REEF-BUILDING CORALS. 375 
and a line of green, interrupted at intervals, is traced along the 
water’s surface. 
The long swell produced by the gentle but steady action of the 
trade wind, always blowing in one direction over a wide area, 
causes breakers which even exceed in violence those of our 
temperate regions, and which never cease to rage. It is im- 
possible to behold these waves without feeling a conviction 
that a low island, though built of the hardest rock, would ulti- 
mately yield, and be demolished by such irresistible forces. Yet 
the insignificant coral-islets stand and are victorious; for here 
another power, antagonistic to the former, takes part in the 
contest. The organic forces separate the atoms of carbonate of 
lime one by one from the foaming breakers, and unite them in 
a symmetrical structure. Let the hurricane tear up its thousand 
huge fragments, yet what will this tell against the accumulated 
labours of myriads of architects at work night and day, month 
after month. Thus do we see the soft and gelatinous body 
of a polyp, through the agency of vital laws, conquering the 
great mechanical power of the waves of an ocean, which neither 
the art of man nor the inanimate works of nature could suc- 
cessfully resist. 
The reef-building corals, so hardy in this respect, are ex- 
tremely sensitive and delicate in others. They absolutely 
require warmth for their existence, and only inhabit seas the 
temperature of which never sinks below 60° Fahr. They also 
require clear and transparent waters. Wherever streams or 
currents are moving or transporting sediment, there no corals 
grow, and for the same reason we find no living zoophytes upon 
sandy or muddy shores. 
As within one cast of the lead coral-reefs rise suddenly like 
walls from the depths of ocean, it was formerly supposed that 
the polyps raised their structures out of the profound abysses of 
the sea; but this epmion could no longer be maintained, after 
Mr. C. Darwin and other naturalists had proved that the litho- 
phytes cannot live at greater depths than twenty or at most 
thirty fathoms, 
Hereupon Quoy and Gaimard broached the theory that corals 
construct their colonies on the summits of mountain ridges, or 
the circular crests of submarine craters, and thus accounted both 
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