by the melting of snows by lava. These
inundations might deposit alluvial matter far and wide over the original
sands, as the country assumed varied shapes, and was modified again and
again by the moving power from below, and the aqueous erosion of the
surface above. At length the Sahara might be fertilized, irrigated by
rivers and streamlets intersecting it in every direction, and covered by
jungle and morasses; so that the animals and plants which now people
Northern Africa would disappear, and the region would gradually become
fitted for the reception of a population of species perfectly dissimilar
in their forms, habits, and organization.

There are always some peculiar and characteristic features in the
physical geography of each large division of the globe; and on these
peculiarities the state of animal and vegetable life is dependent. If,
therefore, we admit incessant fluctuations in the physical geography, we
must, at the same time, concede the successive extinction of terrestrial
and aquatic species to be part of the economy of our system. When some
great class of _stations_ is in excess in certain latitudes, as, for
example, in wide savannahs, arid sands, lofty mountains, or inland seas,
we find a corresponding development of species adapted for such
circumstances. In North America, where there is a chain of vast inland
lakes of fresh water, we find an extraordinary abundance and variety of
aquatic birds, fresh-water fish, testacea, and small amphibious
reptiles, fitted for such a climate. The greater part of these would
perish if the lakes were destroyed,--an event that might be brought
about by some of the least of those important revolutions contemplated
in geology. It might happen that no fresh-water lakes of corresponding
magnitude might then exist on the globe; or that, if they occurred
elsewhere, they might be situated in New Holland, Southern Africa,
Eastern Asia, or some region so distant as to be quite inaccessible to
the North American species; or they might be situated within the
tropics, in a climate uninhabitable by creatures fitted for a temperate
zone; or, finally, we may presume that they would be pre-occupied by
_indigenous_ tribes.

A vivid description has been given by Mr. Darwin and Sir W. Parish of
the great droughts which have sometimes visited the Pampas of South
America, for three or four years in succession, during which an
incredible number of wild animals, cattle, horses, and birds