Wallace's Line

The Wallace Line or Wallace's Line is a boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace that separates the ecozones of Asia and Wallacea, a transitional zone between Asia and Australia. West of the line are found organisms related to Asiatic species; to the east, a mixture of species of Asian and Australian origin is present. Wallace noticed this clear division during his travels through the East Indies in the 19th century.

The line runs through Indonesia, between Borneo and Sulawesi (Celebes), and through the Lombok Strait between Bali and Lombok. The distance between Bali and Lombok is small, about 35 kilometres (22 mi). The distributions of many bird species observe the line, since many birds do not cross even the smallest stretches of open ocean water. Some bats have distributions that cross the line, but other mammals are generally limited to one side or the other; an exception is the crab-eating macaque. Other groups of plants and animals show differing patterns, but the overall pattern is striking and reasonably consistent. Flora do not follow the Wallace Line to the same extent as fauna.

Antonio Pigafetta had recorded the biological contrasts between the Philippines and the Maluku Islands (Spice Islands) (on opposite sides of the line) in 1521 during the continuation of the voyage of Ferdinand Magellan, after Magellan had been killed on Mactan. Moreover, as noted by Wallace himself, the observations in faunal differences between the two regions had already been made earlier by George Windsor Earl. In Earl's pamphlet On the Physical Geography of South-Eastern Asia and Australia, published in 1845, he described how shallow seas connected islands on the west (Sumatra, Java, etc.) with the Asian continent and with similar wildlife, and islands on the east such as New Guinea were connected to Australia and were characterised by the presence of marsupials. Wallace used his extensive travel in the region to propose a line to the east of Bali since "all the islands eastward of Borneo and Java formed part of an Australian or Pacific Continent, from which they were separated." The name 'Wallace's Line' was first used by Thomas Huxley in an 1868 paper to the Zoological Society of London, but showed the line to the west of the Philippines. Wallace's studies in Indonesia demonstrated the emerging theory of evolution, at about the same time as Joseph Dalton Hooker and Asa Gray published essays also supporting Darwin's hypothesis.

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