An invisible line through Indonesia splits Asian and Australian animals

While mapping species across the Malay Archipelago, Alfred Russel Wallace noticed that islands only tens of kilometers apart had starkly different wildlife -- tigers and monkeys to the west, marsupials and cockatoos to the east -- separated by deep ocean straits that persisted even when ice-age sea levels dropped. The boundary, now called the Wallace Line, tracks the edge of two tectonic plates rather than any visible feature.

— Alfred Russel Wallace, The Malay Archipelago — 1869
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