PRE FA TOR Y NOTE. xxiii 



against them. I noted at Diego Garcia in many places, but 

 particularly at the east end of East Islet, that a strong and 

 direct ocean current is most unfavourable to coral growth, and 

 that the reef is barren and suffering rapid erosion at such spots 

 as allow the whole force of the current to fall directly upon 

 them. As the current parts and flows round the obstacle, 

 one meets with a reef covered with debris, but barren of 

 live coral ; further on, as the current moderates in force, 

 one finds a few growing heads of coral ; and, finally, at 

 the further end of the reef, where the current has abated 

 its force considerably, there is a luxuriant bed of living 

 corals and Alcyonaria. This can be seen in perfection on 

 the southern reef of East Islet. Dr. Hickson tells me that 

 he has observed the same facts at Celebes, that direct and 

 strong currents are unfavourable to coral growth, that moderate 

 tangential currents are extremely favourable, and sluggish or 

 still water again unfavourable. This view, which both of us 

 can support by many observations, is much at variance with 

 the old accepted saying that corals grow best where the 

 breakers are the heaviest. It appeared to me that heavy 

 breakers are not favourable to coral growth, because of the quan- 

 tity of shingle which they dash against the soft-bodied polypes. 

 Some massive forms might withstand the force of breakers 

 and violent currents if the polypes could be sufficiently pro- 

 tected from the shingle, but the branching madrepores are soon 

 broken off and swept away, and even the more massive mcean- 

 drina soon follows, for whilst the surface of the colony grows 

 the base is dead, is soon riddled by boring sponges, serpulas, 

 etc., and is no longer able to bear the strain put upon it. The 

 great mass then breaks off, and is rolled along the reef, pound- 

 ing other corals in its course.' 



The whole question is still under consideration, and the 

 reader must judge for himself which of these theories he will 

 accept. But it may be stated that Professor Dana 1 — a dis- 

 tinguished authority and student of corals — has advanced his 

 opinion that ' all the hypotheses of objection to Darwin's 

 1 Am. Journ. Sci., 1885, p. 190. 



