says) be of immense
antiquity, probably several thousand years old, so that Pharaoh may have
looked upon these same individuals in the Red Sea."[1112] They certainly
imply, as he remarks, that the reef on which they grow has increased at
a very slow rate. After collecting more than 100 species, he found none
of them covered with parasitic zoophytes, nor any instance of a living
coral growing on another living coral. To this repulsive power which
they exert whilst living, against all others of their own class, we owe
the beautiful symmetry of some large Meandrinae, and other species which
adorn our museums. Yet Balani and Serpulae can attach themselves to
living corals, and holes are excavated in them by saxicavous mollusca.

At the island called Taaopoto, in the South Pacific, the anchor of a
ship, wrecked about 50 years before, was observed in seven fathoms
water, still preserving its original form, but entirely incrusted by
coral.[1113] This fact would seem to imply a slow rate of augmentation;
but to form a correct estimate of the average rate must be very
difficult, since it must vary not only according to the species of
coral, but according to the circumstances under which each species may
be placed; such, for example, as the depth from the surface, the
quantity of light, the temperature of the water, its freedom from sand
or mud, or the absence or presence of breakers, which is favorable to
the growth of some kinds and is fatal to that of others. It should also
be observed that the apparent stationary condition of some coral reefs,
which according to Beechey have remained for centuries at the same depth
under water, may be due to subsidence, the upward growth of the coral
having been just sufficient to keep pace with the sinking of the solid
foundation on which the zoophytes have built. We shall afterwards see
how far this hypothesis is borne out by other evidence in the regions of
annular reefs or atolls.

In one of the Maldive islands a coral reef, which, within a few years,
existed on an islet bearing cocoa-nut trees, was found by Lieutenant
Prentice, "_entirely covered with live coral and madrepore_." The
natives stated that the islet had been washed away by a change in the
currents, and it is clear that a coating of growing coral had been
formed in a short time.[1114] Experiments, also, of Dr. Allan, on the
east coast of Madagascar, prove the possibility of coral growing to a
thickness of three feet in abou