he main-land, to
Key West, at a distance of twelve miles from the coast, which does not,
however, close the series, for sixty miles farther west stands the
group of the Tortugas, isolated in the Gulf of Mexico. Though they
seem disconnected, these islands are parts of a submerged Coral Reef,
concentric with the shore of the peninsula and continuous underneath the
water, but visible above the surface at such points of the summit as
have fully completed their growth.

This demands some explanation, since I have already said that no Coral
growth can continue after it has reached the line of high-water. But we
have not finished the history of a Coral wall, when we have followed it
to the surface of the ocean. It is true that its normal growth ceases
there, but already a process of partial decay as begun that insures its
further increase. Here, as elsewhere, destruction and construction go
hand in hand, and the materials that are broken or worn away from one
part of the Reef help to build it up elsewhere. The Corals which form
the Reef are not the only beings that find their home there: many other
animals--Shells, Worms, Crabs, Star-Fishes, Sea-Urchins--establish
themselves upon it, work their way into its interstices, and seek a
shelter in every little hole and cranny made by the irregularities of
its surface. In the Zoological Museum at Cambridge there are some large
fragments of Coral Reef which give one a good idea of the populous
aspect that such a Reef would present, could we see it as it actually
exists beneath the water. Some of these fragments consist of a
succession of terraces, as it were, in which are many little miniature
caves, where may still be seen the Shells or Sea-Urchins which made
their snug and sheltered homes in these recesses of the Reef.

We must not consider the Reef as a solid, massive structure throughout.
The compact kinds of Corals, giving strength and solidity to the wall,
may be compared to the larger trees in a forest, which give it shade and
density; but between these grow all kinds of trailing vines, ferns and
mosses, wild flowers and low shrubs, that till the spaces between the
larger trees with a thick underbrush. The Coral Reef also has its
underbrush of the lighter, branching, more brittle kinds, that fill its
interstices and fringe the summit and the sides with their delicate,
graceful forms. Such an intricate underbrush of Coral growth affords an
excellent retreat for many animals th