esert, without including Bornou and Darfour, extends,
according to the calculation of Humboldt, over 194,000 square leagues;
an area nearly three times as great as that of France. In a small
portion of so vast a space, we may infer from analogy that there were
many peculiar species of plants and animals which must have been
banished by the sand, and their habitations invaded by the camel, and by
birds and insects formed for the arid sands.

There is evidently nothing in the nature of the catastrophe to favor the
escape of the former inhabitants to some adjoining province; nothing to
weaken, in the bordering lands, that powerful barrier against
emigration--pre-occupancy. Nor, even if the exclusion of a certain group
of species from a given tract were compensated by an extension of their
range over a new country, would that circumstance tend to the
conservation of species in general; for the extirpation would merely
then be transferred to the region so invaded. If it be imagined, for
example, that the aboriginal quadrupeds, birds, and other animals of
Africa, emigrated in consequence of the advance of drift-sand, and
colonized Arabia, the indigenous Arabian species must have given way
before them, and have been reduced in number or destroyed.

Let us next suppose that, in some central or more elevated parts of the
great African desert, the upheaving power of subterranean movements
should be exerted throughout an immense series of ages, accompanied, at
certain intervals, by volcanic eruptions, such as gave rise at once, in
1755, to a mountain 1600 feet high, on the Mexican plateau. When the
continued repetition of these events had caused a mountain-chain, it is
obvious that a complete transformation in the state of the climate would
be brought about throughout a vast area.

We may imagine the summits of the new chain to rise so high as to be
covered, like Mount Atlas, for several thousand feet, with snow, during
a great part of the year. The melting of these snows, during the
greatest heat, would cause the rivers to swell in the season when the
greatest drought now prevails; the waters, moreover, derived from this
source, would always be of lower temperature than the surrounding
atmosphere, and would thus contribute to cool the climate. During the
numerous earthquakes and volcanic eruptions supposed to accompany the
gradual formation of the chain, there would be many floods caused by the
bursting of temporary lakes, and