Which biogeographic line separates Asian wildlife from Australian wildlife?

The Wallace Line, running between Bali and Lombok and up through the Makassar Strait - islands a few miles apart with completely different animals on them. Tigers, monkeys and woodpeckers on one side; marsupials, cockatoos and megapodes on the other.

The cause is under the water rather than on it. Ice ages dropped sea level enough to join Borneo, Java, Sumatra and Bali to mainland Asia as one landmass, and to join Australia to New Guinea as another, but the strait between them is deep and never dried - so there was always a crossing, and only good swimmers, fliers and drifters made it. Wallace noticed the boundary from the birds, in the 1850s, without knowing any of this.

He is the other man who worked out natural selection. Wallace sent Darwin an essay from the Malay Archipelago in 1858 laying out essentially the same theory, which is what finally forced Darwin - who had been sitting on twenty years of notes - to publish; their work was read jointly to the Linnean Society that summer. Wallace was collecting specimens for sale at the time because he had no private income, and had already lost several years of them when the ship home caught fire and sank.