e been upheaved since they were formed, yet the sea-
bottom, while they were being formed, was sinking and not rising.
This is a fact which was first pointed out by Mr. Darwin, from the
observations which he made in the world-famous Voyage of the Beagle;
and the observations of subsequent great naturalists have all gone to
corroborate his theory.

It was supposed at first, you must understand, that when a coral
island rose steeply to the surface of the sea out of blue water,
perhaps a thousand fathoms or more, that fact was plain proof that
the little coral polypes had begun at the bottom of the sea, and, in
the course of ages, built up the whole island an enormous depth.

But it soon came out that that theory was not correct; for the coral
polypes cannot live and build save in shallow water--say in thirty to
forty fathoms. Indeed, some of the strongest and largest species
work best at the very surface, and in the cut of the fiercest surf.
And so arose a puzzle as to how coral rock is often found of vast
thickness, which Mr. Darwin explained. His theory was, and there is
no doubt now that it is correct, that in these cases the sea-bottom
is sinking; that as it sinks, carrying the coral beds down with it,
the coral dies, and a fresh live crop of polypes builds on the top of
the houses of their dead ancestors: so that, as the depression goes
on, generation after generation builds upwards, the living on the
dead, keeping the upper surface of the reef at the same level, while
its base is sinking downward into the abyss.

Applying this theory to the coral reef of the Pacific Ocean, the
following interesting facts were made out:

That where you find an Island rising out of deep water, with a ring
of coral round it, a little way from the shore--or, as in Eastern
Australia, a coast with a fringing reef (the Flinders reef of
Australia is eleven thousand miles long)--that is a pretty sure sign
that that shore, or mountain, is sinking slowly beneath the sea.
That where you find, as you often do in the Pacific, a mere atoll, or
circular reef of coral, with a shallow pond of smooth water in the
centre, and deep sea round, that is a pretty sure sign that the
mountain-top has sunk completely into the sea, and that the corals
are going on building where its peak once was.

And more. On working out the geography of the South Sea Islands by
the light of this theory of Mr. Darwin's, the following