1888.] and the Coral Formations of the Indian Ocean. 453 



influence of currents setting in from the ocean. Yet, as I have 

 already shown, it is to the growth of coral alone that the shoaling of 

 this part of the lagoon can be attributed. These facts are a sufficient 

 argument against the idea that currents teeming with pelagic life are 

 the prime factors in determining coral growth. It must be remem- 

 bered that we are very ignorant about the food of corals. Th^re are 

 very few accounts of the food found in their digestive cavities, and it is 

 purely an assumption to speak of their feeding only on pelagic organ- 

 isms. I have in another placs reported the presence of vegetable 

 matter in the curiously modified digestive stomodaeum of Eupfiyllui, 

 and T have no doubt, after what Dr. Hickson has told me of the 

 relations of corals to mangrove swamps in Celebes, that far more 

 corals are vegetable feeders than has hitherto been supposed. The 

 lagoon of an atoll is always full of decaying vegetable matter derived 

 from the shore bushes and palms, and. it seems likely that there is a 

 ronnexion between the richer coral growth along the shores of a lagoon 

 and the supply of vegetable debris from the shore. 



My observations incline me to the belief that the most important 

 circumstances affecting coral growth are the direction and velocity of 

 currents. My observations are confirmed in every particular by those 

 made by Dr. Hickson in Celebes, and communicated by him to the 

 British Association in 1887. 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, and 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, 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 quantity of shingle which they dash against the soft-bodied 

 polyps. Some massive forms might withstand the force of breakers 



vol. xliii. 2 K 



