ations from the Cambrian upwards.

_Value of the Preceding Estimate of Geological Time._--It is not of course
supposed that the calculation here given {114} makes any approach to
accuracy, but it is believed that it does indicate the _order_ of magnitude
of the time required. We have a certain number of data, which are not
guessed but the result of actual measurement; such are, the amount of solid
matter carried down by rivers, the width of the belt within which this
matter is mainly deposited, and the maximum thickness of the known
stratified rocks.[45] A considerable but unknown amount of denudation is
effected by the waves of the ocean eating away coast lines. This was once
thought to be of more importance than sub-aerial denudation, but it is now
believed to be comparatively slow in its action.[46] Whatever it may be,
however, it adds to the rate of formation of new strata, and its omission
from the calculation is again on the side of making the lapse of time
greater rather than less than the true amount. Even if a considerable
modification should be needed in some of the assumptions it has been
necessary to make, the result must still show that, so far as the time
required for the formation of the known stratified rocks, the hundred
million years allowed by physicists is not only ample, but will permit of
even more than an equal period anterior to the lowest Cambrian rocks, as
demanded by Mr. Darwin--a demand supported and enforced by the arguments,
taken from independent standpoints, of Professor Huxley and Professor
Ramsay.

_Organic Modification Dependent on Change of Conditions._--Having {115}
thus shown that the physical changes of the earth's surface may have gone
on much more rapidly and occupied much less time than has generally been
supposed, we have now to inquire whether there are any considerations which
lead to the conclusion that organic changes may have gone on with
corresponding rapidity.

There is no part of the theory of natural selection which is more clear and
satisfactory than that which connects changes of specific forms with
changes of external conditions or environment. If the external world
remains for a moderate period unchanged, the organic world soon reaches a
state of equilibrium through the struggle for existence; each species
occupies its place in nature, and there is then no inherent tendency to
change. But almost any change whatever in the external world disturbs this
equilibrium,