Alfred Russel Wallace (Figure 1) was a 19th century British naturalist who co-originated the theory of evolution by natural selection with Charles Darwin. He travelled widely in Southeast Asia and the East Indies, collecting many thousands of specimens of animals and plants, which he sent back to the UK for study and classification.
While studying fauna in what was then known as the Malay Archipelago (the collection of islands that lies between mainland Indochina and Australia), Wallace noticed a clear distinction between the bird, insect, and bat species present in what we now identify as Borneo, Bali, Java, and Sumatra, and those found in Sulawesi, Lombok, Papua New Guinea, and the rest of Australasia. He first described his findings in 1852 and published an extensive analysis of the region in a book called The Malay Archipelago following his return from the region.
Figure 2 shows a map onto which Wallace drew a line to distinguish between the different groups of species occurred. It forms a natural geographical demarcation between the 'Indo-Malayan' and 'Austro-Malayan’ regions and became known as ‘Wallace’s Line’. The northern part of the line was later modified and extended by the biologist and anthropologist Thomas Henry Huxley (1825–1895), which you can see in Figure 3.
The distance between Bali and Lombok is only about 35km. Wallace found it strange that there should be such a distinct difference in the groups of fauna observed in the two locations, despite the modest distance between them. However, these two groups of islands were probably distinct land masses when sea levels were lower during the last ice age (18–25,000 years ago) than they are now. The ocean channel between the two regions is particularly deep, being a remnant of the separation of the Australian land mass from the rest of Asia as Australia moved away from Pangaea up to 50 Ma. This was enough of a barrier to prevent the interchange of animals, especially the mammals. Differences in climate and land mass size will also have limited the ability of species to colonize even closely adjacent land masses.