All of the animals living in Asia and its surrounding seas and islands are considered the fauna of Asia. Since there is no natural biogeographic boundary in the west between Europe and Asia, the term "fauna of Asia" is somewhat elusive. Temperate Asia is the eastern part of the Palearctic realm (which in turn is part of the Holarctic), and its south-eastern part belongs to the Indomalayan realm (previously called the Oriental region). Asia shows a notable diversity of habitats, with significant variations in rainfall, altitude, topography, temperature and geological history, which is reflected in its richness of animal life.