geographic region, regardless of size, thus the cougar is endemic to the Americas, however, endemism is normally used only where there is a considerable restriction in the area of distribution. All species are not endemics, some species may be cosmopolitan. All endemics are not necessarily rare; some might be common where they occur. All rare species are not necessarily endemics, some may have a large range but be rare throughout this range. Endemism is caused by historical and ecological factors. Vicariant events caused by drifting continents, dispersal and extinction are some possible historical factors. Ecological factors can explain the present limits on a distribution. Endemic species are especially likely to develop in geographically and biologically isolated areas such as islands and remote island groups, including Hawaii, the Galápagos Islands and Socotra, because of the potential for isolation and therefore evolution through allopatric speciation. Darwin's finches in the Galápagos archipelago are examples of species endemic to islands. Similarly, isolated mountainous regions like the Ethiopian Highlands, or large bodies of water far from other lakes, like Lake Baikal, can also have high rates of endemism. The stability of a region's climate and habitat through time may also contribute to high rates of endemism (especially paleoendemism), acting as refuges for species during times of climate change like Ice Ages. These changes may have caused species to repeatedly restrict their ranges into these refuges, leading to regions with many small-ranged species. In many cases biological factors, such as low rates of dispersal or returning to the spawning area (philopatry), can cause a particular group of organisms to have high speciation rates and thus many endemic species. for example, cichlids in the East African Rift Lakes have diversified into many more endemic species than the other fish families in the same lakes, possibly due to such factors. Plants that become endemic on isolated islands are often those which have a high rate of dispersal and are able to reach such islands by being dispersed by birds. While birds are less likely to be endemic to a region based on their ability to disperse via flight, there are over 2,500 species which are considered endemic, meaning that the species is restricted to an area less than 5 million hectares. Microorganisms were traditionally not believed to form endemics. The hypothesis 'everything is everywhere', first stated in Dutch by Lourens G.M. Baas Becking in 1934, describes the theory