THE FORMATION OF THE MALDIVES AND LACCADIVES. ERT 
mainly influenced by the amount of carbonic acid (CO.) in the water, the circulation of 
the water, and the quantity and intensity of the light. Temperature is not a factor of 
vital importance in coral-reef areas. It undoubtedly assists the metabolism, but excess of 
heat appears to be practically unknown on reefs. I have measured in pools at Minikoi, 
abounding in corals, temperatures up to 133° F. at low tide; save that the corals tended 
to expand as soon as the colder tidal waters entered the pool, I could not subsequently 
see any difference in them to those completely covered with the sea water. So far as cold 
is concerned I have been unable to trace any death of reef-corals to this source, nor have 
I found any definite evidence of such recorded. In reference to the carbonic acid the 
“Challenger” results showed an excess in deep water over shallow, but in any case within 
50 fathoms of the surface, well within the wave limits, there could not be much difference 
in this, the water thoroughly mixing. The real factor is the light, which to reach the 
commensal algae has to penetrate the tissues of the polyps as well as the water. An 
intense light is evidently required, and even on the Equator 30 fathoms is almost beyond 
its limits in the Maldives. On it depends mainly the metabolism, and our dredgings indicate 
that its action is not great and of little effect below 15 or 20 fathoms. Hence Darwin's 
conclusion “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”! appears to be literally 
true and amply borne out by our results. 
The accounts of nullipores, the depths at which they live, are extremely unsatisfactory, 
algae being generally much neglected by zoologists, to whom the practical investigation of 
coral-reef conditions and formation has been, by almost universal consent, relegated. My 
own investigations have not unfortunately been carried out on such a scale that I can give 
or indicate any answer to the question of the importance of these forms in building up 
a reef beyond a depth of 40 to 50 fathoms. To this depth nullipores certainly may and 
do grow against the open sea in great luxuriance. Their species or facies appear to be 
the same from the surface down to this depth, and there is no alteration in their texture, 
no decrease of specific gravity, so far as I could ascertain. At the same time their growth 
is always extremely slow as compared with corals, and they do not nearly so readily cover 
a vacant area. Only the smallest growths are found in the oldest cleared out boat-channels 
through the reefs to the shores of the islands of the different atolls*, and where beacons 
or other fixed marks have been placed by the islanders on the reefs, they have not been 
in any case, that I saw, appreciably covered at their bases by nullipores, although these 
organisms may clothe the whole basal reef on which they are fixed. Their metabolism also 
like that of corals must decrease with greater depth, and, although they very certainly are 
an important contributory agency in building up banks from upwards of 200 fathoms, I 
must reject them absolutely as in any case capable alone of furnishing a sufficient amount 
of material to raise the foundations to sufficient depths for the reef-corals to take possession®. 
It remains then to consider what may be the chief contributory agents in building 
up our banks to the surface from their common plateau 150 or 200 fathoms below. In 
1 Coral Reefs, 3rd ed., p. 115 (1889). that this is properly done. The coral, etc. obtained is either 
= Most inhabited islands have one or two boat-passages 
through the reef around them communicating with the 
boat-channel inside, where it exists. These passages have 
by Maldivan law to be cleared out every three years from 
every scrap of coral or nullipore that may be growing in 
them, and they are inspected by Government officers to see 
taken on shore to be burned into lime, or more often sunk in 
the lagoon of the atoll outside. 
3 The importance of nullipores is rather in consolidating 
the corals together. For this purpose they are especially 
efficient, since their skeletons are scarcely affected by boring 
organisms. 
23—2 
