ISLAND, a piece of land surrounded by water (0.E. ieg =isle +land). The words "island," "isle" are sometimes used for a piece of land isolated during high tide or surrounded by marsh; e.g., Hayling Island, Isle of Ely ; the spelling "island," accepted before 1700, is due to a false connection with "isle" (Fr. ile, Lat. insula). Islands are usually classified as continental or oceanic. The former may result from the submergence of coastal high lands of which the summits remain above water or by the sea cutting through the isthmus (q.v.) of a peninsula, or through some zone of weakness between a coastal headland and the main land. In all cases their structure is essentially that of the main land and they occur on the continental shelf (q.v.). Examples are provided by the smaller islands off the coasts of Britain and Ireland, while the larger islands are in their turn continental islands of Europe. Oceanic islands have various origins. The majority of the islands of the Malay Archipelago may be regarded either as continental or oceanic, but such a group as the Caroline islands in their vicinity are truly oceanic. The ocean floor has marked irregularities, and where any individual peak, structurally belonging to the ocean bed, rises above the surface, an oceanic island is produced. Its projection above sea-level may be conse quent on submarine volcanic activity, or on the buckling of the ocean bed by earth movements. Coral islands (see ATOLL) are
usually oceanic islands and are frequently clustered upon sub marine plateaux in shallow water, but occasionally represent the crowns of isolated peaks rising from deep water.
Island life exhibits features of special interest. The sea is a barrier to some forms of life, but acts as a carrier to others which, once established in their new home, frequently develop new features in their isolated surroundings. A long-established sea barrier results in a marked difference between the fauna and flora even of adjacent islands; and this difference can be made to in dicate the origin of an island. Wallace noticed that to the west of a line running between Bali and Lombok, and between Borneo and Celebes, the islands were biologically Asiatic, but to the east of the line (Wallace's line), notwithstanding the narrow Bali strait, the flora and fauna were Australian. Any detailed map of the biological regions of the globe will illustrate the importance of islands in the determination of the boundaries of faunal and floral types. Oceanic islands are usually colonized by but few animal forms, chiefly sea-birds and insects, but they are often clothed with abundant vegetation, the seeds of which have been carried by air and water currents, birds, etc., but the variety of plants is by no means so great as on the mainland.