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Moreover, at Keeling atoll, and probably in many other cases, the different kinds of corals would have to encroach on each other; thus the Nulliporae cannot increase outwards without encroaching on the Porites and Millepora complanata, as is now taking place; nor these latter without encroaching on the strongly branched Madreporet, the Millepora alcicornis, and some Astraeas; nor these again without a foundation being formed for them within the requisite depth, by the accumulation of sediment.
The corals, also, on the margin of Keeling Island occurred in zones; thus the Porites and Millepora complanata grow to a large size only where they are washed by a heavy sea, and are killed by a short exposure to the air; whereas, three species of Nullipora also live amidst the breakers, but are able to survive uncovered for a part of each tide; at greater depths, a strong Madrepora and Millepora alcicornis are the commonest kinds, the former appearing to be confined to this part, beneath the zone of massive corals, minute encrusting corallines and other organic bodies live.
We shall presently see that there are other organic productions, fitted to bear a somewhat longer exposure to the air and sun. Next, but much inferior in importance to the Porites, is the Millepora complanata.
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