Corals and coral islands.

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CORALS AND CORAL ISLANDS.

V. THE REEF-FORMING CORALS AND THE CAUSES INFLUENCING THEIR GROWTH AND DISTRIBUTION.
I. DISTRIBUTION IN LATITUDE.
Reef-forming species are the warm-water corals of the
globe. A general survey of the facts connected with the temperature of the ocean in coral-reef seas appears to sustain the
conclusion that they are confined to waters which, through even
the'coldest winter month, have a mean temperature not below
68~ F. Under the equator, the surface waters in the hotter
part of the ocean have the temperature of 850 F. in the Pacific,
and 83~ F. in the Atlantic. The range from 68~ F. to 85~ F.
is, therefore, not too great for reef:making species.
An isothermal line, crossing the ocean where this wintertemperature of the sea is experienced, one north of the equator,
and another south, bending in its course toward or from the
equator wherever the marine currents change its position,
will include all the growing reefs of the world; and the area
of waters may be properly called the coral-reef seas.
This isothermal boundary line, the isocryme (or cold-water
line) of 68~ F., extends, through mid-ocean, near the parallel of'
28~; but in the vicinity of the continents it varies greatly from
this, as explained beyond in the course of remarks on the geographical distribution of reefs. It is to be observed that the
temperature of 68Q F. is a temporary extreme-not that under
which the polyps will flourish. Except for a short period, the
waters near the limits of the coral seas are much warmer; the
mean for the year is about 73i~ F. in the North Pacific, and
70~ F. in the South; from which it may be inferred that the
summer mean would be as high at least as 78~ and 740 F.
Over the sea thus limited coral reefs grow luxuriantly, yet