Sect. III. REEF-BUILDING CORALS LIVE. Ill 
these islands, the limit above assigned would be found 
never to vary, but I conceive the facts are sufficient to 
show that the exceptions would be few. The circum- 
stance of a gradual change, in the two cases, from a 
field of clean coral to a smooth sandy bottom, is far 
more important in indicating the depth at which the 
larger kinds of coral flourish, than almost any number 
of separate observations on the depth at which certain 
species have been dredged up. For we can understand 
the gradation only as a prolonged struggle against 
unfavourable conditions. If a person were to find the 
soil clothed with turf on the banks of a stream of 
water, but on going to some distance on one side of it 
he observed the blades of grass growing thinner and 
thinner with intervening patches of sand, until he 
entered a desert of sand, he would safely conclude, 
especially if changes of the same kind were noticed in 
other places, that the presence of the water was abso- 
lutely necessary to the formation of a thick bed of 
turf : so may we conclude, with the same feeling of 
certainty, that thick beds of coral are formed only at 
small depths beneath the surface of the sea. 
I have endeavoured to collect every fact which 
might either invalidate or corroborate this conclusion. 
Captain Moresby, whose opportunities for observation 
during his survey of the Maldiva and Chagos Archi- 
pelagoes were unrivalled, informs me, that the upper 
part or zone of the steep-sided reefs on the inner and 
outer coasts of the atolls in both groups, invariably 
consisted of coral, and the lower parts of sand. At 
