CORAL ISLAXDS. 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, Milltpora, 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 de'bris, 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 circular lagune. 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. 



