86 CORAL-REEFS. 



islands in the Pacific, the shores of which are surrounded 

 by gigantic walls of coral-rock. With the exception of 

 Bermuda, there is not a single coral-reef in the central 

 expanse of the Atlantic Ocean. It will, perhaps, be 

 suggested that the quantity of carbonate of lime in different 

 parts of the sea, may regulate the presence of reefs. But 

 this cannot be the case, for at Ascension, the waves charged 

 to excess precipitate a thick layer of calcareous matter on 

 the tidal rocks ; and at St. Jago, in the Cape Verdes, 

 carbonate of lime not only is abundant on the shores, but 

 it forms the chief part of some upraised post-tertiary strata. 

 The apparently capricious distribution, therefore, of coral- 

 reefs, cannot be explained by any of these obvious causes ; 

 but as the study of the terrestrial and better known half of 

 the world must convince every one that no station capable 

 of supporting life is lost, — nay more, that there is a struggle 

 for each station, between the different orders of nature, — 

 we may conclude that in those parts of the intertropical sea, 

 in which there are no coral-reefs, there are other organic 

 bodies supplying the place of the 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 

 Holothuriae which prey on the tenderer parts of the corals. 

 On the other hand, the polypifers in their turn must prey 

 on some other organic beings ; the decrease of which from 

 any cause would cause a proportionate destruction of the 

 living coral. 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 quite 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 



