Sect. n. 
RATE OF GROWTH. 
103 
all adapted to the stations they occupy, and hold their 
places, like other organic beings, by a struggle one 
with another and with external nature ; hence we may 
infer that their growth would be slow except under 
peculiarly favourable circumstances. Almost the only 
natural condition, allowing a quick upward growth of 
the whole surface of a reef, would be a slow subsidence 
of the area in which it stood ; — if, for instance, Keeling 
atoll were to subside two or three feet, can we doubt 
that the projecting margin of live coral, about half an 
inch in thickness, which surrounds the dead upper sur- 
faces of the mounds of Porites, would in this case form 
a concentric layer over them, and the reef thus increase 
upwards, instead of, as at present, outwards ? The 
Nulliporae are now encroaching on the Porites and 
Millepora, but in this case might we not confidently 
expect that the latter would, in their turn, encroach 
on the Nulliporae ? After a subsidence of this kind, the 
sea would gain on the islets, and the great fields of dead 
but upright corals in the lagoon would be covered by a 
sheet of clear water ; and might we not then expect 
that these reefs would rise to the surface, as they an- 
ciently did when the lagoon was less confined by islets, 
and as they did within a period of ten years in the 
schooner-channel cut by the inhabitants. In one of the 
Maldiva atolls, a reef, which within a very few years 
existed as an islet bearing cocoa-nut trees, was found 
by Lieut. Prentice ‘ entirely covered with live coral and 
Madrepore.’ The natives believe that the islet was 
washed away by a change in the currents, but if, 
