456 NOTES. 



of a coral as one foot in three and a half years, and another observation, 

 in Port Darwin, giyes one foot in twelve years. On the other hand, how- 

 ever, there are other islands which prove that the upward growth of 

 corals is certainly never so rapid, and is often remarkably slow. In the 

 Sandwich Mands — which, according to Dana, are sinking — all the corals 

 live at several fathoms below the level of the water, and the case is the 

 same in the Galapagos and the Gulf of Panama. Here, by assuming a 

 subsidence, the growth upwards is less rapid than the rate of subsidence, 

 and it must be even slower, much slower, if we assume an upheaval as 

 going on in these islands. Hence I regard it as quite possible that 

 under certain circumstances a subsidence may be combined with the 

 formation of atolls, and even that it may once have been the sole 

 cause of their formation ; but I cannot admit that subsidence is alone 

 sufficient to explain all the conditions and relations of coral-reefs, or 

 even of predominant importance. 



The following letter from Mr. Charles Darwin to the author refers 

 to the subject under consideration : — 



' October 2, 1879. 



' ITy dear Professor Semper, — I thank you for your extremely kind 

 letter of the 19th and for the proof-sheets. I believe that I understand 

 all, excepting one or two sentences where my imperfect knowledge of 

 German has interfered. This is my sole excuse for the mistake which 

 I made in the second edition of my Coral-book. Tour account of the 

 Pelew Islands is a fine addition to our knowledge on coral reefs. I 

 have very little to say on the subject: even if I had formerly read your 

 account and seen your maps, but had known nothing of the proofs of 

 recent elevation, and of your belief that the islands have not since 

 subsided, I have no doubt that I should have considered them as formed 

 during subsidence. But I should have been much troubled in my mind 

 by the sea not being so deep as it usually is round atolls, and by the 

 reef on one side sloping so gradually beneath the sea ; for this latter 

 fact, as far as my memory serves me, is a very unusual and almost un- 

 paralleled case. I always foresaw that a bank at the proper depth 

 beneath the surface would give rise to a reef which could not be dis- 

 tinguished from an atoll formed during subsidence. I must still adhere 

 to my opinion that the atolls and barrier-reefs in the middle of the 

 Pacific and Indian Oceans indicate subsidence ; 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 very little value. 

 Future observers must decide between us. It will be a strange fact if 

 there has not been subsidence of the bed of the great oceans, and if 

 this has not affected the forms of the coral reefs. 



•' Yours very sincerely, 



' Ghaeleb Dabwin.' 



