only in one of two ways:
either the sea must have risen over those areas which are now covered by
atolls and encircling reefs; or, the land upon which the sea rests must
have been depressed to a corresponding extent.

If the sea has risen, its rise must have taken place over the whole
world simultaneously, and it must have risen to the same height over all
parts of the coral zone. Grounds have been shown for the belief that the
general level of the sea may have been different at different times; it
has been suggested, for example, that the accumulation of ice about the
poles during one of the cold periods of the earth's history necessarily
implies a diminution in the volume of the sea proportioned to the amount
of its water thus permanently locked up in the Arctic and Antarctic
ice-cellars; while, in the warm periods, the greater or less
disappearance of the polar ice-cap implies a corresponding addition of
water to the ocean. And no doubt this reasoning must be admitted to be
sound in principle; though it is very hard to say what practical effect
the additions and subtractions thus made have had on the level of the
ocean; inasmuch as such additions and subtractions might be either
intensified or nullified, by contemporaneous changes in the level of the
land. And no one has yet shown that any such great melting of polar ice,
and consequent raising of the level of the water of the ocean, has taken
place since the existing atolls began to be formed.

In the absence of any evidence that the sea has ever risen to the extent
required to give rise to the encircling reefs and the atolls, Mr. Darwin
adopted the opposite hypothesis, viz., that the land has undergone
extensive and slow depression in those localities in which these
structures exist.

It seems, at first, a startling paradox, to suppose that the land
is less fixed than the sea; but that such is the case is the uniform
testimony of geology. Beds of sandstone or limestone, thousands of feet
thick, and all full of marine remains, occur in various parts of the
earth's surface, and prove, beyond a doubt, that when these beds
were formed, that portion of the sea-bottom which they then occupied
underwent a slow and gradual depression to a distance which cannot have
been less than the thickness of those beds, and may have been very much
greater. In supposing, therefore, that the great areas of the Pacific
and of the Indian Ocean, over which atolls and encircling reefs ar