CORAL INSECTS AND CORAL ISLANDS--POLYNESIA--OPERATIONS OF THE CORAL
INSECT--GROWTH OF CORAL REEFS.
Many of the large and beautiful islands that stud the Pacific Ocean,
like emeralds in a field of blue, are _artificial_; that is to say, they
were made by artists--they were actually _built_ by _artisans_!
These artisans are the coral insects; and as they not only affect the
face of the sea by raising large islands above it, but also, in
consequence of their labours, assist in causing the circulation of the
ocean, we think they are justly entitled to very special attention.
The great archipelago called Polynesia covers an area of the Pacific
nearly 5000 miles in length, and not far short of 2000 in breadth. Some
of the islands of this group are of volcanic origin, and some are
crystal; but by far the greater number are of coral formation--the work
of those curious little insects, which are so small that they inhabit a
dwelling sometimes little larger than a pin-point.
The manner in which these islands are made is, to some extent, a matter
of uncertainty. The most generally received opinion is, that the
insects fasten round the summit of a submarine mountain, and build
upwards until they reach the surface of the sea, where they die, and
their labours cease. As, however, the sea is sometimes unfathomable
close to those islands, it has been supposed that the submarine islands
on which the corallines began to build have gradually subsided, and
that, as they did so, the insects always built a little more, so as to
keep the top of their structures on a level with the sea. Above the sea
they cannot build. To be washed by the waves is essential to their
existence.
We do not think this a very satisfactory theory, because it supposes a
prolonged subsiding of these islands, and then an unaccountably sudden
stoppage. For although the corallines might continue to build during
the whole time of subsidence, it were utterly impossible that the coral
_island_, with its luxuriant herbage, could be formed until that
subsidence should have ceased. The manner in which the islands are
formed makes this obvious.
When the coral reef, as it is called, reaches the surface, it advances
no further. Soon the action of the waves breaks off the branches of the
upper portions of coral, which are tossed upon the reef, and pulverised
into fine sand. This goes on increasing until the island rises a little
above the waves.
When this happens, birds alight there; sea-drift is carried thither;
seeds are blown to it by the wind; and gradually a few green blades
arise. From this little beginning it is easy to conceive the process by
which at last a flourishing island springs up. At the same time, it is
not easy to see how such islands could ever be formed on the supposition
that the submarine rocks on which they were founded were perpetually
subsiding.
But be that as it may, we have no difficulty in understanding the fact
that the coral insect does build those islands. It possesses the power
of secreting the lime held in solution by sea water, and depositing the
same on the rocks below the waves. The coral rock is the edifice of the
coralline. The insect itself is a soft and very minute worm, which,
when washed by the waves, thrusts its head out of its tiny little door,
and spreading abroad its numerous feelers, so that it resembles a
beautiful little star, moves these about as if enjoying itself--though,
doubtless, it is actually engaged in the process of manufacturing its
little atom of coral rock.
It is extremely interesting to think of the immense power of _union_
thus exhibited. Singly, those little creatures could not produce a
sufficient result to attract the attention of any creature save such as
chanced to come in direct and close contact with its little cell.
United, they have formed vast islands, which have become the abode of
man, and which, in the aggregate, form no inconsiderable portion of the
globe.
The consideration of this leads us to perceive that God has ordained
that units cannot, separately, accomplish much; and that united effort,
in order to be successful, requires the harmonious action of units. "A
house divided against itself cannot stand." The innumerable and
eminently beautiful isles of the Pacific had never stood where they now
stand if the curious, and separately insignificant, little architects
that reared them had not wrought unitedly upon a fixed and systematic
plan--each insect working its utmost from the hour of its birth until
that of its death.
There are various kinds of coral insects, which form varied species of
coral rock. Some kinds of coral assume the form of rounded masses; some
are like a branching shrub; others are in layers, or thin plates; and
some are shaped like the human brain, from which they derive their
name--brainstones. These different kinds differ also in colour, and
thus present a beautiful appearance when seen at the bottom of clear and
shallow water.
In regard to the rate at which the corallines build their cells there is
some diversity of opinion--some asserting that the process is
imperceptible, while others state as positively that it is rapid. There
can be no doubt that some localities and positions are more favourable
to the growth of coral than others. Dr Allan, while at Madagascar,
made several experiments to test this. He selected several masses of
coral, each weighing about ten pounds, and of different species. These
he placed three feet below the surface of the sea, and staked them in to
prevent removal. In a little more than six months they were found to
have risen nearly to the surface, and to have attached themselves to the
solid rock.
There is also a case mentioned of a ship in the Persian Gulf which, in
the course of twenty months, had her copper encased with living coral to
the thickness of two feet.
On the other hand, it is asserted, and we doubt not with equal truth,
that many reefs do not seem to increase in size in the course of many
years.
When a coral reef has reached the surface, the formation of an island
instantly begins; but it necessarily takes a long time ere this island
becomes habitable by man. Among the first plants that raise their heads
to the sea-breeze is the graceful cocoa-nut palm. This tree is
exceedingly hardy, and is found growing on reefs which are so low that
at a distance the trees seem to be standing on the surface of the water.
Indeed many of them spring out of the pure white sand, and their roots
are washed perpetually by the salt spray. Nevertheless, the fruit of
such trees is sweet and good.
Coral islands of the kind we have just described seldom rise more than a
few feet above the level of the sea; but most of them are clothed with
luxuriant vegetation.
We might easily fill a volume on the subject of the ocean's inhabitants,
small and great; but we think the few to which we have made reference is
sufficient for the purpose of showing that one set of creatures accounts
for that strange luminosity of the ocean which is seen at times in all
marine parts of the globe, while another set accounts not only for the
sudden appearance of coral islands in the sea where no such islands
existed in days of old, but also, partly, for that circulation of the
waters of the ocean which is absolutely necessary to the wellbeing of
all the creatures on this earth.
There are other animals in the sea, besides medusae, which assist in
giving luminosity to its waters; and there are other insects, besides
corallines, which extract its lime, destroy its equilibrium, and assist
in causing its perpetual motion; but the two species which we have
described are the best types of the respective classes to which they
belong.