, have
perished from want of food and water. Several hundred thousand animals
were drowned in the Parana alone, having rushed into the river to drink,
and being too much exhausted by hunger to escape.[986] Such droughts are
often attended in South America and other hot climates by wide-spreading
conflagrations, caused by lightning, which fires the dried grass and
brush-wood. Thus quadrupeds, birds, insects, and other creatures, are
destroyed by myriads. How many species, both of the animal and vegetable
world, which once flourished in the country between the valley of the
Parana and the Straits of Magellan, may not have been annihilated, since
the first drought or first conflagration began!

To pursue this train of reasoning farther is unnecessary; the geologist
has only to reflect on what has been said of the habitations and
stations of organic beings in general, and to consider them in relation
to those effects which were contemplated in the second book, as
resulting from the igneous and aqueous causes now in action, and he will
immediately perceive that, amidst the vicissitudes of the earth's
surface, species cannot be immortal, but must perish, one after the
other, like the individuals which compose them. There is no possibility
of escaping from this conclusion, without resorting to some hypothesis
as violent as that of Lamarck, who imagined, as we have before seen,
that species are each of them endowed with indefinite powers of
modifying their organization, in conformity to the endless changes of
circumstances to which they are exposed.

_Effects of a general Alteration in Climate on the Distribution of
Species._

Some of the effects which must attend every general alteration of
_climate_ are sufficiently peculiar to claim a separate consideration
before concluding the present chapter.

I have before stated that, during seasons of extraordinary severity,
many northern birds, and in some countries many quadrupeds, migrate
southwards. If these cold seasons were to become frequent, in
consequence of a gradual and general refrigeration of the atmosphere,
such migrations would be more and more regular, until, at length, many
animals, now confined to the arctic regions, would become the tenants of
the temperate zone; while the inhabitants of the temperate zone would
approach nearer to the equator. At the same time, many species
previously established on high mountains would begin to descend, in
every latitude, tow