the largest unit of zoogeographic regionalization (seeZOOLOGICAL REGIONALIZATION). The land areas of the earth are divided into three realms: Arctogea, Neogea, and Notogea. Some zoogeographers use the term “region” instead of realm, for example, the Holarctic region. The Pacific Ocean is also divided into three realms: the realm of the cold and temperate sea of the northern hemisphere, the tropical realm, and the realm of the cold and temperate seas of the southern hemisphere.