84 CONDITIONS FAVOURABLE TO Ch. IV. 



reef-building polypifers. It has been shown in the 

 chapter on Keeling atoll that there are some species of 

 large fish, and the whole tribe of Holothuria?, which 

 prey on the tenderer parts of the corals. On the other 

 hand, the polypifers in their turn must prey on other 

 organic beings ; and they would suffer by the diminu- 

 tion of their prey through any cause. The relations, 

 therefore, which determine the formation of reefs on 

 any shore, by the vigorous growth of the efficient kinds 

 of coral, must be very complex, and with our imperfect 

 knowledge inexplicable. From these considerations, 

 we may infer that changes in the condition of the sea, 

 not obvious to our senses, might destroy all the coral- 

 reefs in one area, and cause them to appear in another : 

 thus, the Pacific or Indian ocean might become as 

 barren of coral-reefs as now is the Atlantic, without our 

 being able to assign any adequate cause for such a 

 change. 1 



It has been a question with some naturalists, which 

 part of a reef is most favourable to the growth of coral. 

 The great mounds of living Porites and of Millepora 

 round Keeling atoll occur exclusively on the extreme 

 verge of the reef, which is washed by a constant suc- 



1 I have left the foregoing paragraphs nearly as they stood in the 

 first edition ; but, as stated in the Preface to the present work, Dana 

 has shown that I have undervalued the importance of the mean tempe- 

 rature of the sea during the coldest season of the year, on the distribu- 

 tion of coral-reefs, as well as perhaps the injurious effects of recent 

 volcanic action. But I cannot see that the absence of coral-reefs round 

 certain islands in the Atlantic, for instance Ascension, St. Paul's Rock, 

 and Fernando Noronha, or from the shores of the Gulf of Panama, is 

 explicable through any known cause. 



