formation of coral reefs would never have been a difficult one。 Nothing
can be easier than to understand how there must have been a time when
the coral polypes came and settled on the shores of this island；
everywhere within the 20 to 25 fathom line； and how； having perched
there； they gradually grew until they built up the reef。
But these are by no means the only sort of coral reefs in the world； on
the contrary； there are very large areas； not only of the Indian ocean；
but of the Pacific； in which many many thousands of square miles are
covered either with a peculiar kind of reef； which is called the
〃encircling reef；〃 or by a still more curious reef which goes by the
name of the 〃atoll。〃 There is a very good picture； which Professor
Roscoe has been kind enough to prepare for me； of one of these atolls；
which will enable you to form a notion of it as a landscape。 You have
in the foreground the waters of the Pacific。 You must fancy yourself
in the middle of the great ocean； and you will perceive that there is
an almost circular island； with a low beach； which is formed entirely
of coral sand； growing upon that beach you have vegetation； which takes；
of course； the shape of the circular land； and then； in the interior of
the circle； there is a pool of water； which is not very deepprobably
in this case not more than eight or nine fathomsand which forms a
strange and beautiful contrast to the deep blue water outside。 This
circular island； or atoll； with a lagoon in the middle； is not a
complete circle； upon one side of it there is a break； exactly like the
entrance into a dock； and； as a matter of course； these circular
islets； or atolls； form most efficient break…waters； for if you can only
get inside your ship is in perfect safety； with admirable anchorage in
the interior。 If the ship were lying within a mile of that beach； the
water would be one or two thousand feet deep； therefore； a section of
that atoll； with the soundings as deep as this all round； would give
you the notion of a great cone； cut off at the top； and with a shallow
cup in the middle of it。 Now； what a very singular fact this is； that
we should have rising from the bottom of the deep ocean a great pyramid；
beside which all human pyramids sink into the most utter
insignificance！ These singular coral limestone structures are very
beautiful； especially when crowned with cocoa…nut trees。 There you see
the long line of land； covered with vegetationcocoa…nut treesand you
have the sea upon the inner and outer sides； with a vessel very
comfortably riding at anchor。 That is one of the remarkable forms of
reef in the Pacific。 Another is a sort of half…way house； between the
atoll and the fringing reef； it is what is called an 〃encircling reef。〃
In this case you see an Island rising out of the sea； and at two or
three miles distance； or more； and separated by a deep channel； which
may be eight to twelve fathoms deep； there is a reef； which encircles
it like a great girdle； and outside that again the water is one or two
thousand feet deep。 I spent three or four years of my life in cruising
about a modification of one of these encircling reefs； called a
〃barrier reef；〃 upon the east coast of Australiaone of the most
wonderful accumulations of coral rock in the world。 It is about 1；100
miles long； and varies in width from one or two to many miles。 It is
separated from the coast of Australia by a channel of about 25 fathoms
deep； while outside； looking toward America； the water is two or three
thousand feet deep at a mile from the edge of the reef。 This is an
accumulation of limestone rock； built up by corals； to which we have no
parallel anywhere else。 Imagine to yourself a heap of this material
more than one thousand miles long； and several miles wide。 That is a
barrier reef； but a barrier reef is merely as it were a fragment of an
encircling reef running parallel to the coast of a great continent。
I told you that the polypes which built these reefs were not able to
live at a greater depth than 20 to 25 fathoms of water； and that is the
reason why the fringing reef goes no farther from the land than it
does。 And for the same reason； if the Pacific could be laid bare we
should have a most singular spectacle。 There would be a number of
mountains with truncated tops scattered over it； and those mountains
would have an appearance just the very reverse of that presented by the
mountains we see on shore。 You know that the mountains on shore are
covered with vegetation at their bases； while their tops are barren or
covered with snow； but these mountains would be perfectly bare at their
bases； and all round their tops they would be covered with a beautiful
vegetation of coral polypes。 And not only would this be the case； but
we should find that for a considerable distance down； all the material
of these atoll and encircling reefs was built up of precisely the same
coral rock as the fringing reef。 That is to say； you have an enormous
mass of coral rock at a depth below the surface of the water where we
know perfectly well that the coral animals could not have lived to form
it。 When those two facts were first put together； naturalists were
quite as much puzzled as I daresay you are； at present； to understand
how these two seeming contradictions could be reconciled； and all sorts
of odd hypotheses were resorted to。 It was supposed that the coral did
not extend so far down； but that there was a great chain of submarine
mountains stretching through the Pacific； and that the coral had grown
upon them。 But only fancy what supposition that was； for you would
have to imagine that there was a chain of mountains a thousand miles or
more long； and that the top of every mountain came within 20 fathoms of
the surface of the sea； and neither rose above nor sunk beneath that
level。 That is highly improbable： such a chain of mountains was never
known。 Then how can you possibly account for the curious circular form
of the atolls by any supposition of this kind？ I believe there was
some one who imagined that all these mountains were volcanoes； and that
the reefs had grown round the tops of the craters； so we all stuck
fast。 I may say 〃we；〃 though it was rather before my time。 And when we
all stick fast； it is just the use of a man of genius that he comes and
shows us the meaning of the thing。 He generally gives an explanation
which is so ridiculously simple that everybody is ashamed that he did
not find it out before； and the way such a discoverer is often rewarded
is by finding out that some one had made the discovery before him！ I
do not mean to say that it was so in this particular instance； because
the great man who played the part of Columbus and the egg on this
occasion had； I believe； always had the full credit which he so well
deserves。 The discoverer of the key to these problems was a man whose
name you know very well in connection with other matters； and I should
not wonder if some of you have heard it said that he was a superficial
kind of person who did not know much about the subject on which he
writes。 He was Mr。 Darwin； and this brilliant discovery of his was made
public thirty years ago； long before he became the celebrated man he
now is； and it was one of the most singular instances of that
astonishing sagacity which he possesses of drawing consequences by way
of deduction from simple principles of natural sciencea power which
has served him in good stead on other occasions。 Well； Mr。 Darwin；
looking at these curious difficulties and having that sort of knowledge
of natural phenomena in general； without which he could not have made a
step towards the solution of the problem； said to himself〃It is
perfectly clear that the coral which forms the base of the atolls and
fringing reefs could not possibly have been formed there if the level
of the sea has always been exactly where it is now； for we know for
certain that these polypes cannot build at a greater depth than 20 to
25 fathoms； and here we find them at 50 to 100 fathoms。〃
That was the first point to make clear。 The second point to deal with
wasif the polypes cannot have built there while the level of the sea
has remained stationary； then one of two things must have
happenedeither the sea has gone up； or the land has gone down。
There is no escape from one of these two alternatives。 Now the
objections to the notion of the sea having gone up are very
considerable indeed； for you will readily perceive that the sea could
not possibly have risen a thousand feet in the Pacific without rising
pretty much the same distance everywhere else； and if it had risen that
height everywhere else since the reefs began to be formed； the
geography of the world in general must have been very different indeed；
at that time； from what it is now。 And we have very good means of
knowing that any such rise as this certainly has not taken place in the
level of the sea since the time that the corals have been building
their houses。 And so the only other alternative was to suppose that
the land had gone down； and at so slow a rate that the corals were able
to grow upward as fast as it went downwa