British naturalist and explorer Alfred Russel Wallace independently invented the theory of evolution by natural selection in 1858, nearly scooping Charles Darwin, who published first. Nevertheless, Wallace's research led him to another important discovery, one that geologists still enshrine on their maps: Wallace's Line. While exploring the vast 2,500-mile Malay Archipelago, Wallace noticed what kinds of animals lived on each island as he traveled farther from the mainland peninsula. He found that he could draw a boundary down the narrow Macasser Straight, which runs a twisted course between the islands of Bali and Lombock, and between Borneo and the Celebes group. Wallace's Line--an ocean channel only 15 miles wide--separates tigers from marsupials and trogons from cockatoos. The animals on either side of it, he wrote in 1858, "differ as much as those of South America and Africa. Yet there is nothing on the map to mark their limits. I believe the western part to be a separated portion of continental Asia, the eastern the fragmentary prolongation of a former Pacific continent." Wallace had no way to observe the sea floor directly, and in his day nothing was known of tectonic plates. On the basis of animal distribution alone he deduced that the eastern island groups must have been separated from the western for much longer than any individual islands were separated from each other. A hundred years later, geologists and oceanographers found the reason and the proof: Wallace's Line traverses an area of intense crustal activity, where the northward-moving Australian plate interacts with the western-moving Pacific (Asian-derived) plate. In addition to bringing two different geographic clusters of animals and plants close together, the plates' enormous pressures on each other and on the Eurasian continent has given rise to the most concentrated volcanic activity on Earth.