if we thought it possible
that the palm or the elephant, which now flourish in equatorial regions,
could ever learn to bear the variable seasons of our temperate zone, or
the rigors of an arctic winter, we might with no less confidence affirm,
that they must perish before they had time to become habituated to such
new circumstances. That they would be displaced by other species as
often as the climate varied, may be inferred from the data before
explained respecting the local extermination of species produced by the
multiplication of others.

Suppose the climate of the highest part of the woody zone of Etna to be
transferred to the sea-shore of the base of the mountain, no botanist
would anticipate that the olive, lemon-tree, and prickly pear (_Cactus
Opuntia_) would be able to contend with the oak and chestnut, which
would begin forthwith to descend to a lower level; or that these last
would be able to stand their ground against the pine, which would also,
in the space of a few years, begin to occupy a lower position. We might
form some kind of estimate of the time which might be required for the
migrations of these plants; whereas we have no data for concluding that
any number of thousands of years would be sufficient for one step in
the pretended metamorphosis of one species into another, possessing
distinct attributes and qualities.

This argument is applicable not merely to _climate_, but to any other
cause of mutation. However slowly a lake may be converted into a marsh,
or a marsh into a meadow, it is evident that before the lacustrine
plants can acquire the power of living in marshes, or the marsh-plants
of living in a less humid soil, other species, already existing in the
region, and fitted for these several stations, will intrude and keep
possession of the ground. So, if a tract of salt water becomes fresh by
passing through every intermediate degree of brackishness, still the
marine mollusks will never be permitted to be gradually metamorphosed
into fluviatile species; because long before any such transformation can
take place by slow and insensible degrees, other tribes, already formed
to delight in brackish or fresh water, will avail themselves of the
change in the fluid, and will, each in their turn, monopolize the space.

It is idle, therefore, to dispute about the abstract possibility of the
conversion of one species into another, when there are known causes so
much more active in their nature, which