218 CORAL AND ATOLLS 



(May 5, 1881): "I have expressly said that a bank at the 

 proper depth would give rise to an atoll, which could not be 

 distinguished from one formed during subsidence." 



It is often overlooked that Darwin ever admitted any 

 alternate hypothesis to that of subsidence, and it is well, there- 

 fore, to quote his own words to show that he fully recognised 

 a state of things which, subsequently brought forward anew, 

 became a rival theory. Not only did he admit that, were 

 banks existent in the ocean of the necessary depth below the 

 surface, they would become the sites of atolls in a coral- 

 bearing sea ; but he further acknowledged that, were signs of 

 elevation and not depression common in atolls, his theory 

 would not be valid. To Semper he wrote : " But I fully agree 

 with you that such cases as that of the Pelew Islands, if of at 

 all frequent occurrence, would make my general conclusions of 

 but very little value." 



The Pelew Islands are peaks in the Caroline group, where 

 Semper found coral reefs raised up from 400 to 500 feet, and 

 only 60 miles away from them are typical atolls. 



Having seen what was Darwin's attitude towards other 

 views of atoll formation, it will be well to consider in some 

 detail his theory of subsidence ; for it is almost entirely in 

 connection with the question of the foundation of the atoll 

 structure that his theory has its individuality and its 

 importance. 



No better idea of it can be given briefly than Darwin's own 

 diagram, which shows at a glance the main points of the 

 explanation that he furnished for all coral formations. 



In the first place, a high oceanic island is pictured, 

 situated in a sea favourable to coral formation ; on the 

 submarine slopes of this island corals will flourish and form 

 a reef around the land. (See Illustration A.) 



This first stage is of course no fictitious state of things, 

 for many instances of high islands surrounded by barriers are 

 known. 



Now if the whole structure be imagined to sink slowly, 

 the corals of the barrier will grow, and compensate for the 



