by the slow and successive
removal and creation of species, we shall be convinced that a rare
combination of circumstances alone can give rise to such a series of
strata as will bear testimony to a gradual passage from one state of
organic life to another. To produce such strata nothing less will be
requisite than the fortunate coincidence of the following conditions:
first, a never-failing supply of sediment in the same region throughout
a period of vast duration; secondly, the fitness of the deposit in every
part for the permanent preservation of imbedded fossils; and, thirdly, a
gradual subsidence to prevent the sea or lake from being filled up and
converted into land.

It will appear in the chapter on coral reefs,[260] that, in certain
parts of the Pacific and Indian Oceans, most of these conditions, if not
all, are complied with, and the constant growth of coral, keeping pace
with the sinking of the bottom of the sea, seems to have gone on so
slowly, for such indefinite periods, that the signs of a gradual change
in organic life might probably be detected in that quarter of the globe,
if we could explore its submarine geology. Instead of the growth of
coralline limestone, let us suppose, in some other place, the continuous
deposition of fluviatile mud and sand, such as the Ganges and
Brahmapootra have poured for thousands of years into the Bay of Bengal.
Part of this bay, although of considerable depth, might at length be
filled up before an appreciable amount of change was effected in the
fish, mollusca, and other inhabitants of the sea and neighboring land.
But, if the bottom be lowered by sinking at the same rate that it is
raised by fluviatile mud, the bay can never be turned into dry land. In
that case one new layer of matter may be superimposed upon another for a
thickness of many thousand feet, and the fossils of the inferior beds
may differ greatly from those entombed in the uppermost, yet every
intermediate gradation may be indicated in the passage from an older to
a newer assemblage of species. Granting, however, that such an unbroken
sequence of monuments may thus be elaborated in certain parts of the
sea, and that the strata happen to be all of them well adapted to
preserve the included fossils from decomposition, how many accidents
must still concur before these submarine formations will be laid open to
our investigation! The whole deposit must first be raised several
thousand feet, in order to bring i