genera of coral-building zoophytes--Their rate of growth--Seldom
flourish at greater depths than twenty fathoms--Atolls or annular
reefs with lagoons--Maldive Isles--Origin of the circular
form--Coral reefs not based on submerged volcanic craters--Mr.
Darwin's theory of subsidence in explanation of atolls, encircling
and barrier reefs--Why the windward side of atolls
highest--Subsidence explains why all atolls are nearly on one
level--Alternate areas of elevation and subsidence--Origin of
openings into the lagoons--Size of atolls and barrier
reefs--Objection to the theory of subsidence
considered--Composition, structure, and stratified arrangement of
rocks now forming in coral reefs--Lime, whence derived--Supposed
increase of calcareous matter in modern epochs
controverted--Concluding remarks.

The powers of the organic creation in modifying the form and structure
of the earth's crust, are most conspicuously displayed in the labors of
the coral animals. We may compare the operation of these zoophytes in
the ocean, to the effects produced on a smaller scale upon the land by
the plants which generate peat. In the case of the Sphagnum, the upper
part vegetates while the lower part is entering into a mineral mass, in
which the traces of organization remain when life has entirely ceased.
In corals, in like manner, the more durable materials of the generation
that has passed away serve as the foundation on which the living animals
continue to rear a similar structure.

The stony part of the lamelliform zoophyte may be likened to an internal
skeleton; for it is always more or less surrounded by a soft animal
substance capable of expanding itself; yet, when alarmed, it has the
power of contracting and drawing itself almost entirely into the cells
and hollows of the hard coral. Although oftentimes beautifully colored
in their own element, the soft parts become when taken from the sea
nothing more in appearance than a brown slime spread over the stony
nucleus.[1111]

The growth of those corals which form reefs of solid stone is entirely
confined to the warmer regions of the globe, rarely extending beyond the
tropics above two or three degrees, except under peculiar circumstances,
as in the Bermuda Islands, in lat. 32 degrees N., where the Atlantic is
warmed by the Gulf stream. The Pacific Ocean, throughout a space
comprehended between the thirtieth parallels of latitude on