lagoon in the middle. Thus you see that every peculiarity and
phenomenon of these different forms of coral reef was explained at once
by the simplest of all possible suppositions, namely, by supposing that
the land has gone down at a rate not greater than that at which the
coral polypes have grown up. You explain a Fringing Reef as a reef
which is formed round land comparatively stationary; an Encircling Reef
as one which is formed round land going down; and an Atoll as a reef
formed upon land gone down; and the thing is so simple that a child may
understand it when it is once explained.

But this would by no means satisfy the conditions of a scientific
hypothesis. No man who is cautious would dream of trusting to an
explanation of this kind simply because it explained one particular set
of facts. Before you can possibly be safe in dealing with Nature--who
is very properly made of the feminine gender, on account of the
astonishing tricks which she plays upon her admirers!--I say before you
can be safe in dealing with Nature, you must get two or three kinds of
cross proofs, so as to make sure not only that your hypothesis fits
that particular set of facts, but that it is not contradicted by some
other set of facts which is just as clear and certain. And it so
happens, that in this case Mr. Darwin supplied the cross proofs as well
as the immediate evidence. You have all heard of volcanoes, those
wonderful vents in the surface of the earth out of which pour masses of
lava, cinders and ashes, and the like. Now, it is a matter of
observation and experience that all volcanoes are placed in areas in
which the surface of the earth is undergoing elevation, or at any rate
is stationary; they are not placed in parts of the world in which the
level of the land is being lowered. They are all indications of a
great subterranean activity, of a something being pushed up, and
therefore naturally the land either gives way and lets it come through,