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APPENDIX II. 
Mr. Bourne passes on to consider the opinion expressed 
by certain authors that the favourable conditions for coral 
growth in the external slopes of a reef consist in the in- 
creased food supply brought by the superficial currents of the 
ocean. This explanation, for reasons given, he regards as 
incomplete, being of opinion that the direction and velocity 
of currents are the most important circumstances. His 
observations, he states, are confirmed in every particular by 
those made by Dr. Hichson in Celebes, and communicated 
by him to the British Association in 1887. 1 Mr. Bourne 
expresses the result of his observations in the following 
words: — 
‘ Corals grow best in places where a moderate current 
flows constantly over them. They are killed in still water 
by the deposition of sediment, and they will not grow in 
places where a strong current sets directly against them. 
I noticed 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, aud 
that the reef is barren and suffering rapid erosion at such 
exposed 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, wl}ere the 
current has abated its force considerably, there is a luxu- 
riant 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 aro 
extremely favourable, and sluggish or still water again 
unfavourable. This view, which both of us can support 
1 The paper is not printed in the volume for that year. 
