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 

 urade 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 opinion 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 Graimard 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 



