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grinding them to powder； throwing them over the reef into the shallow
water inside。 But the heavier the surf beats upon them； the stronger the
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polypes outside grow； repairing their broken houses； and building up fresh
coral on the dead coral below； because it is in the fresh sea… water that
beats upon the surf that they find most lime with which to build。 And as
they build they form a barrier against the surf； inside of which； in water
still as glass； the weaker and more delicate things can grow in safety； just
as these very Encrinites may have grown； rooted in the lime…mud； and
waving their slender arms at the bottom of the clear lagoon。 Such mighty
builders are these little coral polypes； that all the works of men are small
compared with theirs。 One single reef； for instance； which is entirely
made by them； stretches along the north…east coast of Australia for nearly a
thousand miles。 Of this you must read some day in Mr。 Jukes's Voyage
of H。M。S。 〃Fly。〃 Every island throughout a great part of the Pacific is
fringed round each with its coral…reef； and there are hundreds of islands of
strange shapes； and of Atolls； as they are called； or ring…islands； which are
composed entirely of coral； and of nothing else。
A ring…island？ How can an island be made in the shape of a ring？
Ah！ it was a long time before men found out that riddle。 Mr。 Darwin
was the first to guess the answer； as he has guessed many an answer beside。
These islands are each a ring； or nearly a ring of coral； with smooth
shallow water inside： but their outsides run down； like a mountain wall；
sheer into seas hundreds of fathoms deep。 People used to believe； and
reasonably enough； that the coral polypes began to build up the islands
from the very bottom of the deep sea。
But that would not account for the top of them being of the shape of a
ring； and in time it was found out that the corals would not build except in
shallow water； twenty or thirty fathoms deep at most； and men were at
their wits' ends to find out the riddle。 Then said Mr。 Darwin； 〃Suppose one
of those beautiful South Sea Islands； like Tahiti； the Queen of Isles； with
its ring of coral… reef all round its shore； began sinking slowly under the
sea。 The land； as it sunk； would be gone for good and all： but the coral…
reef round it would not； because the coral polypes would build up and up
continually upon the skeletons of their dead parents； to get to the surface
of the water； and would keep close to the top outside； however much the
land sunk inside； and when the island had sunk completely beneath the sea；
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what would be left？ What must be left but a ring of coral reef； around
the spot where the last mountain peak of the island sank beneath the sea？〃
And so Mr。 Darwin explained the shapes of hundreds of coral islands in
the Pacific； and proved； too； some strange things besides （he proved； and
other men； like Mr。 Wallace； whose excellent book on the East Indian
islands you must read some day； have proved in other ways） that there was
once a great continent； joined perhaps to Australia and to New Guinea； in
the Pacific Ocean； where is now nothing but deep sea； and coral…reefs
which mark the mountain ranges of that sunken world。
But how does the coral ever rise above the surface of the water and
turn into hard stone？
Of course the coral polypes cannot build above the high…tide mark； but
the surf which beats upon them piles up their broken fragments just as a
sea…beach is piled up； and hammers them together with that water hammer
which is heavier and stronger than any you have ever seen in a smith's
forge。 And then； as is the fashion of lime； the whole mass sets and
becomes hard； as you may see mortar set； and so you have a low island a
few feet above the sea。 Then sea…birds come to it； and rest and build； and
seeds are floated thither from far lands； and among them almost always
the cocoa… nut； which loves to grow by the sea…shore； and groves of cocoa
palms grow up upon the lonely isle。 Then； perhaps； trees and bushes are
drifted thither before the trade…wind； and entangled in their roots are seeds
of other plants； and eggs or cocoons of insects； and so a few flowers and a
few butterflies and beetles set up for themselves upon the new land。 And
then a bird or two； caught in a storm and blown away to sea finds shelter
in the cocoa…grove； and so a little new world is set up； in which （you must
remember always） there are no four…footed beasts； nor snakes； nor lizards；
nor frogs； nor any animals that cannot cross the sea。 And on some of those
islands they may live （indeed there is reason to believe they have lived）； so
long； that some of them have changed their forms； according to the laws of
Madam How； who sooner or later fits each thing exactly for the place in
which it is meant to live； till upon some of them you may find such strange
and unique creatures as the famous cocoa…nut crab； which learned men call
Birgus latro。 A great crab he is； who walks upon the tips of his toes a
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foot high above the ground。 And because he has often nothing to eat but
cocoa…nuts； or at least they are the best things he can find； cocoa…nuts he
has learned to eat； and after a fashion which it would puzzle you to imitate。
Some say that he climbs up the stems of the cocoa…nut trees； and pulls the
fruit down for himself； but that； it seems； he does not usually do。 What he
does is this： when he finds a fallen cocoa…nut； he begins tearing away the
thick husk and fibre with his strong claws； and he knows perfectly well
which end to tear it from； namely； from the end where the three eye…holes
are； which you call the monkey's face； out of one of which you know； the
young cocoa…nut tree would burst forth。 And when he has got to the eye…
holes； he hammers through one of them with the point of his heavy claw。
So far； so good： but how is he to get the meat out？ He cannot put his
claw in。 He has no proboscis like a butterfly to insert and suck with。 He