Imagine the explorer who passes through Southeast Asia. An intrepid adventure, to be sure, as the Malay Archipelago is aptly named. Thousands of islands are strewn haphazardly throughout the sea. This particular explorer starts in Singapore, hopeful and excited, and begins to island-hop her way southward. She passes through Borneo and takes note of the great orangutans who bellow like foghorns in the jungle. In Sumatra and Java she may pause to philosophize on the elusive and sublime tigers or their tapir prey.
Our explorer gathers her notes in Bali, perhaps taking a little longer than is strictly necessary to enjoy the island, before setting off to the nearby island of Lombok, only fifteen miles away. Excited, she hurries off into the jungle, watchful for tigers and monkeys. However, there are none to be found. There are no wolves, otters, bears, or squirrels. No woodpeckers or thrushes are found here. In their place are cockatoos, brush-turkeys, opossums. She moves on to Sulawesi, Flores, and Timor, confused. The familiar calls and shapes have been replaced. Flightless cassowaries stalk through dense thickets. Small kangaroos bound and abound.
What happened? Our explorer crossed what is known as the Wallace Line. It is named after Alfred Russel Wallace, Darwin’s lesser-known co-author of natural selection (although that is a story for another time). Working here in the middle of the 19th century Wallace was interested in the stark contrast of animals between the east and the west. He solved the problem by proposing that Australia and Asia were once separated by a much emptier, perhaps wider, stretch of sea. However, islands eventually rose between them creating the vast Malay Archipelago. Naturally creatures from each continent travelled forth into the unoccupied land, either through land bridges or by island hopping. Tigers stalked downwards from China, kangaroos moved outwards from New Guinea. The two worlds crossed at this imaginary line. Whether due to the final watery barrier or due to competition from the established populations, the ecosystems met, stopped, and did not mix.
The Wallace Line helped to strengthen the field of zoogeography and inadvertently pushed forth the idea of plate tectonics and movement of the continents. It is the most famous legacy of Alfred Russel Wallace and his dedication to natural observation. We can still experience the phenomenon today, as our explorer once did. You cross the Wallace Line and, in turn, cross into a new world.
Sources: Second Shot, which provides a first hand account of crossing the line, and Krakatoa by Simon Winchester for a detailed history.