84 
CONDITIONS FAVOURABLE TO 
Ch IV. 
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 fully explained by 
any of the above obvious causes ; hut, as the study of 
the terrestrial and better-known half of the world, 
must convince everyone that no station capable of 
supporting life is lost,— nay more, that there is a 
struggle for each station between different organisms, 
— we may conclude that in those parts of the inter- 
tropical sea in which there are no coral-reefs, there 
are other organic beings, 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 Holotliuriae, 1 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 
1 [See Appendix ii.] 
