Wallace's hypothetical line between Australasian and Southeast Asian fauna.

The Wallace Line is a hypothetical line which separates the zoogeographical regions of Asia and Australasia. It is named after Alfred Russel Wallace who noticed the apparent boundary during his travels through the East Indies in the 19th Century. The line runs through the Malay Archipelago, between Borneo and the Celebes and east of Bali. West of the line are found mostly species that are related to Asiatic species, to the east mostly species that are related to Australian species. This was also noted in Antonio Pigafetta's journal during the voyage of Ferdinand Magellan in 1521, from the contrast between the Philippines and the Spice Islands.

In the mid-20th century, geological studies of plate tectonics showed there is an Indo-Australian plate that has the Wallace Line as a boundary, resulting in a large drop in the sea floor at precisely the same point. This means that it has never been possible for a land bridge to form in the region, hence the zoological distribution.

One interesting note is that many (but by no means all) bird species also observe the line, as many birds refuse to cross even the smallest stretches of open water.

Reference

• Penny van Oosterzee, Where Worlds Collide: the Wallace Line, 1997

See also

Australia-New Guinea