a fringing reef; the land has gone slowly down; the
consequence is the lagoon has deepened until its depth is now 25 fathoms
and the corals have grown up at the outer edge until you have that
prodigious accumulation which forms the barrier reef at present. Now let
this process go on further still; let us take the land a further step
down, so as to submerge even the peak. The coral, still growing up, will
cover the surface of the land, and you will have an atoll reef; that is
to say, a more or less circular or oval ring of coral rock with a 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 l