ards the middle regions; and those which were
confined to the flanks of mountains would make their way into the
plains. Analogous changes would also take place in the vegetable
kingdom.

If, on the contrary, the heat of the atmosphere be on the increase, the
plants and animals of low grounds would ascend to higher levels, the
equatorial species would migrate into the temperate zone, and those of
the temperate into the arctic circle.

But although some species might thus be preserved, every great change of
climate must be fatal to many which can find no place of retreat when
their original habitations become unfit for them. For if the general
temperature be on the rise, then there is no cooler region whither the
polar species can take refuge; if it be on the decline, then the animals
and plants previously established between the tropics have no resource.
Suppose the general heat of the atmosphere to increase, so that even the
arctic region became too warm for the musk-ox, and rein-deer, it is
clear that they must perish; so if the torrid zone should lose so much
of its heat, by the progressive refrigeration of the earth's surface, as
to be an unfit habitation for apes, boas, bamboos, and palms, these
tribes of animals and plants, or, at least; most of the species now
belonging to them, would become extinct, for there would be no warmer
latitudes for their reception.

It will follow, therefore, that as often as the climates of the globe
are passing from the extreme of heat to that of cold--from the summer to
the winter of the great year before alluded to[987]--the migratory
movement will be directed constantly from the poles towards the equator;
and for this reason the species inhabiting parallel latitudes, in the
northern and southern hemispheres, must become widely different. For I
assume, on grounds before explained, that the original stock of each
species is introduced into one spot of the earth only, and,
consequently, no species can be at once indigenous in the arctic and
antarctic circles.

But when, on the contrary, a series of changes in the physical geography
of the globe, or any other supposed cause, occasions an elevation of the
general temperature,--when there is a passage from the winter to one of
the vernal or summer seasons of the great cycle of climate,--then the
order of the migratory movement is inverted. The different species of
animals and plants direct their course from the equator towards the
p