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Charles Darwin |
Charles Robert Darwin
(1809–1882). The Voyage of the Beagle.
Vol. 29, pp. 466-475 of
The Harvard Classics
Many amazing things
happen in the Malay jungles. For example, Darwin tells about a crab
that climbs trees and walks down the trunks for an occasional bath in
a pool.
Chapter
XX
During another day I visited West
Islet, on which the vegetation was perhaps more luxuriant than on any
other. The cocoa-nut trees generally grow separate, but here the
young ones flourished beneath their tall parents, and formed with
their long and curved fronds the most shady arbours. Those alone who
have tried it, know how delicious it is to be seated in such shade,
and drink the cool pleasant fluid of the cocoa-nut. In this island
there is a large bay-like space, composed of the finest white sand:
it is quite level and is only covered by the tide at high water; from
this large bay smaller creeks penetrate the surrounding woods. To see
a field of glittering white sand, representing water, with the
cocoa-nut trees extending their tall and waving trunks around the
margin, formed a singular and very pretty view.
I have before
alluded to a crab which lives on the cocoa-nuts; it is very common on
all parts of the dry land, and grows to a monstrous size: it is
closely allied or identical with the Birgos latro. The front pair of
legs terminate in very strong and heavy pincers, and the last pair
are fitted with others weaker and much narrower. It would at first be
thought quite impossible for a crab to open a strong cocoa-nut
covered with the husk; but Mr. Liesk assures me that he has
repeatedly seen this effected. The crab begins by tearing the husk,
fibre by fibre, and always from that end under which the three
eyeholes are situated; when this is completed, the crab commences
hammering with its heavy claws on one of the eye-holes till an
opening is made. Then turning round its body, by the aid of its
posterior and narrow pair of pincers, it extracts the white
albuminous substance. I think this is as curious a case of instinct
as ever I heard of, and likewise of adaptation in structure between
two objects apparently so remote from each other in the scheme of
nature, as a crab and a cocoa-nut tree. The Birgos is diurnal in its
habits; but every night it is said to pay a visit to the sea, no
doubt for the purpose of moistening its branchiæ. The young are
likewise hatched, and live for some time, on the coast. These crabs
inhabit deep burrows, which they hollow out beneath the roots of
trees; and where they accumulate surprising quantities of the picked
fibres of the cocoa-nut husk, on which they rest as on a bed. The
Malays sometimes take advantage of this, and collect the fibrous mass
to use as junk. These crabs are very good to eat; moreover, under the
tail of the larger ones there is a mass of fat, which, when melted,
sometimes yields as much as a quart bottle full of limpid oil. It has
been stated by some authors that the Birgos crawls up the cocoa-nut
trees for the purpose of stealing the nuts: I very much doubt the
possibility of this; but with the Pandanus 1 the
task would be very much easier. I was told by Mr. Liesk that on these
islands the Birgos lives only on the nuts which have fallen to the
ground.
Captain Moresby
informs me that this crab inhabits the Chagos and Seychelle groups,
but not the neighbouring Maldiva archipelago. It formerly abounded at
Mauritius, but only a few small ones are now found there. In the
Pacific, this species, or one with closely allied habits, is
said 2 to inhabit a single coral island, north
of the Society group. To show the wonderful strength of the front
pair of pincers, I may mention, that Captain Moresby confined one in
a strong tin-box, which had held biscuits, the lid being secured with
wire; but the crab turned down the edges and escaped. In turning down
the edges, it actually punched many small holes quite through the
tin!
I was a good deal surprised by finding
two species of coral of the genus Millepora (M. complanata and
alcicornis), possessed of the power of stinging. The stony branches
or plates, when taken fresh from the water, have a harsh feel and are
not slimy, although possessing a strong and disagreeable smell. The
stinging property seems to vary in different specimens: when a piece
was pressed or rubbed on the tender skin of the face or arm, a
pricking sensation was usually caused, which came on after the
interval of a second, and lasted only for a few minutes. One day,
however, by merely touching my face with one of the branches, pain
was instantaneously caused; it increased as usual after a few
seconds, and remaining sharp for some minutes, was perceptible for
half an hour afterwards. The sensation was as bad as that from a
nettle, but more like that caused by the Physalia or Portuguese
man-of-war. Little red spots were produced on the tender skin of the
arm, which appeared as if they would have formed watery pustules, but
did not. M. Quoy mentions this case of the Millepora; and I have
heard of stinging corals in the West Indies. Many marine animals seem
to have this power of stinging: besides the Portuguese man-of-war,
many jelly-fish, and the Aplysia or seaslug of the Cape de Verd
Islands, it is stated in the voyage of the Astrolabe, that an Actinia
or sea-anemone, as well as a flexible coralline allied to Sertularia,
both possess this means of offence or defence. In the East Indian
sea, a stinging sea-weed is said to be found.
Two species of fish, of the genus
Scarus, which are common here, exclusively feed on coral: both are
coloured of a splendid bluish-green, one living invariably in the
lagoon, and the other amongst the outer breakers. Mr. Liesk assured
us, that he had repeatedly seen whole shoals grazing with their
strong bony jaws on the tops of the coral branches: I opened the
intestines of several, and found them distended with yellowish
calcareous sandy mud. The slimy disgusting Holuthuriæ (allied to our
star-fish), which the Chinese gourmands are so fond of, also feed
largely, as I am informed by Dr. Allan, on corals; and the bony
apparatus within their bodies seems well adapted for this end. These
Holuthuriæ, the fish, the numerous burrowing shells, and nereidous
worms, which perforate every block of dead coral, must be very
efficient agents in producing the fine white mud which lies at the
bottom and on the shores of the lagoon. A portion, however, of this
mud, which when wet resembled pounded chalk, was found by Professor
Ehrenberg to be partly composed of siliceous-shielded infusoria.
April 12th.—In
the morning we stood out of the lagoon on our passage to the Isle of
France. I am glad we have visited these islands: such formations
surely rank high amongst the wonderful objects of this world. Captain
Fitz Roy found no bottom with a line 7200 feet in length, at the
distance of only 2200 yards from the shore; hence this island forms a
lofty submarine mountain, with sides steeper even than those of the
most abrupt volcanic cone. The saucer-shaped summit is nearly ten
miles across; and every single atom, 3 from the
least particle to the largest fragment of rock, in this great pile,
which however is small compared with very many other lagoon-islands,
bears the stamp of having been subjected to organic arrangement. We
feel surprise when travellers tell us of the vast dimensions of the
Pyramids and other great ruins, but how utterly insignificant are the
greatest of these, when compared to these mountains of stone
accumulated by the agency of various minute and tender animals! This
is a wonder which does not at first strike the eye of the body, but,
after reflection, the eye of reason.
I will now give a
very brief account of the three great classes of coral-reefs; namely,
Atolls, Barrier, and Fringing-reefs, and will explain my views 4 on
their formation. Almost every voyager who has crossed the Pacific has
expressed his unbounded astonishment at the lagoon-islands, or as I
shall for the future call them by their Indian name of atolls, and
has attempted some explanation. Even as long ago as the year 1605,
Pyrard de Laval well exclaimed, “C’est une merveille de voir
chacun de ces atollons, environné d’un grand banc de pierre tout
autour, n’y ayant point d’artifice humain.” The accompanying
sketch of Whitsunday Island in the Pacific, copied from Capt.
Beechey’s admirable Voyage, gives but a faint idea of the singular
aspect of an atoll: it is one of the smallest size, and has its
narrow islets united together in a ring. The immensity of the ocean,
the fury of the breakers, contrasted with the lowness of the land and
the smoothness of the bright green water within the lagoon, can
hardly be imagined without having been seen.
The earlier voyagers fancied that the
coral-building animals instinctively built up their great circles to
afford themselves protection in the inner parts; but so far is this
from the truth, that those massive kinds, to whose growth on the
exposed outer shores the very existence of the reef depends, cannot
live within the lagoon, where other delicately-branching kinds
flourish. Moreover, on this view, many species of distinct genera and
families are supposed to combine for one end; and of such a
combination, not a single instance can be found in the whole of
nature. The theory that has been most generally received is, that
atolls are based on submarine craters; but when we consider the form
and size of some, the number, proximity, and relative positions of
others, this idea loses its plausible character: thus Suadiva atoll
is 44 geographical miles in diameter in one line, by 34 miles in
another line; Rimsky is 54 by 20 miles across, and it has a strangely
sinuous margin; Bow atoll is 30 miles long, and on an average only 6
in width; Menchicoff atoll consists of three atolls united or tied
together. This theory, moreover, is totally inapplicable to the
northern Maldiva atolls in the Indian Ocean (one of which is 88 miles
in length, and between 10 and 20 in breadth), for they are not
bounded like ordinary atolls by narrow reefs, but by a vast number of
separate little atolls; other little atolls rising out of the great
central lagoon-like spaces. A third and better theory was advanced by
Chamisso, who thought that from the corals growing more vigorously
where exposed to the open sea, as undoubtedly is the case, the outer
edges would grow up from the general foundation before any other
part, and that this would account for the ring or cup-shaped
structure. But we shall immediately see, that in this, as well as in
the crater-theory, a most important consideration has been
overlooked, namely, on what have the reef-building corals, which
cannot live at a great depth, based their massive structures?
Numerous soundings
were carefully taken by Captain Fitz Roy on the steep outside of
Keeling atoll, and it was found that within ten fathoms, the prepared
tallow at the bottom of the lead, invariably came up marked with the
impression of living corals, but as perfectly clean as if it had been
dropped on a carpet of turf; as the depth increased, the impressions
became less numerous, but the adhering particles of sand more and
more numerous, until at last it was evident that the bottom consisted
of a smooth sandy layer: to carry on the analogy of the turf, the
blades of grass grew thinner and thinner, till at last the soil was
so sterile, that nothing sprang from it. From these observations,
confirmed by many others, it may be safely inferred that the utmost
depth at which corals can construct reefs is between 20 and 30
fathoms. Now there are enormous areas in the Pacific and Indian
Ocean, in which every single island is of coral formation, and is
raised only to that height to which the waves can throw up fragments,
and the winds pile up sand. Thus Radack group of atolls is an
irregular square, 520 miles long and 240 broad; the Low Archipelago
is elliptic-formed, 840 miles in its longer, and 420 in its shorter
axis: there are other small groups and single low islands between
these two archipelagoes, making a linear space of ocean actually more
than 4000 miles in length, in which not one single island rises above
the specified height. Again, in the Indian Ocean there is a space of
ocean 1500 miles in length, including three archipelagoes, in which
every island is low and of coral formation. From the fact of the
reef-building corals not living at great depths, it is absolutely
certain that throughout these vast areas, wherever there is now an
atoll, a foundation must have originally existed within a depth of
from 20 to 30 fathoms from the surface. It is improbable in the
highest degree that broad, lofty, isolated, steep-sided banks of
sediment, arranged in groups and lines hundreds of leagues in length,
could have been deposited in the central and profoundest parts of the
Pacific and Indian Oceans, at an immense distance from any continent,
and where the water is perfectly limpid. It is equally improbable
that the elevatory forces should have uplifted throughout the above
vast areas, innumerable great rocky banks within 20 to 30 fathoms, or
120 to 180 feet, of the surface of the sea, and not one single point
above that level; for where on the whole surface of the globe can we
find a single chain of mountains, even a few hundred miles in length,
with their many summits rising within a few feet of a given level,
and not one pinnacle above it? If then the foundations, whence the
atoll-building corals sprang, were not formed of sediment, and if
they were not lifted up to the required level, they must of necessity
have subsided into it; and this at once solves the difficulty. For as
mountain after mountain, and island after island, slowly sank beneath
the water, fresh bases would be successively afforded for the growth
of the corals. It is impossible here to enter into all the necessary
details, but I venture to defy 5 any one to
explain in any other manner how it is possible that numerous islands
should be distributed throughout vast areas-all the islands being
low-all being built of corals, absolutely requiring a foundation
within a limited depth from the surface.
Before explaining how atoll-formed
reefs acquire their peculiar structure, we must turn to the second
great class, namely, Barrier-reefs. These either extend in straight
lines in front of the shores of a continent or of a large island, or
they encircle smaller islands; in both cases, being separated from
the land by a broad and rather deep channel of water, analogous to
the lagoon within an atoll. It is remarkable how little attention has
been paid to encircling barrier-reefs; yet they are truly wonderful
structures. The following sketch represents part of the barrier
encircling the island of Bolabola in the Pacific, as seen from one of
the central peaks. In this instance the whole line of reef has been
converted into land; but usually a snow-white line of great breakers,
with only here and there a single low islet crowned with cocoa-nut
trees, divides the dark heaving waters of the ocean from the
light-green expanse of the lagoon-channel. And the quiet waters of
this channel generally bathe a fringe of low alluvial soil, loaded
with the most beautiful productions of the tropics, and lying at the
foot of the wild, abrupt, central mountains.
Encircling barrier-reefs are of all
sizes, from three miles to no less than forty-four miles in diameter;
and that which fronts one side, and encircles both ends, of New
Caledonia, is 400 miles long. Each reef includes one, two, or several
rocky islands of various heights; and in one instance, even as many
as twelve separate islands. The reef runs at a greater or less
distance from the included land; in the Society archipelago generally
from one to three or four miles; but at Hogoleu the reef is 20 miles
on the southern side, and 14 miles on the opposite or northern side,
from the included islands. The depth within the lagoon-channel also
varies much; from 10 to 30 fathoms may be taken as an average; but at
Vanikoro there are spaces no less than 56 fathoms or 363 feet deep.
Internally the reef either slopes gently into the lagoon-channel, or
ends in a perpendicular wall sometimes between two and three hundred
feet under water in height: externally the reef rises, like an atoll,
with extreme abruptness out of the profound depths of the ocean. What
can be more singular than these structures? We see an island, which
may be compared to a castle situated on the summit of a lofty
submarine mountain, protected by a great wall of coral-rock, always
steep externally and sometimes internally, with a broad level summit,
here and there breached by a narrow gateway, through which the
largest ships can enter the wide and deep encircling moat.
As far as the actual reef of coral is
concerned, there is not the smallest difference, in general size,
outline, grouping, and even in quite trifling details of structure,
between a barrier and an atoll. The geographer Balbi has well
remarked, that an encircled island is an atoll with high land rising
out of its lagoon; remove the land from within, and a perfect atoll
is left.
But what has caused these reefs to
spring up at such great distances from the shores of the included
islands? It cannot be that the corals will not grow close to the
land; for the shores within the lagoon-channel, when not surrounded
by alluvial soil, are often fringed by living reefs; and we shall
presently see that there is a whole class, which I have called
Fringing Reefs from their close attachment to the shores both of
continents and of islands. Again, on what have the reef-building
corals, which cannot live at great depths, based their encircling
structures? This is a great apparent difficulty, analogous to that in
the case of atolls, which has generally been overlooked. It will be
perceived more clearly by inspecting the above sections, which are
real ones, taken in north and south lines, through the islands with
their barrier-reefs, of Vanikoro, Gambier, and Maurua; and they are
laid down, both vertically and horizontally, on the same scale of a
quarter of an inch to a mile.
1. Vanikoro. 2. Gambier Islands. 3.
Maurua. The horizontal shading shows the barrier-reefs and
lagoon-channels. The inclined shading above the level of the sea (AA)
shows the actual form of the land; the inclined shading below this
line, shows its probable prolongation under water.
It should be observed that the
sections might have been taken in any direction through these
islands, or through many other encircled islands, and the general
features would have been the same. Now, bearing in mind that
reef-building coral cannot live at a greater depth than from 20 to 30
fathoms, and that the scale is so small that the plummets on the
right hand show a depth of 200 fathoms, on what are these
barrier-reefs based? Are we to suppose that each island is surrounded
by a collar-like submarine ledge of rock, or by a great bank of
sediment, ending abruptly where the reef ends?
If the sea had formerly eaten deeply
into the islands, before they were protected by the reefs, thus
having left a shallow ledge round them under water, the present
shores would have been invariably bounded by great precipices; but
this is most rarely the case. Moreover, on this notion, it is not
possible to explain why the corals should have sprung up, like a
wall, from the extreme outer margin of the ledge, often leaving a
broad space of water within, too deep for the growth of corals. The
accumulation of a wide bank of sediment all round these islands, and
generally widest where the included islands are smallest, is highly
improbable, considering their exposed positions in the central and
deepest parts of the ocean. In the case of the barrier-reef of New
Caledonia, which extends for 150 miles beyond the northern point of
the islands, in the same straight line with which it fronts the west
coast, it is hardly possible to believe that a bank of sediment could
thus have been straightly deposited in front of a lofty island, and
so far beyond its termination in the open sea. Finally, if we look to
other oceanic islands of about the same height and of similar
geological constitution, but not encircled by coral-reefs, we may in
vain search for so trifling a circumambient depth as 30 fathoms,
except quite near to their shores; for usually land that rises
abruptly out of water, as do most of the encircled and non-encircled
oceanic islands, plunges abruptly under it. On what then, I repeat,
are these barrier-reefs based? Why, with their wide and deep
moat-like channels, do they stand so far from the included land? We
shall soon see how easily these difficulties disappear.
Note 1. See
Proceedings of Zoological Society, 1832, p. 17.
Note 2.
Tyerman and Bennett. Voyage, etc., vol. ii. p. 33.
Note 3. I
exclude, of course, some soil which has been imported here in vessels
from Malacca and Java, and likewise some small fragments of pumice,
drifted here by the waves. The one block of greenstone, moreover, on
the northern island must be excepted.
Note 4. These
were first read before the Geological Society in May, 1837, and have
since been developed in a separate volume on the “Structure and
Distribution of Coral Reefs.”
Note 5. It is
remarkable that Mr. Lyell, even in the first edition of his
“Principles of Geology,” inferred that the amount of subsidence
in the Pacific must have exceeded that of elevation, from the area of
land being very small relatively to the agents there tending to form
it, namely, the growth of coral and volcanic action.
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