FORMATION OF CORAL REEFS AND ISLANDS. 195 



Mr. Darwin, in discussing the origin of the finer calcareous: 

 mud (op, cit., p. 14), supposes that it is derived in part from 

 fishes and Holothurians ; and other authors have thrown out 

 the same suggestion. He cites as a fact, on the authority of 

 Mr. Liesk, that certain fish browse on the Hving zoophytes ; 

 and from Mr. Allan, of Forres, he learned also that Holothu- 

 rians subsisted on them. With regard to the facts here stated, 

 no positive asertion can be made. Small fish swarm about the 

 branching clumps, and when disturbed, seek shelter at once 

 among the branches, where they are safe from pursuit. The 

 author has often witnessed this, and never saw reason to sup- 

 pose that they clustered about the coral for any other purpose. 

 It is an undoubted fact, as stated by Mr. Darwm, that frag- 

 ments of coral and sand may be found in the stomachs of these 

 animals, but this is not sufficient evidence of their browsing on 

 the coral. Fish so carefully avoid polyps of all kinds because 

 of their power of stinging (as illustrated on p. 16), that we 

 should wait for further and direct evidence on this point. The 

 conclusion deduced by him from the facts, may be justly 

 doubted. The fish and Holothurians, though numerous, are 

 quite inadequate for the supply ; and, moreover, we have, as 

 explained above, an abundant source of the finest coral ma- 

 terial without such aid. Motion of particle over particle will 

 necessarily wear to dust, even though the particles be diamonds; 

 and this incessant grinding acdon about reefs accounts satis- 

 factorily for the deposits of coral mud, however great their extent. 



The coral world, as we thus perceive, is planted, like the 

 land with a variety of shrubs and smaller plants, and the ele- 

 ments and natural decay are producing gradual accumulations 

 of material, like those of vegetation. The history of the 

 growing reef has consequently its counterpart among the 

 ordinary occurrences of the land about us. 



The progress of the coral formation is like its commence- 

 ment. The same causes continue, with similar results, and the 

 reader might easily supply the details from the facts already 

 presented. The production of debris will necessarily continue 



