The Himalayas, stretching from east to west, form a barrier that largely prevents the movement of fauna southward or northward. Thus, Asia north of the Himalayas, with parts of western Asia and most of East Asia, belongs to the Palearctic (Old World) subregion of the Holarctic zoogeographic region (roughly, the Northern Hemisphere north of the tropics). Asia south of the Himalayas is called the Oriental, or Indian, region. The boundary dividing those zones east and west of the Himalayas is not well marked, however, as the mountain chains there often have a north-south trend facilitating migration of animals between them.