1 1 8 Charles Darwin. 



sediment he considered impossible, as had this been 

 the case the atolls would have shown lofty pinnacles 

 and rocks elevated above the surface. 



From a consideration of this he formulated a 

 theory of subsidence, which he says " at once solves 

 the difficulty." He assumed that these islands were 

 formed in areas of subsidence, that mountains and 

 plateaux were sinking or subsiding, and that the 

 coral was, in brief, forming at a sufficiently rapid 

 rate to keep it at the surface, — a belief in which 

 Dana, the American naturalist, joined and which 

 was generally accepted. Professor Semper was one 

 o'f the first to suggest another theory, and the follow- 

 ing letter from Darwin, written many years later, 

 may be read with interest in this connection as illus- 

 trating the extreme courtesy with which he met 

 those who differed with him, and the evident desire 

 to cooperate with others in bringing out the great 

 truths of nature at whatever cost : 



" October 2, 1879. 



" My 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. Your 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 



