_Intensity of aqueous causes._--The great problem considered in the
preceding chapters, namely, whether the former changes of the earth made
known to us by geology, resemble in kind and degree those now in daily
progress, may still be contemplated from several other points of view.
We may inquire, for example, whether there are any grounds for the
belief entertained by many, that the intensity both of aqueous and of
igneous forces, in remote ages, far exceeded that which we witness in
our own times.

First, then, as to aqueous causes: it has been shown, in our history of
the science, that Woodward did not hesitate, in 1695, to teach that the
entire mass of fossiliferous strata contained in the earth's crust had
been deposited in a few months; and, consequently, as their mechanical
and derivative origin was already admitted, the reduction of rocky
masses into mud, sand, and pebbles, the transportation of the same to a
distance, and their accumulation elsewhere in regular strata, were all
assumed to have taken place with a rapidity unparalleled in modern
times. This doctrine was modified by degrees, in proportion as different
classes of organic remains, such as shells, corals, and fossil plants,
had been studied with attention. Analogy led every naturalist to assume,
that each full-grown individual of the animal or vegetable kingdom, had
required a certain number of months or years for the attainment of
maturity, and the perpetuation of its species by generation; and thus
the first approach was made to the conception of a common standard of
time, without which there are no means whatever of measuring the
comparative rate at which any succession of events has taken place at
two distinct periods. This standard consisted of the average duration of
the lives of individuals of the same genera or families in the animal
and vegetable kingdoms; and the multitude of fossils dispersed through
successive strata implied the continuance of the same species for many
generations. At length the idea that species themselves had had a
limited duration, arose out of the observed fact that sets of strata of
different ages contained fossils of distinct species. Finally, the
opinion became general, that in the course of ages, one assemblage of
animals and plants had disappeared after another again and again, and
new tribes had started into life to replace them.

_Denudation._--In addition to the proofs derived from organic remains,
the