the land had gone down； and at so slow a rate that the corals were able
to grow upward as fast as it went downward。 You will see at once that
this is the solution of the mystery； and nothing can be simpler or more
obvious when you come to think about it。 Suppose we start with a coral
sea and put in the middle of it an island such as the Mauritius。 Now
let the coral polypes come and perch on the shore and build a fringing
reef； which will stop when they come to 20 or 25 fathoms； and you will
have a fringing reef like that round the island in the illustration。 So
long as the land remains stationary； so long as it does not descend so
long will that reef be unable to get any further out； because the
moment the polype embryos try to get below they die。 But now suppose
that the land sinks very gradually indeed。 Let it subside by slow
degrees； until the mountain peak； which we have in the middle of it；
alone projects beyond the sea level。 The fringing reef would be
carried down also； but we suppose that the sinking is so slow that the
coral polypes are able to grow up as fast as the land is carried down；
consequently they will add layer upon layer until they form a deep cup；
because the inner part of the reef grows much more slowly than the
outer part。 Thus you have the reef forming a bed thicker upon the
flanks of the island； but the edge of the reef will be very much further
out from the land； and the lagoon will be many times deeper； in short；
your fringing reef will be converted into an encircling reef。 And if；
instead of this being an island； it were a great continent like
Australia； then you will have the phenomenon of a barrier reef which I
have described。 The barrier reef of Australia was originally 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 Naturewho
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；
or else is raised up by its violence。 And so Mr。 Darwin； being
desirous not to merely put out a flashy hypothesis； but to get at the
truth of the matter； said to himself； 〃If my notion of this matter is
right； then atolls and encircling reefs； inasmuch as they are dependent
upon subsidence； ought not to be found in company with volcanoes； and；
'vice versa'； volcanoes ought not to be found in company with atolls；
but they ought to be found in company with fringing reefs。〃 And if you
turn to Mr。 Darwin's great work upon the coral reefs； you will see a
very beautiful chart of the world； which he prepared with great pains
and labour； showing the distribution on the one hand of the reefs； and
on the other of the volcanoes； you will find that in no case does the
atoll accompany the volcano； or the volcano burst up among the atolls。
It is most instructive to look at the great area of the Pacific on the
map； and see the great masses of atolls forming in one region of it a
most enormous belt； running from north…west to south…east； while the
volcanoes； which are very numerous in that region； go round the margin；
so that we can picture the Pacific to ourselves a section of a kind of
very shallow basinshallow in proportion to its width； with the atolls
rising from the bottom of it； and at the margins the volcanoes。 It is
exactly as if you had taken a flat mass and lifted up the edges of it；
the subterranean force which lifted up the edges shows itself in
volcanoes； and as the edges have been raised； the middle part of the
mass has gone down。 In other words； the facts of physical geography
precisely and exactly correspond with the hypothesis which accounts for
the infinite varieties of coral reefs。
One other point； before I conclude； about this matter。 These reefs； as
you have just perceived； are in a most singular and unexpected manner
indications of physical changes of elevations and depressions going on
upon the surface of the globe。 I dare say it may have surprised you to
hear me talk in this familiar sort of way of land going up and down； but
it is one of the universal lessons of geology that the land is going
down and going up； and has been going up and down； in all sorts of
places and to all sorts of distances； through all recorded time。
Geologists would be quite right in maintaining the seeming paradox that
the stable thing in the world is the fluid sea and the shifting thing
is the solid land。 That may sound a very hard saying at first； but the
more you look into geology； the more you will see ground for believing
that it is not a mere paradox。
In an unexpected manner； again； these reefs afford us not only an
indication of change of place； but they afford an indication of lapse
of time。 The reef is a timekeeper of a very curious character； and you
can easily understand why。 The coral polype； like everything else；
takes a certain time to grow to its full size； it does not do it in a
minute； just as a child takes a certain time to grow into a man so does
the embryo polype take time to grow into a perfect polype and form its
skeleton。 Consequently every particle of coral limestone is an
expression of time。 It must have taken a certain time to separate the
lime from the sea water。 It is not possible to arrive at an accurate
computation of the time it must have taken to form these coral islands；
because we lack the necessary data； but we can form a rough calculation；
which leads to very curious and striking results。 The computations of
the rate at which corals grow are so exceedingly variable； that we must
allow the widest possible margin for error； and it is better in this
case to make the allowance upon the side of excess。 I think that
anybody who knows anything about the matter will tell you that I am
making a computation far in excess of what is probable； if I say that
an inch of coral limestone may be added to one of these reefs in the
course of a year。 I think most naturalists would be inclined to laugh
at me for making such an assumption； and would put the growth at
certainly not more than half that amount。 But supposing it is so； what
a very curious notion of the antiquity of some of these great living
pyramids comes out by a very simple calculation。 There is no doubt
whatever that the sea faces of some of them are fully a thousand feet
high； and if you take the reckoning of an inch a year； that will give
you 12；000 years for the age of that particular pyramid or cone of
coral limestone； 12；000 long years have these creatures been labouring
in conditions which must have been substantially the same as they are
now； otherwise the polypes could not have continued their work。 But I
believe I very much understate both the height of some of these masses；
and overstate the amount which these animals can form in the course of a
year； so that you might very safely double the period as the time
during which the Pacific Ocean