Critiques and Addresses by Thomas Henry Huxley
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Thomas Henry Huxley >> Critiques and Addresses
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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?
These and other puzzling facts remained insoluble until the
publication, in the year 1840, of Mr. Darwin's famous work on
coral reefs; in which a key was given to all the difficult problems
connected with the subject, and every difficulty was shown to be
capable of solution by deductive reasoning from a happy combination of
certain well-established geological and biological truths. Mr.
Darwin, in fact, showed, that so long as the level of the sea remains
unaltered in any area in which coral reefs are being formed, or if the
level of the sea relatively to that of the land is falling, the
only reefs which can be formed are fringing reefs. While if, on the
contrary, the level of the sea is rising relatively to that of the
land, at a rate not faster than that at which the upward growth of
the coral can keep pace with it, the reef will gradually pass from the
condition of a fringing, into that of an encircling or barrier reef.
And, finally, that if the relative level of the sea rise so much that
the encircled land is completely submerged, the reef must necessarily
pass into the condition of an atoll.
For, suppose the relative level of the sea to remain stationary, after
a fringing reef has reached that distance from the land at which
the depth of water amounts to 150 feet. Then the reef cannot extend
seaward by the migration of coral germs, because these coral germs
would find the bottom of the sea to be too deep for them to live in.
And the only manner in which the reef could extend outwards, would
be by the gradual accumulation, at the foot of its seaward face, of a
talus of coral fragments torn off by the violence of the waves, which
talus might, in course of time, become high enough to bring its upper
surface within the limits of coral growth, and in that manner provide
a sort of factitious sea-bottom upon which the coral embryos might
perch. If, on the other hand, the level of the sea were slowly and
gradually lowered, it is clear that the parts of its bottom originally
beyond the limit of coral growth, would gradually be brought within
the required distance of the surface, and thus the reef might be
indefinitely extended. But this process would give rise neither to an
encircling reef nor to an atoll, but to a broad belt of upheaved
coral rock, increasing the dimensions of the dry land, and continuous
seawards with the fresh fringing reef.
Suppose, however, that the sea-level rose instead of falling, at the
same slow and gradual rate at which we know it to be rising in some
parts of the world--not more, in fact, than a few inches, or, at
most, a foot or two, in a hundred years. Then, while the reef would
be unable to extend itself seaward, the sea-bottom outside it being
gradually more and more removed from the depth at which the life of
the coral polypes is possible, it would be able to grow upwards
as fast as the sea rose. But the growth would take place almost
exclusively around the circumference of the reef, this being the only
region in which the coral polypes would find the conditions favourable
for their existence. The bottom of the lagoon would be raised, in the
main, only by the coral _debris_ and coral mud, formed in the manner
already described; consequently, the margins of the reef would
rise faster than the bottom, or, in other words, the lagoon would
constantly become deeper. And, at the same time, it would gradually
increase in breadth; as the rising sea, covering more and more of the
land, would occupy a wider space between the edge of the reef and what
remained of the land. Thus the rising sea would eventually convert a
large island with a fringing reef, into a small island surrounded by
an encircling reef. And it will be obvious that when the rising of the
sea has gone so far as completely to cover the highest points of the
island, the reef will have passed into the condition of an atoll.
But how is it possible that the relative level of the land and sea
should be altered to this extent? Clearly, only in one of two ways:
either the sea must have risen over those areas which are now covered
by atolls and encircling reefs; or, the land upon which the sea rests
must have been depressed to a corresponding extent.
If the sea has risen, its rise must have taken place over the whole
world simultaneously, and it must have risen to the same height over
all parts of the coral zone. Grounds have been shown for the belief
that the general level of the sea may have been different at different
times; it has been suggested, for example, that the accumulation of
ice about the poles during one of the cold periods of the earth's
history, necessarily implies a diminution in the volume of the sea
proportioned to the amount of its water thus permanently locked up in
the Arctic and Antarctic ice-cellars; while, in the warm periods,
the greater or less disappearance of the polar ice-cap implies a
corresponding addition of water to the ocean. And no doubt this
reasoning must be admitted to be sound in principle; though it is very
hard to say what practical effect the additions and subtractions thus
made have had on the level of the ocean; inasmuch as such additions
and subtractions might be either intensified or nullified, by
contemporaneous changes in the level of the land. And no one has yet
shown that any such great melting of polar ice, and consequent raising
of the level of the water of the ocean, has taken place since the
existing atolls began to be formed.
In the absence of any evidence that the sea has ever risen to the
extent required to give rise to the encircling reefs and the atolls,
Mr. Darwin adopted the opposite hypothesis, viz. that the land has
undergone extensive and slow depression in those localities in which
these structures exist.
It seems, at first, a startling paradox, to suppose that the land
is less fixed than the sea; but that such is the case is the uniform
testimony of geology. Beds of sandstone or limestone, thousands of
feet thick, and all full of marine remains, occur in various parts of
the earth's surface, and prove, beyond a doubt, that when these beds
were formed, that portion of the sea-bottom which they then occupied
underwent a slow and gradual depression to a distance which cannot
have been less than the thickness of those beds, and may have been
very much greater. In supposing, therefore, that the great areas of
the Pacific and of the Indian Ocean, over which atolls and encircling
reefs are found scattered, have undergone a depression of some
hundreds, or, it may be, thousands of feet, Mr. Darwin made a
supposition which had nothing forced or improbable, but was entirely
in accordance with what we know to have taken place over similarly
extensive areas, in other periods of the world's history. But Mr.
Darwin subjected his hypothesis to an ingenious indirect test. If
his view be correct, it is clear that neither atolls, nor encircling
reefs, should be found in those portions of the ocean in which we have
reason to believe, on independent grounds, that the sea-bottom has
long been either stationary, or slowly rising. Now it is known that,
as a general rule, the level of the land is either stationary, or is
undergoing a slow upheaval, in the neighbourhood of active volcanoes;
and, therefore, neither atolls nor encircling reefs ought to be found
in regions in which volcanoes are numerous and active. And this turns
out to be the case. Appended to Mr. Darwin's great work on coral
reefs, there is a map on which atolls and encircling reefs are
indicated by one colour, fringing reefs by another, and active
volcanoes by a third. And it is at once obvious that the lines of
active volcanoes lie around the margins of the areas occupied by the
atolls and the encircling reefs. It is exactly as if the upheaving
volcanic agencies had lifted up the edges of these great areas, while
their centres had undergone a corresponding depression. An atoll area
may, in short, be pictured as a kind of basin, the margins of which
have been pushed up by the subterranean forces, to which the craters
of the volcanoes have, at intervals, given vent.
Thus we must imagine the area of the Pacific now covered by the
Polynesian Archipelago, as having been, at some former time,
occupied by large islands, or, may be, by a great continent, with the
ordinarily diversified surface of plain, and hill, and mountain chain.
The shores of this great land were doubtless fringed by coral reefs;
and, as it slowly underwent depression, the hilly regions, converted
into islands, became, at first, surrounded by fringing reefs, and
then, as depression went on, these became converted into encircling
reefs, and these, finally, into atolls, until a maze of reefs and
coral-girdled islets took the place of the original land masses.
Thus the atolls and the encircling reefs furnish us with clear, though
indirect, evidence of changes in the physical geography of large parts
of the earth's surface; and even, as my lamented friend, the late
Professor Jukes, has suggested, give us indications of the manner in
which some of the most puzzling facts connected with the distribution
of animals have been brought about. For example, Australia and New
Guinea are separated by Torres Straits, a broad belt of sea 100 or
120 miles wide. Nevertheless, there is in many respects a curious
resemblance between the land animals which inhabit New Guinea and
the land animals which inhabit Australia. But, at the same time, the
marine shell-fish which are found in the shallow waters of the shores
of New Guinea, are quite different from those which are met with upon
the coasts of Australia. Now, the eastern end of Torres Straits is
full of atolls, which, in fact, form the northern termination of the
Great Barrier Reef which skirts the eastern coast of Australia. It
follows, therefore, that the eastern end of Torres Straits is an area
of depression, and it is very possible, and on many grounds highly
probable, that, in former times, Australia and New Guinea were
directly connected together, and that Torres Straits did not exist.
If this were the case, the existence of cassowaries and of marsupial
quadrupeds, both in New Guinea and in Australia, becomes intelligible;
while the difference between the littoral molluscs of the north and
the south shores of Torres Straits is readily explained by the great
probability that, when the depression in question took place, and
what was, at first, an arm of the sea became converted into a strait
separating Australia from New Guinea, the northern shore of this new
sea became tenanted with marine animals from the north, while the
southern shore was peopled by immigrants from the already existing
marine Australian fauna.
Inasmuch as the growth of the reef depends upon that of successive
generations of coral polypes, and as each generation takes a certain
time to grow to its full size, and can only separate its calcareous
skeleton from the water in which it lives at a certain rate, it is
clear that the reefs are records not only of changes in physical
geography, but of the lapse of time. It is by no means easy, however,
to estimate the exact value of reef-chronology, and the attempts which
have been made to determine the rate at which a reef grows vertically,
have yielded anything but precise results. A cautious writer, Mr.
Dana, whose extensive study of corals and coral reefs makes him an
eminently competent judge, states his conclusion in the following
terms:--
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