Coral and Coral Reefs
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Thomas H. Huxley >> Coral and Coral Reefs
This etext was prepared by Amy E. Zelmer.
CORAL AND CORAL REEFS*
by Thomas H. Huxley
*[Foonote] A Lecture delivered in Manchester, November 4th, 1970.
THE subject upon which I wish to address you to-night is the structure
and origin of Coral and Coral Reefs. Under the head of "coral" there
are included two very different things; one of them is that substance
which I imagine a great number of us have champed when we were very
much younger than we are now,--the common red coral, which is used so
much, as you know, for the edification and the delectation of children
of tender years, and is also employed for the purposes of ornament for
those who are much older, and as some think might know better. The
other kind of coral is a very different substance; it may for
distinction's sake be called the white coral; it is a material which
most assuredly not the hardest-hearted of baby farmers would give to a
baby to chew, and it is a substance which is to be seen only in the
cabinets of curious persons, or in museums, or, may be, over the
mantelpieces of sea-faring men. But although the red coral, as I have
mentioned to you, has access to the very best society; and although the
white coral is comparatively a despised product, yet in this, as in
many other cases, the humbler thing is in reality the greater; the
amount of work which is done in the world by the white coral being
absolutely infinite compared with that effected by its delicate and
pampered namesake. Each of these substances, the white coral and the
red, however, has a relationship to the other. They are, in a
zoological sense, cousins, each of them being formed by the same kind
of animals in what is substantially the same way. Each of these bodies
is, in fact, the hard skeleton of a very curious and a very simple
animal, more comparable to the bones of such animals as ourselves than
to the shells of oysters or creatures of that kind; for it is the
hardening of the internal tissue of the creature, of its internal
substance, by the deposit in the body of a material which is exceedingly
common, not only in fresh but in sea water, and which is specially
abundant in those waters which we know as "hard," those waters, for
example, which leave a "fur" upon the bottom of a tea-kettle. This
"fur" is carbonate of lime, the same sort of substance as limestone and
chalk. That material is contained in solution in sea water, and it is
out of the sea water in which these coral creatures live that they get
the lime which is needed for the forming of their hard skeleton.
But now what manner of creatures are these which form these hard
skeletons? I dare say that in these days of keeping aquaria, of
locomotion to the sea-side, most of those whom I am addressing may have
seen one of those creatures which used to be known as the "sea
anemone," receiving that name on account of its general resemblance, in
a rough sort of way, to the flower which is known as the "anemone"; but
being a thing which lives in the sea, it was qualified as the "sea
anemone." Well, then, you must suppose a body shaped like a short
cylinder, the top cut off, and in the top a hole rather oval than round.
All round this aperture, which is the mouth, imagine that there are
placed a number of feelers forming a circle. The cavity of the mouth
leads into a sort of stomach, which is very unlike those of the higher
animals, in the circumstance that it opens at the lower end into a
cavity of the body, and all the digested matter, converted into
nourishment, is thus distributed through the rest of the body. That is
the general structure of one of these sea anemones. If you touch it it
contracts immediately into a heap. It looks at first quite like a
flower in the sea, but if you touch it you find that it exhibits all
the peculiarities of a living animal; and if anything which can serve as
its prey comes near its tentacles, it closes them round it and sucks
the material into its stomach and there digests it and turns it to the
account of its own body.
These creatures are very voracious, and not at all particular what they
seize; and sometimes it may be that they lay hold of a shellfish which
is far too big to be packed into that interior cavity, and, of course,
in any ordinary animal a proceeding of this kind would give rise to a
very severe fit of indigestion. But this is by no means the case in the
sea anemone, because when digestive difficulties of this kind arise he
gets out of them by splitting himself in two; and then each half builds
itself up into a fresh creature, and you have two polypes where there
was previously one, and the bone which stuck in the way lying between
them! Not only can these creatures multiply in this fashion, but they
can multiply by buds. A bud will grow out of the side of the body (I
am not speaking of the common sea anemone, but of allied creatures)
just like the bud of a plant, and that will fashion itself into a
creature just like the parent. There are some of them in which these
buds remain connected together, and you will soon see what would be the
result of that. If I make a bud grow out here, and another on the
opposite side, and each fashions itself into a new polype, the
practical effect will be that before long you will see a single polype
converted into a sort of tree or bush of polypes. And these will all
remain associated together, like a kind of co-operative store, which is
a thing I believe you understand very well here,--each mouth will help
to feed the body and each part of the body help to support the
multifarious mouths. I think that is as good an example of a
zoological co-operative store as you can well have. Such are these
wonderful creatures. But they are capable not only of multiplying in
this way, but in other ways, by having a more ordinary and regular kind
of offspring. Little eggs are hatched and the young are passed out by
the way of the mouth, and they go swimming about as little oval bodies
covered with a very curious kind of hairlike processes. Each of these
processes is capable of striking water like an oar; and the consequence
is that the young creature is propelled through the water. So that you
have the young polype floating about in this fashion, covered by its
'vibratile cilia', as these long filaments, which are capable of
vibration are termed. And thus, although the polype itself may be a
fixed creature unable to move about, it is able to spread its offspring
over great areas. For these creatures not only propel themselves, but
while swimming about in the sea for many hours, or perhaps days, it
will be obvious that they must be carried hither and thither by the
currents of the sea, which not unfrequently move at the rate of one or
two miles an hour. Thus, in the course of a few days, the offspring of
this stationary creature may be carried to a very great distance from
its parent; and having been so carried it loses these organs by which
it is propelled, and settles down upon the bottom of the sea and grows
up again into the form and condition of its parents. So that if you
suppose a single polype of this kind settled upon the bottom of the
sea, it may by these various methods--that is to say, by cutting itself
in two, which we call "fission," or by budding; or by sending out these
swimming embryos,--multiply itself to an enormous extent, and give rise
to thousands, or millions, of progeny in a comparatively short time;
and these thousands, or millions, of progeny may cover a very large
surface of the sea bottom; in fact, you will readily perceive that, give
them time, and there is no limit to the surface which they may cover.
Having understood thus far the general nature of these polypes, which
are the fabricators both of the red and white coral, let us consider a
little more particularly how the skeletons of the red coral and of the
white coral are formed. The red coral polype perches upon the sea
bottom, it then grows up into a sort of stem, and out of that stem there
grow branches, each of which has its own polypes; and thus you have a
kind of tree formed, every branch of the tree terminated by its
polype. It is a tree, but at the end of the branches there are open
mouths of polypes instead of flowers. Thus there is a common soft body
connecting the whole, and as it grows up the soft body deposits in its
interior a quantity of carbonate of lime, which acquires a beautiful
red or flesh colour, and forms a kind of stem running through the whole,
and it is that stem which is the red coral. The red coral grows
principally at the bottom of the Mediterranean Sea, at very great
depths, and the coral fishers, who are very adventurous seamen, take
their drag nets, of a peculiar kind, roughly made, but efficient for
their purpose, and drag them along the bottom of the sea to catch the
branches of the red coral, which become entangled and are thus brought
up to the surface. They are then allowed to putrefy, in order to get
rid of the animal matter, and the red coral is the skeleton that is
left.
In the case of the white coral, the skeleton is more complete. In the
red coral, the skeleton belongs to the whole; in the white coral there
is a special skeleton for every one of these polypes in addition to
that for the whole body. There is a skeleton formed in the body of
each of them, like a cup divided by a number of radiating partitions
towards the outside; and that cup is formed of carbonate of lime, only
not stained red, as in the case of the red coral. And all these cups
are joined together into a common branch, the result of which is the
formation of a beautiful coral tree. This is a great mass of
madrepore, and in the living state every one of the ends of these
branches was terminated by a beautiful little polype, like a sea
anemone, and all the skeleton was covered by a soft body which united
the polypes together. You must understand that all this skeleton has
been formed in the interior of the body, to suit the branched body of
the polype mass, and that it is as much its skeleton as our own bones
are our skeleton. In this next coral the creature which has formed the
skeleton has divided itself as it grew, and consequently has formed a
great expansion; but scattered all over this surface there were polype
bodies like those I previously described. Again, when this great cup
was alive, the whole surface was covered with a beautiful body upon
which were set innumerable small polype flowers, if we may so call
them, often brilliantly coloured; and the whole cup was built up in the
same fashion by the deposit of carbonate of lime in the interior of the
combined polype body, formed by budding and by fission in the way I
described. You will perceive that there is no necessary limit to this
process. There is no reason why we should not have coral three or four
times as big; and there are certain creatures of this kind that do
fabricate very large masses, or half spheres several feet in diameter.
Thus the activity of these animals in separating carbonate of lime from
the sea and building it up into definite shapes is very considerable
indeed.
Now I think I have said sufficient--as much as I can without taking you
into technical details, of the general nature of these creatures which
form coral. The animals which form coral are scattered over the seas
of all countries in the world. The red coral is comparatively limited,
but the polypes which form the white coral are widely scattered. There
are some of them which remain single, or which give rise to only small
accumulations; and the skeletons of these, as they die, accumulate upon
the bottom of the sea, but they do not come to much; they are washed
about and do not adhere together, but become mixed up with the mud of
the sea. But there are certain parts of the world in which the coral
polypes which live and grow are of a kind which remain, adhere
together, and form great masses. They differ from the ordinary polypes
just in the same way as those plants which form a peat-bog or
meadow-turf differ from ordinary plants. They have a habit of growing
together in masses in the same place; they are what we call
"gregarious" things; and the consequence of this is, that as they die
and leave their skeletons, those skeletons form a considerable solid
aggregation at the bottom of the sea, and other polypes perch upon
them, and begin building upon them, and so by degrees a great mass is
formed. And just as we know there are some ancient cities in which you
have a British city, and over that the foundations of a Roman city; and
over that a Saxon city, and over that again a modern city, so in these
localities of which I am speaking, you have the accumulations of the
foundations of the houses, if I may use the term, of nation after nation
of these coral polypes; and these accumulations may cover a very
considerable space, and may rise in the course of time from the bottom
to the surface of the sea.
Mariners have a name which they apply to all sorts of obstacles
consisting of hard and rocky matter which comes in their way in the
course of their navigation; they call such obstacles "reefs," and they
have long been in the habit of calling the particular kind of reef,
which is formed by the accumulation of the skeletons of dead corals, by
the name of "coral reefs," therefore, those parts of the world in which
these accumulations occur have been termed by them "coral reef areas,"
or regions in which coral reefs are found. There is a very notable
example of a simple coral reef about the island of Mauritius, which I
dare say you all know, lies in the middle of the Indian Ocean. It is a
very considerable and beautiful island, and is surrounded on all sides
by a mass of coral, which has been formed in the way I have described;
so that if you could get upon the top of one of the peaks of the island,
and look down upon the Indian Ocean, you would see that the beach round
the Island was continued outward by a kind of shallow terrace, which is
covered by the sea, and where the sea is quite shallow; and at a
distance varying from three-quarters of a mile to a mile and a half from
the proper beach, you would see a line of foam or surf which looks most
beautiful in contrast with the bright green water in the inside, and
the deep blue of the sea beyond. That line of surf indicates the point
at which the waters of the ocean are breaking upon the coral reef which
surrounds the island. You see it sweep round the island upon all
sides, except where a river may chance to come down, and that always
makes a gap in the shore.
There are two or three points which I wish to bring clearly before your
notice about such a reef as this. In the first place, you perceive it
forms a kind of fringe round the island, and is therefore called a
"fringing reef." In the next place, if you go out in a boat, and take
soundings at the edge of the reef, you find that the depth of the water
is not more than from 20 to 25 fathoms--that is about 120 to 150 feet.
Outside that point you come to the natural sea bottom; but all inside
that depth is coral, built up from the bottom by the accumulation of the
skeletons of innumerable generations of coral polypes. So that you see
the coral forms a very considerable rampart round the island. What the
exact circumference may be I do not remember, but it cannot be less
than 100 miles, and the outward height of this wall of coral rock
nowhere amounts to less than about 100 or 150 feet.
When the outward face of the reef is examined, you find that the upper
edge, which is exposed to the wash of the sea, and all the seaward
face, is covered with those living plant-like flowers which I have
described to you. They are the coral polypes which grow, flourish, and
add to the mass of calcareous matter which already forms the reef. But
towards the lower part of the reef, at a depth of about 120 feet, these
creatures are less active, and fewer of them at work; and at greater
depths than that you find no living coral polype at all; and it may be
laid down as a rule, derived from very extensive observation, that
these reef-building corals cannot live in a greater depth of water than
about 120 to 150 feet. I beg you to recollect that fact, because it is
one I shall have to come back to by and by, and to show to what very
curious consequences that rule leads. Well then, coming back to the
margin of the reef, you find that part of it which lies just within the
surf to be coated by a very curious plant, a sort of seaweed, which
contains in its substance a very great deal of carbonate of lime, and
looks almost like rock; this is what is called the nulli pore. More
towards the land, we come to the shallow water upon the inside of the
reef, which has a particular name, derived from the Spanish or the
Portuguese--it is called a "lagoon," or lake. In this lagoon there is
comparatively little living coral; the bottom of it is formed of coral
mud. If we pounded this coral in water, it would be converted into
calcareous mud, and the waves during storms do for the coral skeletons
exactly what we might do for this coral in a mortar; the waves tear off
great fragments and crush them with prodigious force, until they are
ground into the merest powder, and that powder is washed into the
interior of the lagoon, and forms a muddy coating at the bottom. Beside
that there are a great many animals that prey upon the coral--fishes,
worms, and creatures of that kind, and all these, by their digestive
processes, reduce the coral to the same state, and contribute a very
important element to this fine mud. The living coral found in the
lagoon, is not the reef building coral; it does not give rise to the
same massive skeletons. As you go in a boat over these shallow pools,
you see these beautiful things, coloured red, blue, green, and all
colours, building their houses; but these are mere tenements, and not to
be compared in magnitude and importance to the masses which are built
by the reef-builders themselves. Now such a structure as this is what
is termed a "fringing reef." You meet with fringing reefs of this kind
not only in the Mauritius, but in a number of other parts of the world.
If these were the only reefs to be seen anywhere, the problem of the
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 deep--probably
in this case not more than eight or nine fathoms--and 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 vegetation--cocoa-nut trees--and 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 Australia--one 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 science--a 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."