108 CORALS AND CORAL ISLANDS. 
V. THE REEF-FORMING CORALS AND THE CAUSES INFLU- 
ENCING 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 tem- 
perature 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 85° F’. in the Pacific, 
and 83° F. inthe 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 winter- 
temperature 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 geo- 
graphical distribution of reefs. It is to be observed that the 
temperature of 68° 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 734° 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 74° F. 
Over the sea thus limited coral reefs grow luxuriantly, yet 
