Back in the mid-19th century, a certain naturalist, having spent a great deal of time sailing around the world, collecting and documenting animal specimens, and thinking very hard about why certain species turned out the way they did, came up with a notion that was—and in some quarters, still is—considered heretical: the idea of survival of the fittest, or as it is more properly known, natural selection. That description may evoke the name Charles Darwin, but it could apply equally to one of his contemporaries, Alfred Russel Wallace.

One could argue, in fact, argue that Wallace was the true originator of the idea for which Darwin was to become famous. The two were at least working on the same general problem at the same time though in different parts of the world, and Wallace’s discoveries prompted Darwin to publish his own findings sooner than he had intended. Darwin was an honorable scholar and gave due credit to Wallace, including a mention in the second paragraph of On the Origin of Species. But because of Darwin’s considerably greater influence, Wallace’s contributions to the science of evolution were given far less fanfare. (Interestingly, although Wallace didn’t invent the expression natural selection, there is some evidence that he coined the phrase survival of the fittest, though he was not the first to publish it, and so even that honor goes to someone else: a British economist named Herbert Spencer.)

Father of Evolution Zoogeography

Wallace is, however, remembered for something that has tremendous implications not only for biology but for geology too: an imaginary line that divides what is now Indonesia roughly in half. Islands to the west of the line include Sumatra, Borneo, Java, and Bali; to the east are Lombok, Sulawesi, Timor, and many others. What Wallace noticed during his extensive travels in the area was that the islands in the western part of the archipelago had animal life similar to that found in continental Asia, while the islands in the eastern part of the chain had species resembling those found in Australia. The Wallace Line was his attempt to draw a boundary between these two regions with very different fauna.

What particularly struck Wallace about his discovery was that some islands that were very far apart had the same distribution of animal species, while some that were close together had much different species. Nowhere was this more striking than between the islands of Bali and Lombok, which are separated by only 22 miles (35km) of water. And yet, numerous species of plants and animals—especially birds—that are found on Bali and other, more distant islands to the north and west were absent on Lombok, which had species found on other islands far to the south and east.

Continental Drifter

In 1859, when Wallace originally drew and publicized the line, he couldn’t explain in any detail why these nearby areas had such different species. His theories about natural selection suggested that quite the opposite should have been the case, and Wallace speculated that ancient geological changes must have been the cause. It would be more than a century before the theory of plate tectonics was developed, but Wallace would have been pleased to know that his line closely matches the boundary between two continental plates that were once far apart. Because some of the islands on each of the plates were once connected to each other and to the mainland by land bridges, animals could freely migrate between them, but no such bridge ever existed between the two plates. It just so happens that in some spots these two geological regions have, over many centuries, moved to within spitting distance of each other.

Wallace himself redrew the line with greater precision in 1863. Since then, it’s been subject to a great deal of discussion, controversy, and additional refinement, as researchers have uncovered more detailed data. It’s also been discovered that the zoogeographical division isn’t quite as clear cut as it once seemed. But the fundamental insight that the division between areas of animal species must have been connected to massive geological shifts put Wallace far ahead of his time and remains a remarkable achievement to this day.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on February 23, 2007.