President's Address. 9 



reef, through wliich the strong ebb and flow of the tide serve 

 to keep open some channels. Thus, fringing-reefs, through 

 the scour of the sea, become barrier-reefs, which retreat from 

 the adjacent coast in proportion to the gentleness of the slope 

 on which they are built. On a steeply-shelving sea-bottom 

 the reefs must, obviously, remain friuging-reefs. 



Dr Semper admitted that possibly many atolls and barrier- 

 reefs were formed during subsidence, and even that the 

 downward movement may, in many cases, have furnished the 

 conditions for starting them into existence. The solution of 

 the problem ought in each case, he thought, to be determined 

 by actual detailed observations. But that the alternate 

 currents of the tides are the main agents in the building of 

 coral reefs could be proved, he maintained, by many cases, 

 which, on the theory of subsidence, must be regarded as 

 exceptional or inexplicable, such as the occurrence of true 

 atolls in the midst of areas of elevation.^ 



In the second edition of his " Coral Islands," published in 

 1874, Darwin briefly referred to Semper's observations. He 

 thought it not improbable that the Pele\v Islands originally 

 subsided, were afterwards upraised, and again subsided ; but 

 admitted that the proximity of fringing-reefs was opposed to 

 his views. He suggested that, if the submarine slope were 

 steep, reefs ^^-hich began as fringing-reefs would continue to 

 be of that form, even during subsidence. There is, however, 

 no admission that any valid objection had been made to his 

 theory, or that true atolls and barrier-reefs might be formed 

 in many places without subsidence. 



In 1868 Professor Semper reiterated his dissent from the 

 prevailing theory of coral-reefs."^ Xext year he reprinted his 

 original paper (which seemed to him to have remained un- 

 known to most naturalists) in a general account of the 

 Philippine Islands,^ wherein he appended some additional 



^ Zeitscli. Wissenscli. Zoologie, 1863, xiii., p. 558. ReiJiiiited in 1869 in 

 " Die Philippineu und ilire Bewohner," with additional notes. 



- Verliaudl. Physik-med. Gesellsch. , Wurzburg : SitzungLer., 1st February 

 1868. 



^ Die Plii]ippinen und ilire Bewolmer, AViirzburg, 1869, pp. 100-109. A 

 brief account of the coral-reefs of the Philippine Islands will be found at pp. 

 19-33. 



