r rises
in proportion, and by the time it has completed its growth the central
island has vanished, and there remains only a ring of Coral Reef, with
here and there a break, perhaps, at some spot where the more prosperous
growth of the Corals has been checked. If, however, as sometimes
happens, there is no such break, and the wall is perfectly
uninterrupted, the sheet of sea-water so inclosed may be changed to
fresh water by the rains that are poured into it. Such a water-basin
will remain salt, it is true, in its lower part, and the fact that it is
affected by the rise and fall of the tides shows that it is not entirely
secluded from communication with the ocean outside; but the salt water,
being heavier, sinks, while the lighter rain-water remains above, and it
is to all appearance actually changed into a fresh-water lake.

I need not dwell here on the further history of such a Coral island, or
follow it through the changes by which the summit of its circular wall
becomes covered with a fertile soil, a tropical vegetation springs up on
it, and it is at last perhaps inhabited by man. There is something very
attractive in the idea of these green rings inclosing sheltered harbors
and quiet lakes in mid-ocean, and the subject has lost none of its
fascination since the mystery of their existence has been solved by the
investigations of several contemporary naturalists who have enabled us
to trace the whole story of their structure. I would refer all who wish
for a more detailed account of them to Charles Darwin's charming
little volume on "Coral Reefs," where their mode of formation is fully
described, and also to James D. Dana's "Geological Report of the United
States Exploring Expedition."

Coral Reefs are found only in tropical regions: although Polyps, animals
of the same class as those chiefly instrumental in their formation,
are found in all parts of the globe, yet the Reef-Building Polyps are
limited to the Tropics. We are too apt to forget that the homes of
animals are as definitely limited in the water as on the land. Indeed,
the subject of the geographical distribution of animals according to
laws that are established by altitude, by latitude and longitude, by
pressure of atmosphere or pressure of water, already alluded to in
a previous article, is exceedingly interesting, and presents a most
important field of investigation. The climatic effect of different
degrees of altitude upon the growth of animals and pla