In Depth › Science Features

Drawing the line at Bali

Alfred Russel Wallace is the 'father of biogeography'. On the centenary of his death, Penny van Oosterzee pays tribute to his significant contribution to evolutionary theory.

In the late 1990s I became obsessed with another man, not my husband. A man from another time, the other man was a lanky, gentle, short-sighted English explorer and naturalist named Alfred Russel Wallace.

In the mid-19th century, Wallace discovered a mysterious line that separated two different faunal universes. This line separated marsupials from tigers, and honeyeaters and cockatoos from barbets and trogons. It became known as the Wallace Line and it is the most famous and most discussed biogeographical boundary in the world. This line is the narrow strait between Bali and Lombok.

From 20th April 1854 to the 1st of April 1862, Wallace trekked the Malay Archipelago, today's Indonesia, wallowing in its wildlife. Situated on the equator and bathed by the tepid water of three great tropical oceans, this area of 13,000 islands displayed an unheard-of, wonderful variety of species.

The first visit by a European naturalist to the Malay Archipelago had occurred only in 1776. The region was steeped in mystique. The richest variety of fruits and the most precious spices were indigenous there. Wallace had heard of the monstrous flowers called rafflesia that are "three-feet-wide and weigh 24 pounds!".

The great green-winged Ornithoptera, which Wallace later dubbed 'the Prince among butterfly tribes', the man-like orangutan whose origin sparked Wallace's curiosity as had no mammal other than man himself, and the bird of paradise, which had never been exhibited in Europe, were other features of the Archipelago's giddy, divergent forms of life.

^ to top

The Wallace Line

Alfred Russel Wallace * Born 8 January 1823. Died 7 November 1913



* British naturalist and explorer known for his work on biogeography and evolutionary theory.



* Wallace spent eight years from 1845 - 1862 exploring the Malay archipelago accumulating a vast collection of insects, birds, mammals, reptiles and shells. His best known zoological discoveries are probably Wallace's golden birdwing butterfly (Ornithoptera croesus) and Wallace's standard-wing bird of paradise (Semioptera wallacei), both from Bacan island, and Rajah Brooke's birdwing butterfly (Trogonoptera brookiana) from Borneo.



* Darwin published Wallace's essay On the Tendency of Varieties to Depart Indefinitely from the Original Type without Wallace's knowledge along with his own writings on the subject in August 1958 under the title On the Tendency of Species to Form Varieties; And On the Perpetuation of Varieties and Species by Natural Means of Selection Darwin then published Origin of the Species in 1859.



* Wallace wrote over 1000 articles and 22 books including The Malay Archipelago, The Geographical Distribution of Animals, Island Life and Darwinism.



Source: * Born 8 January 1823. Died 7 November 1913* British naturalist and explorer known for his work on biogeography and evolutionary theory.* Wallace spent eight years from 1845 - 1862 exploring the Malay archipelago accumulating a vast collection of insects, birds, mammals, reptiles and shells. His best known zoological discoveries are probably Wallace's golden birdwing butterfly (Ornithoptera croesus) and Wallace's standard-wing bird of paradise (Semioptera wallacei), both from Bacan island, and Rajah Brooke's birdwing butterfly (Trogonoptera brookiana) from Borneo.* Darwin published Wallace's essay On the Tendency of Varieties to Depart Indefinitely from the Original Type without Wallace's knowledge along with his own writings on the subject in August 1958 under the title On the Tendency of Species to Form Varieties; And On the Perpetuation of Varieties and Species by Natural Means of Selection Darwin then published Origin of the Species in 1859.* Wallace wrote over 1000 articles and 22 books including The Malay Archipelago, The Geographical Distribution of Animals, Island Life and Darwinism.Source: The Alfred Russel Wallace website

The strait between Bali and Lombok is 25 kilometres wide, slicing between two volcanoes, perfect cones rising out of the mists.

Crossing from Bali to Lombok, Wallace watched them, in 1856, glowing with the rays of the evening sun at the end of a tropical day. The giant swells of the Indian Ocean, squeezing through the deep strait, dumped a relieved Wallace onto the steep beach of Ampanam Bay.

Walking up the beach, away from the thunderous sound of the rollers, he heard the raucous call of the helmeted friarbird, an Australian honeyeater. Where were the oriental barbets, fruit-thrushes and woodpeckers, the same Asian birds he had seen in Malaysia, Borneo and Bali, the latter clearly visible across the strait?

Instead, the forests of Lombok echoed with the loud strangled screams of Australian cockatoos, and honey-eaters flitted through the trees. Wallace's explanation was that he believed 'the western part to be a separated portion of continental Asia, the eastern the fragmentary prolongation of a former pacific continent.'

This sentence marks the birth of biogeography: to know where species come from you need to know not only the species' evolutionary history, but also the geological history of the region where they occurred. Today we know that the Wallace Line marks the edge of the Eurasian continent and the beginning, not of a Pacific continent, but of a changing constellation of oceanic islands.

The problem for this father of biogeography was that the theory of evolution had not yet been discovered. But Wallace was onto it. The constant discovery of new species like a biological eruption, altered his perceptions of humanity and its place in the universe. In 1854 he had already penned what became known as the Sarawak Law, which said, simply enough, that a new species came into existence close to a pre-existing closely related species.

Of course such common sense was heresy in Wallace's day. In the early- and mid-19th century virtually all scientists accepted the traditional Christian concept of a Creator God. The Bible was regarded as a literal tome, a real account of the origins of the natural world. Myth was fact. So was the view of Aristotle who held that different types of animals were arranged up a ladder, with humanity, being the pinnacle of divine success, on the top rung. Only by the Hand Of God could this static situation change and an animal be spirited up the ladder.

But Wallace thought the unthinkable: Man was not at the centre of world, any more than the Earth was at the centre of the universe. Wallace knew full well the implications of questioning the origin of species. He knew that he was grappling with the whole question of the organic world and its connection with a time past and with Man. He was grappling with basic beliefs, with religion and with ethics.

Einstein's theory of relativity could hardly affect anyone's personal belief. The Copernican revolution — that the Earth revolves around the Sun, rather than the other way around — and Newton's world view of motion and gravity, required some readjustment of traditional beliefs. The mystery of the origin of species required a rejection of basic belief systems; in a sense the murder of God.

^ to top

Natural selection

Wallace's one great advantage was that he knew what he was talking about. Daily, new species of butterflies were swept into his net; hourly, new beetles crept over his fingers; he watched dazzling new birds created rainbows in the trees.

With this constant bombardment of the 'facts', Wallace recognised that species were the fundamental units of evolution, being both the products of speciation and the things which are thought to speciate. He saw that species were so numerous and their modifications of form and structure so varied, that the tree of life could not possibly be a ladder. Rather the lines of relationships were, he wrote 'as intricate as the twigs of a gnarled oak or the vascular system of the human body.'

In May 1858, Wallace, who was in the Maluku islands in eastern Indonesia, suffering a feverish bout of malaria, wrote a paper in one of his lucid moments. It was titled On the Tendency of Varieties to Depart Indefinitely from the Original Type. It outlined in a clear and succinct manner the origin of species by natural selection, explaining the tendency of species to diverge from a common ancestor.

"In a sudden flash of insight, it was thought out in a few hours, was written down with such a sketch of its various applications and developments as occurred to me at the moment, then copied on thin letter paper."

He sent the essay away to the one person he thought might be interested: Charles Darwin.

About the authorPenny van Oosterzee is an ecologist researching ecosystems and climate change mitigation as an adjunct senior research fellow at James Cook University, and University Fellow at Charles Darwin University. She is a popular science writer with multiple award-winning books including Where Worlds Collide: The Wallace Line, an e-book published by Barnes and Noble.

^ to top