But how is it possible that the relative level of the land and sea
should be altered to this extent? Clearly, 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