|Alfred Russel Wallace oceanography biogeography|
Alfred Russel Wallace drew a line east of Bali, east of Borneo, east of Java, marking the separation from Australia of animals from Asia. West of his line, during the farthest glacial advances, ocean levels were 390 feet lower, connecting the islands on the Sunda Shelf to the Asian mainland, but deep water along the Wallace line separated tigers from koalas and kangeroos.
Islands persisted east of the Wallace line even when the oceans were at their lowest, separated both from Asia and from the Sahul Shelf that surrouds Australia and connects Australia across the Arafura Sea to New Guinea. The islands of Sulawesi, Maluku, Halmahera, Seram, Buru, Lombok, Sumbawa, Sumba, Komodo, Flores, Nusa Tenggara Barat, Nusa Tenggara Timur, Timor, Sumba, Wetar, and hundreds of other islands remained separate from both Asia and Australia.
The sweet potato spread across the Pacific without the benefit of boats. Not so the koala, who hugs its gum tree, or the kangaroo, who hops and swims but would not cross the Wallace line. Why do some animals cross the water? Why do some think better? Many birds migrate across oceans but some are more like me, preferring smaller and safer waters.