The Wallace Line
Bali is the physical end of what was once mainland Asia. Observing that a great contrast exists between the animal life of Bali and that of the islands to the east, the great 19th century English naturalist Sir Alfred Russel Wallace suggested that the treacherous, 24-km-wide strait separating Bali from the neighboring island of Lombok is an important divide, a biologically impassable line cleaving Asia from Australia.
"In just two hours," he suggested, "you can pass from one great division of the earth to the other, differing as essentially in their animal life as Europe does from America."
During the last ice age, Wallace theorized, the sea level around the Greater Sundas fell enough to enable animals to travel overland from the Asian mainland, fanning out through the archipelago until they reached the deep trench of the Lombok Strait and could go no farther. While the Selat Bali ("Bali Strait") separating Bali from Java has a maximum depth of 60 meters, the ocean depths between Bali and Lombok exceed 1,300 meters.
Wallace's book, The Malay Archipelago, published in 1869 contemporary and parallel with Charles Darwin's work, advanced a theory of evolution based on Wallace's examination of the flora and fauna of the region. His imagined line dividing the Asian and Australian regions on either side of the Lombok Strait has since become known as the Wallace Line.
The differences between Bali and Lombok are obvious. Bali is lush, equatorial, smothered in the luxuriant vegetation of tropical Asia, while Lombok is wind-blown and dry like the Australian plains.
Bali, Java, and islands west of Bali are characterized by the monkeys, squirrels, rabbits, tigers, elephants, bears, sheep, oxen, horses, orangutans, and pythons found in the dense tropical forests and jungles of Asia.
On the islands east of Bali begin the parrots and other peculiar bird species, marsupials like wombats and kangaroos, the platypus, and giant lizards of the Australian region. Some "leakage" occurs, i.e., monkeys are found in Sumba.
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