'In this archipelago there are two distinct faunas […] yet there is nothing on the map or on the face of the islands to mark their limits.'
Alfred Russel Wallace
Alongside his work in developing the theory of evolution by natural selection, Wallace (and his vast collection of specimens) contributed to the growth of a new field: evolutionary biogeography.
Evolutionary biogeography studies the distribution of plants and animals around the world, and supplies key evidence that evolution is taking place.
Wallace himself famously encountered this process when crossing from the island of Bali to the island of Lombok, both in modern-day Indonesia. He noted a big difference in the species found on either side of the narrow strait between the islands, and realised that he had crossed an invisible dividing line - now known as the Wallace Line.
We know today that the Wallace Line arises because the species on either side were isolated from each other in the past, and thus have different evolutionary histories.