PREFA TOR Y NOTE. xvii 



scoriae alone remained to mark its former site — or if submarine 

 and more than ioo feet below the surface, then it would tend to 

 reach the bathymetrical zone at which the polypes live by 

 accumulation on its summit of the dead shells of foraminifera, 

 molluscs and other testaceous organisms. Then, on such a 

 peak, it is evident that the coral-polypes, growing upwards, 

 would assume the shape of an atoll. The windward side of a 

 reef thus formed grows faster than the lagoon-side, because it is 

 on that side that the currents bring food to the polypes ; and as 

 the atoll grows outward so the lagoon enlarges, owing to its 

 water containing carbonic acid, derived from the decay of the 

 polypes and the sea-weed brought in by the tides, which 

 dissolves the dead coral and removes in solution the calcium 

 carbonate of which it consists. The size of the lagoon can then 

 be taken as a general index of the age of the reef. Similarly a 

 fringing-reef may be formed round an island which has not 

 undergone, or is not undergoing, subsidence and become con- 

 verted by extension outwards, on a tains of its own debris, into 

 a barrier-reef; provided that, pari passu with the outward 

 growth, the littoral side of the reef has its channel widened by 

 the solvent action of the carbonic acid in the water obtained 

 by the disintegration of the dead polypes. Darwin did not 

 live to bring out a third edition of his book, and was 

 therefore unable to criticise this theory which had been 

 advanced by Murray. In fact, his only contribution to the 

 controversy was a letter 1 which he wrote on May 5, 1881, to 

 Alexander Agassiz, and which, as it shows in a great measure 

 that either his theory had been misrepresented or his work 

 had not been given the justice which it deserved, must be 

 reproduced here. The letter is as follows: — 'You will have 

 seen Mr. Murray's views on the formation of atolls and barrier- 

 reefs. Before publishing my book, I thought long over the 

 same view, but only as far as ordinary marine organisms are 

 concerned, for at that time little was known of the multitude 

 of minute oceanic organisms. I rejected this view, as from 

 the few dredgings made in the Beagle^ in the south temperate 

 1 Life and Letters of Charles Darwin, vol. iii. p. 183. 



