oles; and the northern and southern hemispheres may become peopled to
a certain limited extent by identical species.

I say limited, because we cannot speculate on the entire transposition
of a group of animals and plants from tropical to polar latitudes, or
the reverse, as a probable or even possible event. We may believe the
mean annual temperature of one zone to be transferable to another, but
we know that the same climate cannot be so transferred. Whatever be the
general temperature of the earth's surface, comparative equability of
heat will characterize the tropical regions; while great periodical
variations will belong to the temperate, and still more to the polar
latitudes. These, and many other peculiarities connected with heat and
light, depend on fixed astronomical causes, such as the motion of the
earth and its position in relation to the sun, and not on those
fluctuations of its surface which may influence the general temperature.

Among many obstacles to such extensive transference of habitations, we
must not forget the immense lapse of time required, according to the
hypothesis before suggested, to bring about a considerable change in
climate. During a period so vast, the other cause of extirpation, before
enumerated, would exert so powerful an influence as to prevent all, save
a very few hardy species, from passing from equatorial to polar regions,
or from the tropics to the pole.[988]

But the power of accommodation to new circumstances is great in certain
species, and might enable many to pass from one zone to another, if the
mean annual heat of the atmosphere and the ocean were greatly altered.
To the marine tribes, especially, such a passage would be possible; for
they are less impeded in their migrations by barriers of land, than are
the terrestrial by the ocean. Add to this, that the temperature of the
ocean is much more uniform than that of the atmosphere investing the
land; so that we may easily suppose that most of the testacea, fish, and
other classes, might pass from the equatorial into the temperate
regions, if the mean temperature of those regions were transposed,
although a second expatriation of these species of tropical origin into
the arctic and antarctic circles would probably be impossible.

Let us now consider more particularly the effect of vicissitudes of
climate in causing one species to give way before the increasing numbers
of some other.

When temperature forms the barrier wh