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 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 eighteen hundred 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 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 the common ro