viii PREFA TOR Y NOTE. 



sounding-lead, at a distance of 2,200 yards from the reefs edge, 

 sinks to a depth of 1,200 fathoms, while the lagoon is only from 

 two to seven fathoms deep. ' Such a basin with its deep, clear 

 channels through the reef — affording (as many of them do) 

 room enough for all the navies of Christendom to ride at 

 anchor — supplies the very perfection of harbour accommodation, 

 even though the surrounding reefs are so low that during storms 

 the breakers outside will dash over the massed wall of coral. In 

 this lagoon marine animals of all sorts — including fishes, and 

 pre-eminently sharks — swarm : but it is seldom that the wind 

 disturbs the water with its smooth, glassy expanse, and 

 curiously enough the openings in the reef are always on the 

 leeward side, that is, in the one least exposed to the prevailing 

 winds, so that while a ship has no difficulty in getting out to 

 sea, it sometimes happens that it does not find an escape from 

 the storm so easy. 5 The commonest localities for atolls are the 

 Indian and Pacific Oceans. Stewart's atoll (Fig. 3) in the 

 Solomon Islands, and the Menschikoff Island may be taken as 

 typical examples. 



Before Darwin wrote, it was universally believed that these 

 atolls were formed by the coral polypes growing upon sub- 

 merged volcanic craters. This theory finds expression and 

 support in the second volume of Lyell's Principles of Geology, 

 a bonk which was published in 1832, and which Darwin read. 

 It is known that he procured the first volume (just then issued) 

 at the suggestion of Henslow previous to his setting out on 

 the voyage of the Beagle, which started from Devonport on 

 December 27, 1831, and probably, too, that kindly, sagacious, 

 sympathetic friend and teacher forwarded him the second 

 volume as soon as published. But whatever the case 

 may be, it is evident that Darwin was acquainted with 

 the prevailing idea, and that his acute and penetrating mind 

 discerned at once its weaknesses, for he tells us in his 

 Autobiography that the main features of his theory were 

 conceived while on the voyage, and thai even previous to seeing 

 ' a true coral-reef.' ' No other work of mine, 5 he says, 1 ' was 

 1 Life and Letters of Charles Darwin, 1888, vol. i. p. 70. 



