CORAL ISLANDS. 201 
the complicated conditions of the phenomena. The explanation 
proposed accounts for the known facts, as well as the present appear- 
ance of the madreporic islands. The circular atolls and fringing reefs 
which are disposed as a sort of girdle, are principally formed of species 
of the genera Porites, Millepora, Astrea, zoantharia which cannot 
exist at any great depth in the ocean, but which swarm on the rocks 
at some few fathoms only below the limits of the tide. These animals, 
by means of their accumulated débris, soon form a sort of coating 
round the island, which constitutes the littoral reefs ; this marginal 
shoulder, according to Mr. Darwin, is the first stage in the existence 
of a coral island. At this point the author introduces a geological 
cause, namely, a great subsiding movement of the soil, in which the 
madreporic colony is sunk under the water. It is evident that after 
submersion the coral will only continue to develop itself on the upper 
surface, and within the limits which its nature prescribes. The madre- 
pores exhibiting their greatest vitality at the points most exposed to 
the fury of the waves, it will be near the outer edge of the reef that 
their development will be most rapid. If the subsidence of the island 
thus surrounded should still continue, as mountain after mountain 
and island after island slowly sink beneath the water, fresh bases would 
be successively afforded for the growth of the corals, and the outer 
edge elevated by their continual labour, thus transforming the space 
into a sort of circularlagune. The coral deposits would thus form an 
isolated girdle, and the lagune, which occupies the centre, would 
become deeper and deeper in proportion to the lowering of the soil. 
This is the second stage of the coral isle. 
The existence of the atolls are thus subordinated to two principal 
conditions—the progressive subsidence of the shore washed by the 
sea, and the existence of coral formed of a hard stony substance, the 
growth and multiplication of which was extremely rapid. 
It follows from this that coral isles cannot exist in all seas; that 
they can only have their birth in the torrid zone, or at least near the 
tropics, for it is only in these regions where the warmth exists, so 
necessary to their development, that the madrepores show themselves 
in greatest abundance. : 
The great field of coral formations, in short, is found in the warm 
parts of the Indian and Pacific Oceans. It is from these oceans, 
as from common centres, round which are ranged the series of coral 
isles and islets, that it will be useful, in concluding this chapter, to 
trace their geographical distribution. We borrow the materials for 
this from Milne-Edwards’s table of their distribution in the principal 
seas of the world. 
