epast, it floats up among the
highest summits of the Andes and is lost to sight beyond them, miles
above the line of perpetual snow, where the temperature must be lower
than that of the Arctics. But even the Condor, sweeping at one flight
from tropic heat to arctic cold, although it passes through greater
changes of temperature, does not undergo such changes of pressure as a
fish that rises from a depth of sixty-four feet to the surface of the
sea; for the former remains within the air that surrounds our globe,
and therefore the increase or diminution of pressure to which it is
subjected must be confined within the limits of one atmosphere, while
the latter, at a depth of sixty-four feet, is under a weight equal to
that of three such atmospheres, which is reduced to one when it reaches
the sea-level. The change is even much greater for those fishes that
come from a depth of several hundred feet. These laws of limitation in
space explain many facts in the growth of Coral Reefs that would be
otherwise inexplicable, and which I will endeavor to make clear to my
readers.

For a long time it was supposed that the Coral animals inhabited very
deep waters, for they were sometimes brought up on sounding-lines from a
depth of many hundreds or even thousands of feet, and it was taken for
granted that they must have had their home where they were found;
but the facts recently ascertained respecting the subsidence of
ocean-bottoms have shown that the foundation of a Coral wall may have
sunk far below the place where it was laid, and it is now proved beyond
a doubt that no Reef-Building Coral can thrive at a depth of more than
fifteen fathoms, though Corals of other kinds occur far lower, and that
the dead Reef-Corals sometimes brought to the surface from much greater
depths are only broken fragments of some Reef that has subsided with
the bottom on which it was growing. But though fifteen fathoms is the
maximum depth at which any Reef-Builder can prosper, there are many
which will not sustain even that degree of pressure, and this fact has,
as we shall see, an important influence on the structure of the Reef.

Imagine now a sloping shore on some tropical coast descending gradually
below the surface of the sea. Upon that slope, at a depth of from ten
to twelve or fifteen fathoms, and two or three or more miles from the
main-land, according to the shelving of the shore, we will suppose that
one of those little Coral animals to whom