Biogeography describes and explains distributions, today and in earlier times. Darwin’s theory suggested that animal groups all evolved from one centre of creation. After him, one theory suggested that northern regions of the globe are cradles of successful, dominant modern groups. Cold climates force mammals to adapt to cold, arid, variable climatic conditions and favour active and enduring animals. These hardy-adapted animals disperse towards equatorial regions. Drops in sea levels enable northern faunas to disperse to southern continents, eventually attaining cosmopolitan distributions and displacing early, less adaptable remnants of ancient faunas that find refuge in the tropics. Another theory suggested that successful dominant groups arose in the Old World tropics and spread over much of the world, not polar faunas. A third theory claimed that a group with a narrow range is usually one of recent origin, and one with a wide range is an old one; consequently, the range of a group may indicate its age. To explain discontinuous distributions, another theory suggested that bridges had once crossed oceans and impassable mountains, and had thereby split ancestral, once broad distribution areas, into smaller areas. This theory depreciated the importance of long-distance dispersal. Suggestions that Earth’s continents have drifted apart, over time, began already four centuries ago. Later, naturalists suggested that Earth’s internal parts are a fluid, the continents drifting upon it. Identical Carboniferous fossils in Europe and America suggested that these continents were then inter-connected, all southern continents once forming a single supercontinent. Others suggested that colliding continents thrust earth’s surface upward, forming mountain ranges. Wegener independently developed the continental drift theory (1912), which modern plate tectonics supports: A huge southern supercontinent had existed 550 million years ago; it merged with a huge northern supercontinent 330 million years ago, forming one single giant supercontinent. 200 million years ago, this giant began breaking up, with continents splitting off; Africa and South America drifted apart 140 million years ago. Life’s lineages evolved on each floating continent, thereby forming the various zoological realms. This explains the distribution of organisms in the Permian and Triassic. Among modern animals, it explains why marsupials are the only mammals native to Australia (except for bats) and why marsupials occur in both Australia and South America. It also explains the distribution of some insects. Some organisms have thus achieved their modern disjunct distributions by the splitting of regions—Vicariance Biogeography. Early biogeographic theory suggested that conditions are favourable in an area’s centre, and unfavourable in marginal zones, where death exceeds reproduction. Excess reproduction near the centre and migration towards margins, maintain marginal populations and an equilibrium between these processes determining range limits. Modern theory applies to islands the fact that species undergo cycles of origin, spread, decline, retreat and extinction, due to processes of range expansion, diversification, adaptation, endemism and extinction. Insular faunas achieve saturation through colonization, with extinctions dynamically balancing immigration. The resulting equilibrium is a dynamic species richness, with turnover in species composition. Immigration rates of new species decrease with increasing numbers of species already present on an island, and extinction rates increase, as the number of species increases. The two processes balance at a point where the immigration rate equals extinction rates, and at this equilibrium, the number of species is constant. These factors determine island biodiversity. From island faunas to faunas in general, the theory predicts continual species turnovers, with ecological communities as open assemblages approaching a dynamic, steady-state species richness. In practice, some studies have found evidence for species equilibrium, on islands and in insular habitats, and for species turnover at points of equilibrium. Others have not.