- [Footnote: free and detached polypifers, and those which form wall-like
structures and rocks.
If we are struck with the great accumulation of building polypifers in some regions of the globe, it is not less surprising to remark the entire absence of their structures in other and often nearly adjoining regions. These differences must be determined by causes which have not yet been thoroughly investigated; such as currents, local temperature of the water, and abundance or deficiency of appropriate food. That certain thin-branched corals, with less deposit of lime on the side opposite to the opening of the mouth, prefer the repose of the interior of the lagoon, is not to be denied; but this preference for the unagitated water must not, as has too often been done (Annales des Sciences Naturelles, 1825, T. vi. p. 277), be regarded as a property belonging to the entire class. According to Ehrenberg's experience in the Red Sea, that of Chamisso in the Atolls of the Marshall Islands east of the Caroline group, the observations of Captain Bird Allen in the West Indies, and those of Capt. Moresby in the Maldives, living Madrepores, Millepores, and species of Astræa and of Meandrina, can support the most violent action of the waves,—"a tremendous surf,"—(Darwin, Coral Reefs, pp. 63-65), and even appear to prefer the most stormy exposure. The living organic forces or powers regulating the cellular structure, which with age acquires the hardness of rock, resist with wonderful success the mechanical forces acting in the shock of the agitated water.
In the Pacific, the Galapagos Islands, and the whole]*
