ods produce very unequal effects on climate; and, so
far as the existence of certain animals and plants depends on climate,
the duration of species would be shortened or protracted, according to
the rate at which the change of temperature proceeded.

For even if we assume that the intensity of the subterranean disturbing
forces is uniform and capable of producing nearly equal amounts of
alteration on the surface of the planet, during equal periods of time,
still the rate of alteration in climate would be by no means uniform.
Let us imagine the quantity of land between the equator and the tropic
in one hemisphere to be to that in the other as thirteen to one, which,
as before stated, represents the unequal proportion of the
extra-tropical lands in the two hemispheres at present. (See figs. 3 and
4, p. 110.) Then let the first geographical change consist in the
shifting of this preponderance of land from one side of the line to the
other; from the southern hemisphere, for example, to the northern. Now
this need not affect the general temperature of the earth. But if, at
another epoch, we suppose a continuance of the same agency to transfer
an equal volume of land from the torrid zone to the temperate and arctic
regions of the northern and southern hemispheres, or into one of them,
there might be so great a refrigeration of the mean temperature _in all
latitudes_, that scarcely any of the pre-existing races of animals would
survive; and, unless it pleased the Author of Nature that the planet
should be uninhabited, new species, and probably of widely different
forms, would then be substituted in the room of the extinct. We ought
not, therefore, to infer that equal periods of time are always attended
by an equal amount of change in organic life, since a great fluctuation
in the mean temperature of the earth, the most influential cause which
can be conceived in exterminating whole races of animals and plants,
must, in different epochs, require unequal portions of time for its
completion.

[PLATE I. _Map showing the extent of surface in Europe which has at one
period or another been covered by the sea since the commencement of the
deposition of the older or Eocene Tertiary strata._]

This map will enable the reader to perceive at a glance the great extent
of change in the physical geography of Europe, which can be proved to
have taken place since some of the older tertiary strata began to be
deposited. The proofs of submerge