary--these and countless other causes
displace, in the c