GEOGRAPHICAL DISTRIBUTION OF ANIMALS
antiquity. It is customary to group islands into two classes, suggested by their mode of formation—the continental and the oceanic. Continental islands are detached portions of continents, from which most of them are separated by shallow seas. They have the geological structure of the continents and are composed of the familiar stratified rocks, such as sandstone, slate, and limestone, and some contain granite-like igneous rocks and volcanic material. The faunas of these continental islands are determined by many factors, of which we may disregard several, such as area and climate, and consider only the distance of the island from the mainland and the geological date of its separation.
Great Britain, Ireland, and the islands of the East Indies or Malay Archipelago are continental islands that lie near the parent continents, are surrounded by comparatively shoal water, and although they were detached from the mainland at different times, they were yet, on the whole, of relatively recent origin as islands. Great Britain is, zoögically speaking, indistinguishable from an equal area of the European continent; the species are so generally identical that the islands must have been separated from the continent during the present geological epoch, the Recent. The great Asiatic islands, Borneo, Sumatra, Java, etc., were detached somewhat earlier and contain more peculiar and more characteristic species, but the difference from those of Asia is not great.
Oceanic islands lie far from any continent and rise abruptly, with steep submarine slopes, from profound depths of the sea. Most oceanic islands are composed of volcanic rocks and coral reefs. Nearly all of them seem to have been submarine volcanoes, which have built up their cones from the sea-floor; and the cones that lie in warm seas are generally capped with coral reefs, which may or may not bury the volcanic pedestal out of sight. The rocks that form the
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