e reference is
sufficient for the purpose of showing that one set of creatures accounts
for that strange luminosity of the ocean which is seen at times in all
marine parts of the globe, while another set accounts not only for the
sudden appearance of coral islands in the sea where no such islands
existed in days of old, but also, partly, for that circulation of the
waters of the ocean which is absolutely necessary to the wellbeing of
all the creatures on this earth.

There are other animals in the sea, besides medusae, which assist in
giving luminosity to its waters; and there are other insects, besides
corallines, which extract its lime, destroy its equilibrium, and assist
in causing its perpetual motion; but the two species which we have
described are the best types of the respective classes to which they
belong.

CHAPTER SIXTEEN.

VOLCANIC ISLANDS--OPINIONS OF THE ANCIENTS--"ATLANTIS"--INSTANCE OF THE
FORMATION OF A VOLCANIC ISLAND--CONCLUSION.

In the last chapter we described the manner in which a certain class of
islands in the South Seas are formed; in the present we will make a few
observations on another class, which have sprung up from the bottom of
the sea, as if by magic, under the irresistible influence of fire.

There are volcanoes in the sea, as well as on the land; and these
volcanoes have in former times up-heaved huge masses of land so as to
form large islands, while in other cases they have caused islands
formerly in existence to subside and disappear.

In the writings of the ancients we find reference made to an island
which, if it ever did exist, now exists no longer. It was situated
opposite the Straits of Gibraltar, was nearly two hundred miles in
length, and was called "Atlantis"--hence the name of the Atlantic Ocean.
Many believe, and with some reason, we think, that this island was not
altogether a myth, although much that is said of it is undoubtedly
fabulous.

Plato tells us that it was a large island in the Western Ocean, situated
before or opposite to the Straits of Gades; and that out of this island
there was an easy passage into some others which lay near a large
continent, exceeding in bigness all Europe and Asia. So far Plato may
have told the truth, and from this passage it is conjectured that the
existence of the continent of America was known to the ancients. But he
goes on, immediately after, to draw upon his imagination, and to tell us
that Neptune settled on this i