The simplest groupings of the world’s animals. The holarctic realm covers the nearctic (most of North America, plus Greenland) and the palearctic (extra-tropical Asia, Europe, and North Africa). The neotropical realm covers Central and South America; the Ethiopian Africa south of the Sahara and Arabia. The oriental realm is tropical Asia, with an ill-defined boundary between it and the Australian realm (New Zealand, Australia, Oceania, and some of South-East Asia). See Smith (2004) Complexity 10, 2 for an evolutionary understanding of faunal realms, albeit in very ornate language.