ogether
of species of two distinct provinces.

_Rate of change of species cannot be uniform._--This observation leads
me to point out one of the most interesting conclusions to which we are
led by the contemplation of the vicissitudes of the inanimate world in
relation to those of the animate. It is clear that, if the agency of
inorganic causes be uniform, as I have supposed, they must operate very
irregularly on the state of organic beings, so that the rate according
to which these will change in particular regions will not be equal in
equal periods of time.

I am not about to advocate the doctrine of general catastrophes
recurring at certain intervals, as in the ancient Oriental cosmogonies,
nor do I doubt that, if very considerable periods of equal duration
could be compared one with another, the rate of change in the living, as
well as in the inorganic world, might be nearly uniform; but if we
regard each of the causes separately, which we know to be at present the
most instrumental in remodelling the state of the surface, we shall find
that we must expect each to be in action for thousands of years, without
producing any extensive alterations in the habitable surface, and then
to give rise, during a very brief period, to important revolutions.

_Illustration derived from subsidences._--I shall illustrate this
principle by a few of the most remarkable examples which present
themselves. In the course of the last century, as we have seen, a
considerable number of instances are recorded of the solid surface,
whether covered by water or not, having been permanently sunk or
upraised by subterranean movements. Most of these convulsions are only
accompanied by temporary fluctuations in the state of limited districts,
and a continued repetition of these events for thousands of years might
not produce any decided change in the state of many of those great
zoological or botanical provinces of which I have sketched the
boundaries.

When, for example, large parts of the ocean and even of inland seas are
a thousand fathoms or upwards in depth, it is a matter of no moment to
the animate creation that vast tracts should be heaved up many fathoms
at certain intervals, or should subside to the same amount. Neither can
any material revolution be produced in South America either in the
terrestrial or the marine plants or animals by a series of shocks on the
coast of Chili, each of which, like that of Penco, in 1751, should
uplift