In the case of land and fresh-water organisms the sea is the chief barrier; in the case of marine organisms, the land. Differences in land forms do not exert great influence on the distribution of living creatures directly, but indirectly such land forms as mountain ranges and internal drainage basins are very potent through their action on soil and climate. A snow-capped mountain ridge or an arid desert forms a barrier between different forms of life which is often more effective than an equal breadth of sea. In this way the surface of the land is divided into numerous natural regions, the flora and fauna of each of which include some distinctive species not shared by the others. The distribution of life is discussed in the various articles in this _Encyclopaedia_ dealing with biological, botanical and zoological subjects.[44] Floral zones. The classification of the land surface into areas inhabited by distinctive groups of plants has been attempted by many phyto-geographers, but without resulting in any scheme of general acceptance. The simplest classification is perhaps that of Drude according to climatic zones, subdivided according to continents. This takes account of--(1) the _Arctic-Alpine_ zone, including all the vegetation of the region bordering on perpetual snow; (2) the _Boreal_ zone, including the temperate lands of North America, Europe and Asia, all of which are substantially alike in botanical character; (3) the _Tropical_ zone, divided sharply into (a) the tropical zone of the New World, and (b) the tropical zone of the Old World, the forms of which differ in a significant degree; (4) the _Austral_ zone, comprising all continental land south of the equator, and sharply divided into three regions the floras of which are strikingly distinct--(a) South American, (b) South African and (c) Australian; (5) the _Oceanic_, comprising all oceanic islands, the flora of which consists exclusively of forms whose seeds could be drifted undestroyed by ocean currents or carried by birds. To these might be added the antarctic, which is still very imperfectly known. Many subdivisions and transitional zones have been suggested by different authors. Vegetation areas.