Every coral-trunk arises by a process of gemmation in accordance with certain laws, and forms one complete structure, each portion being formed by a great number of organically distinct individual animals. In the group of Phyto-corals these cannot separate themselves spontaneously, but remain united with one another by lamellæ of carbonate of lime. Hence each coral-trunk by no means possesses a central point of common vitality.[[KT]] The propagation of coral-animals, according to the difference of the orders, is by eggs, spontaneous division or gemmation. This last kind of propagation presents the greatest variety of forms in the development of individuals.
The Coral-reefs (or, as Dioscorides designates them, sea-plants, a forest of stony-trees, Lithodendra), are of three kinds; namely, Coast-reefs, (shore-reefs, fringing-reefs), which are directly connected with continental or insular coasts, as on the north-east coast of New Holland, between Sandy Cape and the dreaded Torres Straits, and almost all the coral-banks of the Red Sea examined for eighteen months by Ehrenberg and Hemprich; Island-surrounding reefs (barrier-reefs, encircling-reefs), as at Vanikoro in the small archipelago of Santa Cruz, north of the New Hebrides, and at Puynipete, one of the Carolinas; and Coral-banks surrounding lagoons (Atolls or Lagoon-islands). This very natural division and nomenclature have been introduced by Charles Darwin, and are most intimately connected with the very ingenious explanation which this intellectual naturalist has given of the gradual origin of these wonderful forms. While, on the one hand, Cavolini, Ehrenberg, and Savigny have completed the scientific anatomical knowledge of the organization of coral-animals, on the other, the geographical and geological relations of coral-islands have been investigated, first by Reinhold and George Forster in Cook’s second voyage, and then, after a long interval, by Chamisso, Péron, Quoy and Gaimard, Flinders, Lütke, Beechey, Darwin, d’Urville, and Lottin.
The coral-animals and their stony cellular scaffoldings belong, for the most part, to the warm tropical seas; and the reefs occur most frequently in the Southern Hemisphere. Thus we find the Atolls or Lagoon Islands crowded together in the so-called coral-sea between the north-east coast of New Holland, New Caledonia, Solomon’s Islands, and the Louisiade Archipelago; in the group of the Low Islands (Low Archipelago), eighty in number; in the Fidji, Ellice, and Gilbert Islands; and in the Indian Ocean, north-east of Madagascar, under the name of the Atoll group of Saya de Malha.
The great Chagos Bank, whose structure and dead coral-trunks have been thoroughly investigated by Captains Moresby and Powell, is the more interesting to us, because we may regard it as a prolongation of the more northern Laccadive and Maldive Islands. I have previously directed attention in another work[[KU]] to the importance of the order of succession of the Atolls, which are exactly in the direction of a meridian as far as 7° south lat., in reference to the general mountain system, and the form of the earth’s surface, in Central Asia. The meridian-chains, which mark the intersection of many mountain-systems running from east to west at the great bend of the Thibetian river Tzang-bo, correspond with the great meridian mountain rampart of the Ghauts and of the more northern Bolor in further or trans-Gangetic India. Here lie the parallel chains of Cochin China, Siam, and Malacca, as well as those of Ava and Arracan, which, after courses of unequal length, all terminate in the gulfs of Siam, Martaban, and Bengal. The bay of Bengal appears like an arrested effort of nature to produce an inland sea. A deep inbreak of the waters, between the simple western system of the Ghauts, and the very complex eastern trans-Gangetic system, has swallowed up a great part of the eastern lowlands, but met with an impediment not so easily overcome in the early existing and extensive table-land of Mysore.
An oceanic inbreak of this nature has given rise to two almost pyramidal peninsulas of very different length and narrowness; and the prolongation of two opposing meridian systems, the mountain system of Malacca in the east, and the Ghauts of Malabar in the west, manifests itself in submarine, symmetrical series of islands, on the one side in the Andaman and Nicobar Islands, which are poor in corals, and on the other in three long-extended archipelagos of Atolls—the Laccadives, the Maldives, and Chagos. The last, called by mariners the Chagos Bank, forms a lagoon, belted by a narrow, and already much broken coral-reef. The length of this lagoon is 88, and its breadth 72 miles. Whilst the enclosed lagoon is only from 17 to 40 fathoms deep, bottom was scarcely found at a depth of 210 fathoms at a small distance from the outer margin of the coral wall, which appears to be now sinking.[[KV]] At the coral-lagoon, known as Keeling-Atoll, south of Sumatra, Captain Fitz-Roy states, that at only 2000 yards from the reef, no soundings were found with 7200 feet of line.
“The forms of coral, which in the Red Sea rise in thick wall-like masses, are Mæandrinæ, Astrææ, Favia, Madrepores (Porites), Pocillopora (Hemprichii), Millepores, and Heteropores. The latter are among the most massive, although they are branched. The deepest coral trunks, which magnified by the refraction of light, appear to the eye to resemble the dome of a cathedral, belong, as far as could be determined, to Mæandrinæ and Astrææ.”[[KW]] A distinction must be made between single and in part free polyp-trunks, and those which form wall-like rocks.
If the accumulation of building polyp-trunks in some regions is so striking, it is no less astonishing to observe the perfect absence of these structures in other and often adjacent regions. Their presence or absence must be determined by certain, still uninvestigated, relations of currents, by the partial temperature of the water, and by the abundance or deficiency of nutriment. That certain delicate-branched corals, with less calcareous deposition on the side opposite to the mouth, prefer the stillness of the interior lagoons, is not to be denied; but this preference for still water must not, as has too often happened,[[KX]] be regarded as a peculiarity of the whole class of these animals. According to the experiences of Ehrenberg and Chamisso in the Red Sea and in the Marshall Islands, which abound in Atolls and lie east of the Caroline Islands, and according to the observations of Captains Bird Allen and Moresby in the West Indies and in the Maldives, we find that living Madrepores, Millepores, Astræas, and Mæandrinas, can support “a tremendous surf;”[[KY]] and indeed seem to prefer localities the most exposed to the action of storms. The vital forces of the organism regulating the cellular structure, which with age acquires a rocky hardness, resist most triumphantly the mechanical forces,—the shock of moving waters.
In the South Pacific there is a perfect absence of coral-reefs at the Galapagos and along the whole of the west coast of the New Continent, notwithstanding their vicinity to the numerous Atolls of the Low Islands, and the Archipelago of Mendaña or the Marquesas. It is true that the current of the South Pacific, which washes the coasts of Chili and Peru. (and whose low temperature I observed in the year 1802,) is only 60°.1 Fahr., while the undisturbed water at the sides of the cold current is from 81°.5 to 83°.7 Fahr. at Punta Parima, where it deflects to the west. Moreover at the Galapagos there are small currents between the islands, having a temperature of only 58°.3 Fahr. But this lower temperature does not prevail further northwards along the coasts of the Pacific from Guayaquil to Guatimala and Mexico, neither does it prevail in the Cape de Verd Islands, on the whole west coast of Africa, or at the small islands of St. Paul, St. Helena, Ascension, and San Fernando Noronha; yet in none of these are there coral-reefs.
If this absence of reefs characterises the western coasts of America, Africa, and New Holland, they are, on the other hand, of frequent occurrence on the eastern coasts of tropical America, on the African coast of Zanzibar, and on the southern coast of New South Wales. The best opportunities I have enjoyed for personally examining coral banks have been in the Gulf of Mexico, and south of the Island of Cuba, in the so-called “Gardens of the King and Queen” (Jardines y Jardinillos del Rey y de la Reyna). It was Christopher Columbus himself who, on his second voyage, in May, 1494, gave this name to this little group of islands, because from the pleasant association of the silver-leaved arborescent Tournefortia gnapholoides, of flowering species of Dolichos, of Avicennia nitida, and mangrove-thickets (Rhizophora), the coral-islands formed as it were an archipelago of floating gardens. “Son Cayos verdes y graciosos llenos de arboledas,” says the admiral. On my voyage from Batabano to Trinidad de Cuba, I remained for several days in these gardens, which lie to the east of the great Isle of Pines, abounding in mahogany, for the purpose of determining the longitude of the different Cayos.
The Cayos Flamenco, Bonito, de Diego Perez, and de Piedras, are coral islands, rising only from 8 to 15 inches above the level of the sea. The upper edge of the reef does not consist merely of dead polyp-trunks, but is rather formed of a true conglomerate, in which angular pieces of coral, lying in various directions, are embedded in a cement composed of granules of quartz. In Cayo de Piedras I saw such embedded masses of coral, some of them measuring upwards of three cubic feet. Several of the West Indian smaller coral islands have fresh water, a phenomenon which merits a careful investigation wherever it occurs (as for instance near Radak in the South Sea),[[KZ]] since it has sometimes been ascribed to hydrostatic pressure, acting from a distant coast (as in Venice, and in the Bay of Xagua, east of Batabano), and sometimes to the filtration of rain-water.[[LA]]