on both
the opposite coasts are wasting away[985], no material alteration
results for thousands of years, save only that there is a progressive
conversion of a small strip of land into water. A few feet only, or a
few yards, are annually removed; but if, at last, the partition should
be broken down, and the tides of the ocean should enter by a direct
passage into the inland sea, instead of going by a circuitous route
through the Cattegat, a body of salt water would sweep up as far as the
Gulfs of Bothnia and Finland, the waters of which are now brackish, or
almost fresh; and this revolution would be attended by the local
annihilation of many species.

Similar consequences must have resulted on a small scale, when the sea
opened its way through the Isthmus of Staveren in the thirteenth
century, forming a union between an inland lake and the ocean, and
opening, in the course of one century, a shallow strait, more than half
as wide as the narrowest part of that which divides England from France.

_Changes in physical geography which must occasion extinction of
species._--It will almost seem superfluous, after I have thus traced the
important modifications in the condition of living beings which flow
from changes of trifling extent, to argue that entire revolutions might
be brought about, if the climate and physical geography of the whole
globe were greatly altered. It has been stated, that species are in
general local, some being confined to extremely small spots, and
depending for their existence on a combination of causes, which, if they
are to be met with elsewhere, occur only in some very remote region.
Hence it must happen that, when the nature of these localities is
changed, the species will perish; for it will rarely happen that the
cause which alters the character of the district will afford new
facilities to the species to establish itself elsewhere.

_African deserts._--If we attribute the origin of a great part of the
desert of Africa to the gradual progress of moving sands driven eastward
by the westerly winds, we may safely infer that a variety of species
must have been annihilated by this cause alone. The sand-flood has been
inundating, from time immemorial, some of the rich lands on the west of
the Nile; and we have only to multiply this effect a sufficient number
of times in order to understand how, in the lapse of ages, a whole group
of terrestrial animals and plants may become extinct.

The African d