Submarine Vents Research Articles

Island Life 1

IN the second half of his volume Mr. Wallace proceeds to apply to the elucidation of the history of the characteristic assemblages of plants and animals in islands, the principles laid down with so much explicitness in the first half. He points out that for the purposes of the naturalist a fundamental difference exists between islands that have once formed part of continents and those which have not. Continental islands are those which, by geological revolutions at more or less remote periods, have been severed from the continental masses in their neighbourhood. They are recognisably portions of the continental ridges of the earth's surface. This relation is usually made strikingly apparent by the chart of soundings between them and the nearest mainland (Fig. 2). Further, in geological structure they resemble parts of the continents, like which they contain both old and new formations, with or without volcanic accumulations. In some cases the evidence of recent severance from the adjacent continent is abundant. In others it is less distinct; for example, where the islands are separated from the nearest land by a depression of a thousand fathoms or more, and where their fauna, though abundant, is of a fragmentary nature, almost all the species being distinct, many of them forming distinct and peculiar genera or families, while many of the characteristic continental orders or families are entirely absent, and in their place come animals to which the nearest allies are to be found only in remote parts of the world. Oceanic islands, on the other hand, exhibit no geological connection with any continental area, but owe their birth either to upheaval of the ocean floor or to the piling up of lavas and tuffs round submarine vents of eruption. Their geological structure is of the simplest kind. As Mr. Darwin long ago showed, they consist of volcanic rocks or of coral reefs, or of volcanic and coralline formations combined. Ancient formations, so characteristic of continental islands, are wholly wanting. These islands lie far removed from a continent, and rise from water of profound depth. Their fauna is in curious keeping with this isolation, for it contains no indigenous land-mammals or amphibians, but abounds in birds and insects, and usually possesses some reptiles. These animals or their ancestors must have reached the islands by crossing the ocean.