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In 1848, Wallace together with another naturalist, Henry Walter Bates, left for Brazil to collect specimens in the Amazon Rainforest. Unfortunately, a large part of his collection got lost when his ship caught fire when he returned to England in 1852.
From 1854 to 1862, he travelled through the Malay Archipelago[?] to collect specimens and study nature.
His studies of the Malay Archipelago led to him arrive independently at a theory of evolution similar to Charles Darwin's. Darwin at that time had not published his theory, but when Wallace sent him a memoir of his theories, asking him to forward it for publication, Charles Lyell and Joseph Hooker advised Darwin that he should publish his own thoughts at the same time. On July 1, 1858, both papers were presented to the Linnean Society[?] of London.
Wallace noted a line seemed to run through the Malay Archipelago, between Borneo and Celebes and east of Bali. This line, which is now known as Wallace's line separates the continents of Asia and Australia zoologically. West of the line mostly species are found that are related to Asiatic species, to the east mostly species that are related to Australian species.
In the mid 20th Century, geological studies of plate tectonics showed there is an Indo-Australian plate that has Wallace's line as a border, 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 bird species also observe the line, as many birds refuse to cross even the smallest stretches of open water.