Sect. III. REEF-BUILDING CORALS LIVE. 
115 
manner in which the clumps of coral gradually became 
infrequent at about the same depth, and wholly dis- 
appeared at a greater depth than 20 fathoms on the 
slope round Keeling atoll, off the reefs in the Pacific 
(according to Dana), on the leeward side of the Mauri- 
tius, and at rather less depth both within and without 
the atolls of the Maldiva and Cliagos Archipelagoes ; 
and when we know that the reefs round these islands do 
not differ from other coral formations in their form and 
structure, we may, I think, conclude that in ordinary 
cases reef-building polypifers do not flourish at greater 
depths than between 20 and 30 fathoms, and rarely at 
above 15 fathoms. 1 
It has been argued 2 — that reefs may possibly rise 
from very great depths through the means of small 
corals first making a platform for the growth of the 
stronger kinds. This, however, is an arbitrary supposi- 
tion : it is not always remembered, that in such cases 
there is an antagonistic power at work, namely, the 
decay of organic bodies when not protected by a cover- 
ing of sediment or by their own rapid growth. We have, 
moreover, no right to calculate on unlimited time for 
1 [The general conclusions of this paragraph do not appear to have 
been disturbed by recent researches, though Mr. Guppy (Proc. Roy. 
Soc. Edin. xiii. p. 857, see Appendix II.) argues in favour of the 
possibility of reefs occasionally beginning to grow at depths of at 
least 50 fathoms ; and in the Reports of the Challenger Expedition 
(Report on the Reef-building Corals, p. 35) cases of species which 
build reefs, living at a depth of 40 fathoms, and in two instances even 
at 70 fathoms, are recorded. Still even here it is admitted that ‘the 
zone of most active growth is from 1 to 20 fathoms.’] 
2 Journal of the Royal Geograph. Soc. 1831, p. 218. 
