physician, Peyssonel, conceived the desire to study these singular
sea-plants, and was sent by the French Government on a mission to the
Mediterranean for that purpose. The pupil undertook the investigation
full of confidence in the ideas of his master, but being able to see
and think for himself, he soon discovered that those ideas by no means
altogether corresponded with reality. In an essay entitled "Traité du
Corail," which was communicated to the French Academy of Science, but
which has never been published, Peyssonel writes:--
"Je fis fleurir le corail dans des vases pleins d'eau de mer,
et j'observai que ce que nous croyons être la fleur de cette
prétendue plante n'était au vrai, qu'un insecte semblable à
une petite Ortie ou Poulpe. J'avais le plaisir de voir remuer
les pattes, ou pieds, de cette Ortie, et ayant mis le vase
plein d'eau où le corail était à une douce chaleur auprès
du feu, tous les petites insectes s'épanouirent ... L'Ortie
sortie étend les pieds, et forme ce que M. de Marsigli et moi
avions pris pour les pétales de la fleur. Le calice de cette
prétendue fleur est le corps même de l'animal avancé et sorti
hors de la cellule."[1]
[Footnote 1: This extract from Peysonnel's manuscript is given by
M. Lacaze Duthiers in his valuable "Histoire Naturelle du Corail"
(1866).]
The comparison of the flowers of the coral to a "petite ortie" or
"little nettle" is perfectly just, but needs explanation. "Ortie de
mer," or "sea-nettle," is, in fact, the French appellation for our
"sea-anemone," a creature with which everybody, since the great
aquarium mania, must have become familiar, even to the limits of
boredom. In 1710, the great naturalist, Réaumur, had written a memoir
for the express purpose of demonstrating that these "orties" are
animals; and with this important paper Peyssonel must necessarily have
been familiar. Therefore, when he declared the "flowers" of the red
coral to be little "orties," it was the same thing as saying that
they were animals of the same general nature as sea-anemones. But
to Peyssonel's contemporaries this was an extremely startling
announcement. It was hard to imagine the existence of such a thing
as an association of animals into a structure with stem and branches
altogether like a plant, and fixed to the soil as a plant is fixed;
and the naturalists of that day preferred not to imagine it. Even
Réaumur could not bring himself to accept the notion, and France being
blessed with Academicians, whose great function (as the late Bishop
Wilson and an eminent modern writer have so well shown) is to cause
sweetness and light to prevail, and to prevent such unmannerly fellows
as Peyssonel from blurting out unedifying truths, they suppressed him;
and, as aforesaid, his great work remained in manuscript, and may
at this day be consulted by the curious in that state, in the
"Bibliothèque du Muséum d'Histoire Naturelle." Peyssonel, who
evidently was a person of savage and untameable disposition, so far
from appreciating the kindness of the Academicians in giving him time
to reflect upon the unreasonableness, not to say rudeness, of making
public statements in opposition to the views of some of the most
distinguished of their body, seems bitterly to have resented the
treatment he met with. For he sent all further communications to the
Royal Society of London, which never had, and it is to be hoped never
will have, anything of an academic constitution; and finally took
himself off to Guadaloupe, and became lost to science altogether.
Fifteen or sixteen years after the date of Peyssonel's suppressed
paper, the Abbé Trembley published his wonderful researches upon the
fresh-water _Hydra_. Bernard de Jussieu and Guettard followed them
up by like inquiries upon the marine sea-anemones and corallines;
Réaumur, convinced against his will of the entire justice of
Peyssonel's views, adopted them, and made him a half-and-half apology
in the preface to the next published volume of the "Mémoires pour
servir à l'Histoire des Insectes;" and, from this time forth,
Peyssonel's doctrine that corals are the work of animal organisms has
been part of the body of established scientific truth.
Peyssonel, in the extract from his memoir already cited, compares the
flower-like animal of the coral to a "poulpe," which is the French
form of the name "polypus,"--"the many-footed,"--which the ancient
naturalists gave to the soft-bodied cuttle-fishes, which, like the
coral animal, have eight arms, or tentacles, disposed around a central
mouth. Réaumur, admitting the analogy indicated by Peyssonel, gave the
name of _polypes_, not only to the sea-anemone, the coral animal, and
the fresh-water _Hydra_, but to what are now known as the _Polyzoa_,
and he termed the skeleton which they fabricate a "_polypier_" or
"polypidom."
The progress of discovery, since Réaumur's time, has made us very
completely acquainted with the structure and habits of all these
polypes. We know that, among the sea-anemones and coral-forming
animals, each polype has a mouth leading to a stomach, which is open
at its inner end, and thus communicates freely with the general cavity
of the body; that the tentacles placed round the mouth are hollow, and
that they perform the part of arms in seizing and capturing prey. It
is known that many of these creatures are capable of being multiplied
by artificial division, the divided halves growing, after a time, into
complete and separate animals; and that many are able to perform a
very similar process naturally, in such a manner that one polype may,
by repeated incomplete divisions, give rise to a sort of sheet,
or turf, formed by innumerable connected, and yet independent,
descendants. Or, what is still more common, a polype may throw out
buds, which are converted into polypes, or branches bearing polypes,
until a tree-like mass, sometimes of very considerable size, is
formed.
This is what happens in the case of the red coral of commerce. A
minute polype, fixed to the rocky bottom of the deep sea, grows up
into a branched trunk. The end of every branch and twig is terminated
by a polype; and all the polypes are connected together by a fleshy
substance, traversed by innumerable canals which place each polype in
communication with every other, and carry nourishment to the substance
of the supporting stem. It is a sort of natural co-operative store,
every polype helping the whole, at the same time as it helps itself.
The interior of the stem, like that of the branches, is solidified
by the deposition of carbonate of lime in its tissue, somewhat in the
same fashion as our own bones are formed of animal matter impregnated
with lime salts; and it is this dense skeleton (usually turned
deep red by a peculiar colouring matter) cleared of the soft animal
investment, as the heart-wood of a tree might be stripped of its bark,
which is the red coral.
In the case of the red coral, the hard skeleton belongs to the
interior of the stem and branches only; but in the commoner white
corals, each polype has a complete skeleton of its own. These
polypes ate sometimes solitary, in which case the whole skeleton is
represented by a single cup, with partitions radiating from its centre
to its circumference. When the polypes formed by budding or division
remain associated, the polypidom is sometimes made up of nothing but
an aggregation of these cups, while at other times the cups are at
once separated and held together, by an intermediate substance, which
represents the branches of the red coral. The red coral polype
again is a comparatively rare animal, inhabiting a limited area, the
skeleton of which has but a very insignificant mass; while the white
corals are very common, occur in almost all seas, and form skeletons
which are sometimes extremely massive.
With a very few exceptions, both the red and the white coral polypes
are, in their adult state, firmly adherent to the sea-bottom; nor do
their buds naturally become detached and locomotive. But, in addition
to budding and division, these creatures possess the more ordinary
methods of multiplication; and, at particular seasons, they give
rise to numerous eggs of minute size. Within these eggs the young are
formed, and they leave the egg in a condition which has no sort of
resemblance to the perfect animal. It is, in fact, a minute oval body,
many hundred times smaller than the full-grown creature, and it
swims about with great activity by the help of multitudes of little
hair-like filaments, called cilia, with which its body is covered.
These cilia all lash the water in one direction, and so drive the
little body along as if it were propelled by thousands of extremely
minute paddles. After enjoying its freedom for a longer or shorter
time, and being carried either by the force of its own cilia, or by
currents which bear it along, the embryo coral settles down to the
bottom, loses its cilia, and becomes fixed to the rock, gradually
assuming the polype form and growing up to the size of its parent.
As the infant polypes of the coral may retain this free and active
condition for many hours, or even days, and as a tidal or other
current in the sea may easily flow at the speed of two or even
more miles in an hour, it is clear that the embryo must often be
transported to very considerable distances from the parent. And it
is easily understood how a single polype, which may give rise to
hundreds, or perhaps thousands, of embryos, may, by this process of
partly active and partly passive migration, cover an immense surface
with its offspring. The masses of coral which may be formed by the
assemblages of polypes which spring by budding, or by dividing, from a
single polype, occasionally attain very considerable dimensions. Such
skeletons are sometimes great plates, many feet long and several feet
in thickness; or they may form huge half globes, like the brainstone
corals, or may reach the magnitude of stout shrubs, or even small
trees. There is reason to believe that such masses as these take a
long time to form, and hence that the age a polype tree, or polype
turf, may attain, may be considerable. But, sooner or later, the coral
polypes, like all other things, die; the soft flesh decays, while the
skeleton is left as a stony mass at the bottom of the sea, where it
retains its integrity for a longer or a shorter time, according as its
position affords it more or less protection from the wear and tear of
the waves.
The polypes which give rise to the white coral are found, as has been
said, in the seas of all parts of the world; but in the temperate and
cold oceans they are scattered and comparatively small in size,
so that the skeletons of those which die do not accumulate in any
considerable quantity. But it is otherwise in the greater part of the
ocean which lies in the warmer parts of the world, comprised within a
distance of about 1,800 miles on each side of the equator. Within the
zone thus bounded, by far the greater part of the ocean is inhabited
by coral polypes, which not only form very strong and large skeletons,
but associate together into great masses, like the thickets and the
meadow turf, or, better still, the accumulations of peat, to which
plants give rise on the dry land. These masses of stony matter, heaped
up beneath the waters of the ocean, become as dangerous to mariners
as so much ordinary rock, and to these, as to common rock ridges, the
seaman gives the name of "reefs."
Such coral reefs cover many thousand square miles in the Pacific and
in the Indian Oceans. There is one reef, or rather great series of
reefs, called the Barrier Reef, which stretches, almost continuously,
for more than 1,100 miles off the east coast of Australia. Multitudes
of the island in the Pacific are either reefs themselves, or are
surrounded by reefs. The Red Sea is in many parts almost a maze of
such reefs; and they abound no less in the West Indies, along the
coast of Florida, and even as far north as the Bahama Islands. But it
is a very remarkable circumstance that, within the area of what we may
call the "coral zone," there are no coral reefs upon the west coast of
America, nor upon the west coast of Africa; and it is a general fact
that the reefs are interrupted, or absent, opposite the mouths of
great rivers. The causes of this apparent caprice in the distribution
of coral reefs are not far to seek. The polypes which fabricate them
require for their vigorous growth a temperature which must not fall
below 68 degrees Fahrenheit all the year round, and this temperature
is only to be found within the distance on each side of the equator
which has been mentioned, or thereabouts. But even within the coral
zone this degree of warmth is not everywhere to be had. On the west
coast of America, and on the corresponding coast of Africa, currents
of cold water from the icy regions which surround the South Pole set
northward, and it appears to be due to their cooling influence that
the sea in these regions is free from the reef builders. Again, the
coral polypes cannot live in water which is rendered brackish by
floods from the land, or which is perturbed by mud from the same
source, and hence it is that they cease to exist opposite the mouths
of rivers, which damage them in both these ways.
Such is the general distribution of the reef-building corals, but
there are some very interesting and singular circumstances to be
observed in the conformation of the reefs, when we consider them
individually. The reefs, in fact, are of three different kinds; some
of them stretch out from the shore, almost like a prolongation of the
beach, covered only by shallow water, and in the case of an island,
surrounding it like a fringe of no considerable breadth. These are
termed "fringing reefs." Others are separated by a channel which may
attain a width of many miles, and a depth of twenty or thirty fathoms
or more, from the nearest land; and when this land is an island, the
reef surrounds it like a low wall, and the sea between the reef and
the land is, as it were, a moat inside this wall. Such reefs as these
are called "encircling" when they surround an island; and "barrier"
reefs, when they stretch parallel with the coast of a continent.
In both these cases there is ordinary dry land inside the reef, and
separated from it only by a narrower or a wider, a shallower or a
deeper, space of sea, which is called a "lagoon," or "inner passage."
But there is a third kind of reef, of very common occurrence in the
Pacific and Indian Oceans, which goes by the name of an "Atoll." This
is, to all intents and purposes, an encircling reef, without anything
to encircle; or, in other words, without an island in the middle
of its lagoon. The atoll has exactly the appearance of a vast,
irregularly oval, or circular, breakwater, enclosing smooth water in
its midst. The depth of the water in the lagoon rarely exceeds twenty
or thirty fathoms, but, outside the reef, it deepens with great
rapidity to 200 or 300 fathoms. The depth immediately outside the
barrier, or encircling, reefs, may also be very considerable; but, at
the outer edge of a fringing reef, it does not amount usually to more
than twenty or twenty-five fathoms; in other words, from 120 to 150
feet.
Thus, if the water of the ocean could be suddenly drained away, we
should see the atolls rising from the sea-bed like vast truncated
cones, and resembling so many volcanic craters, except that their
sides would be steeper than those of an ordinary volcano. In the case
of the encircling reefs, the cone, with the enclosed island, would
look like Vesuvius with Monte Nuovo within the old crater of Somma;
while, finally, the island with a fringing reef would have the
appearance of an ordinary hill, or mountain, girded by a vast parapet,
within which would lie a shallow moat. And the dry bed of the Pacific
might afford grounds for an inhabitant of the moon to speculate
upon the extraordinary subterranean activity to which these vast and
numerous "craters" bore witness!
When the structure of a fringing reef is investigated, the bottom of
the lagoon is found to be covered with fine whitish mud, which results
from the breaking up of the dead corals. Upon this muddy floor there
lie, here and there, growing corals, or occasionally great blocks of
dead coral, which have been torn by storms from the outer edge of
the reef, and washed into the lagoon. Shell-fish and worms of various
kinds abound; and fish, some of which prey upon the coral, sport in
the deeper pools. But the corals which are to be seen growing in the
shallow waters of the lagoon are of a different kind from those which
abound on the outer edge of the reef, and of which the reef is built
up. Close to the seaward edge of the reef, over which, even in calm
weather, a surf almost always breaks, the coral rock is encrusted with
a thick coat of a singular vegetable organism, which contains a great
deal of lime--the so-called _Nullipora_. Beyond this, in the part of
the edge of the reef which is always covered by the breaking waves,
the living, true, reef--polypes make their appearance; and, in
different forms, coat the steep seaward face of the reef to a depth of
100 or even 150 feet. Beyond this depth the sounding-lead rests, not
upon the wall-like face of the reef, but on the ordinary shelving
sea-bottom. And the distance to which a fringing reef extends from the
land corresponds with that at which the sea has a depth of twenty or
five-and-twenty fathoms.
If, as we have supposed, the sea could be suddenly withdrawn from
around an island provided with a fringing reef, such as the Mauritius,
the reef would present the aspect of a terrace, its seaward face,
100 feet or more high, blooming with the animal flowers of the coral,
while its surface would be hollowed out into a shallow and irregular
moat-like excavation.
The coral mud, which occupies the bottom of the lagoon, and with which
all the interstices of the coral skeletons which accumulate to form
the reef are filled up, does not proceed from the washing action of
the waves alone; innumerable fishes, and other creatures which prey
upon the coral, add a very important contribution of finely-triturated
calcareous matter; and the corals and mud becoming incorporated
together, gradually harden and give rise to a sort of limestone rock,
which may vary a good deal in texture. Sometimes it remains friable
and chalky, but, more often, the infiltration of water, charged with
carbonic acid, dissolves some of the calcareous matter, and deposits
it elsewhere in the interstices of the nascent rock, thus glueing
and cementing the particles together into a hard mass; or it may even
dissolve the carbonate of lime more extensively, and re-deposit it in
a crystalline form. On the beach of the lagoon, where the coral sand
is washed into layers by the action of the waves, its grains become
thus fused together into strata of a limestone, so hard that they
ring when struck with a hammer, and inclined at a gentle angle,
corresponding with that of the surface of the beach. The hard parts
of the many animals which live upon the reef become imbedded in this
coral limestone, so that a block may be full of shells of bivalves and
univalves, or of sea-urchins; and even sometimes encloses the eggs of
turtles in a state of petrifaction. The active and vigorous growth of
the reef goes on only at the seaward margins, where the polypes are
exposed to the wash of the surf, and are thereby provided with an
abundant supply of air and of food. The interior portion of the reef
may be regarded as almost wholly an accumulation of dead skeletons.
Where a river comes down from the land there is a break in the reef,
for the reasons which have been already mentioned.
The origin and mode of formation of a fringing reef, such as that just
described, are plain enough. The embryos of the coral polypes have
fixed themselves upon the submerged shore of the island, as far out as
they could live, namely, to a depth of twenty or twenty-five fathoms.
One generation has succeeded another, building itself up upon the dead
skeletons of its predecessor. The mass has been consolidated by
the infiltration of coral mud, and hardened by partial solution and
redeposition, until a great rampart of coral rock 100 or 150 feet high
on its seaward face has been formed all round the island, with only
such gaps as result from the outflow of rivers, in the place of
sally-ports.
The structure of the rocky accumulation in the encircling reefs and
in the atolls is essentially the same as in the fringing reef. But, in
addition to the differences of depth inside and out, they present
some other peculiarities. These reefs, and especially the atolls, are
usually interrupted at one part of their circumference, and this part
is always situated on the leeward side of the reef, or that which is
the more sheltered side. Now, as all these reefs are situated within
the region in which the trade-winds prevail, it follows that, on the
north side of the equator, where the trade-wind is a north-easterly
wind, the opening of the reef is on the south-west side: while in the
southern hemisphere, where the trade-winds blow from the south-east,
the opening lies to the north-west. The curious practical result
follows from this structure, that the lagoons of these reefs really
form admirable harbours, if a ship can only get inside them. But the
main difference between the encircling reefs and the atolls, on the
one hand, and the fringing reefs on the other, lies in the fact of the
much greater depth of water on the seaward faces of the former. As a
consequence of this fact, the whole of this face is not, as it is in
the case of the fringing reef, covered with living coral polypes. For,
as we have seen, these polypes cannot live at a greater depth than
about twenty-five fathoms; and actual observation has shown that
while, down to this depth, the sounding-lead will bring up branches of
live coral from the outer wall of such a reef, at a greater depth it
fetches to the surface nothing but dead coral and coral sand. We must,
therefore, picture to ourselves an atoll, or an encircling reef, as
fringed for 100 feet, or more, from its summit, with coral polypes
busily engaged in fabricating coral; while, below this comparatively
narrow belt, its surface is a bare and smooth expanse of coral sand,
supported upon and within a core of coral limestone. Thus, if the bed
of the Pacific were suddenly laid bare, as was just now supposed, the
appearance of the reef-mountains would be exactly the reverse of that
presented by many high mountains on land. For these are white with
snow at the top, while their bases are clothed with an abundant and
gaudily-coloured vegetation. But the coral cones would look grey and
barren below, while their summits would be gay with a richly-coloured
parterre of flower-like coral polypes.
The practical difficulties of sounding upon, and of bringing up
portions of, the seaward face of an atoll or of an encircling reef,
are so great, in consequence of the constant and dangerous swell which
sets towards it, that no exact information concerning the depth to
which the reefs are composed of coral has yet been obtained. There is
no reason to doubt, however, that the reef-cone has the same structure
from its summit to its base, and that its sea-wall is throughout
mainly composed of dead coral.
And now arises a serious difficulty. If the coral polypes cannot live
at a greater depth than 100 or 150 feet, how can they have built up
the base of the reef-cone, which may be 2,000 feet, or more, below the
surface of the sea?
In order to get over this objection, it was at one time supposed that
the reef-building polypes had settled upon the summits of a chain
of submarine mountains. But what is there in physical geography
to justify the assumption of the existence of a chain of mountains
stretching for 1,000 miles or more, and so nearly of the same height,
that none should rise above the level of the sea, nor fall 150 feet
below that level?
How again, on this hypothesis, are atolls to be accounted for, unless,
as some have done, we take refuge in the wild supposition that every
atoll corresponds with the crater of a submarine volcano? And what
explanation does it afford of the fact that, in some parts of the
ocean, only atolls and encircling reefs occur, while others present
none but fringing reefs?