The causes of this apparent caprice in the distribution of coral reefs
are not far to seek. The polypes which fabricate them require for their
vigorous growth a temperature which must not fall below 68 degrees
Fahrenheit all the year round, and this temperature is only to be
found within the distance on each side of the equator which has been
mentioned, or thereabouts. But even within the coral zone this degree of
warmth is not everywhere to be had. On the west coast of America, and on
the corresponding coast of Africa, the currents of cold water from the
icy regions which surround the South Pole set northward, and it appears
to be due to their cooling influence that the sea in these regions is
free from the reef builders. Again, the coral polypes cannot live in
water which is rendered brackish by floods from the land, or which is
perturbed by mud from the same source, and hence it is that they cease
to exist opposite the mouths of rivers, which damage them in both these
ways.
Such is the general distribution of the reef-building corals, but there
are some very interesting and singular circumstances to be observed in
the conformation of the reefs, when we consider them individually. The
reefs, in fact, are of three different kinds; some of them stretch out
from the shore, almost like a prolongation of the beach, covered only
by shallow water, and in the case of an island, surrounding it like a
fringe of no considerable breadth.
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