Daily Dose · Maps/Sep 24, 2026/Biogeography · Wallace Line
The Line in the Water
Between Bali and Lombok runs one of the sharpest natural borders on Earth. Tigers, rhinos and apes on one side; marsupials on the other. Nothing marks it but a strip of deep sea.
35km
of water across the Lombok Strait — roughly the gap Alfred Russel Wallace identified in 1859 as the frontier between the animals of Asia and those of Australia.
Wallace Line (1859)Huxley's variant (1868)Weber Line (1904)Lydekker Line (1896)
Schematic map. Teal = continental shelf shallower than ~120 m, exposed as dry land in the last ice age. Green land = Asian shelf; olive = Wallacea; ochre = Australian shelf.
35 kmBali → Lombok
250 mLombok Strait max depth
−120 mIce-age sea level
125,660Wallace's specimens
01 · The gap
Thirty-five kilometres, two continents' worth of animals
The Lombok Strait is 20 km wide at its narrowest and 40 km at its northern mouth — a morning's ferry ride.
Detail · Lombok Strait (schematic)
Birds, which could cross it in minutes, largely respect it: many species will not fly over even narrow stretches of open water. Mammals sit almost entirely on one side or the other. The crab-eating macaque is one of only a few mammals found on both.
Wallace put it bluntly in 1859: the islands east of Borneo and Java belonged to “an Australian or Pacific continent,” even though they looked like neighbours on any map.
02 · The ice-age test
The sea fell 120 metres. The strait stayed open.
In the Pleistocene, ocean levels were up to 120 m lower. The shallow shelves became land bridges; the deep strait never did.
Depth scale · one hue, shallow → deep
Everything on the map shaded teal — the Java Sea, the Gulf of Thailand, the Arafura Sea — sits above the ice-age waterline. Bali joined Java, Java joined Borneo and Sumatra: a single Asian landmass, Sundaland. Australia and New Guinea fused into Sahul. Between them, Wallacea stayed islands.
03 · One naturalist
Eight years, 14,000 miles, 125,660 specimens
Wallace found the line by collecting — from 1854 to 1862, overwhelmingly insects.
Wallace's collection by group · Source: The Malay Archipelago (1869), preface
The Natural History Museum in London credits the haul with more than 5,000 species new to Western science. The same fieldwork produced his independent theory of evolution by natural selection, sent to Darwin in 1858.
It was the pattern across thousands of specimens — not any single animal — that revealed where one fauna ended and the other began.
04 · A line that keeps moving
Everyone who looked drew it somewhere else
Wallacea is less a line than a zone of transition — so naturalists kept redrawing the edge.
Huxley renamed it “Wallace's Line” in 1868 and moved its northern end west of the Philippines. Lydekker (1896) traced the edge of the Australian shelf. Weber's line (1904) marks the “faunal balance,” where Asian and Australian species are roughly even. Ernst Mayr revisited it in 1944.
05 · One-way traffic
Why Asia's animals crossed more easily than Australia's
A 2023 study in Science modelled 20,000+ vertebrates over 30 million years to explain the asymmetry.
20,000+birds, mammals, reptiles and amphibians analysed (Skeels et al., 2023)
45 Myrago the Australian Plate began drifting north toward Asia (ETH Zurich)
2marsupials made it as far as Sulawesi; none reached Borneo
Asian lineages evolved in humid tropics much like the stepping-stone islands of Wallacea, so they could establish themselves on the way east. Australian lineages had largely adapted to drier conditions and struggled to settle westward. The line reflects climate history as much as water.
06 · The stakes
A hotspot built by isolation
Wallacea covers some 346,782 km², and much of its life lives nowhere else.
Share of species endemic to Wallacea · Source: CEPF (2014)
The same deep water that kept continents' faunas apart left islands such as Sulawesi, Flores and Halmahera to evolve their own species. Isolation made them unique, and it makes them fragile: when a species found on one island is lost there, it is lost everywhere.
Why it matters · what to notice on the map
Read the colours, not the coastlines
Teal is the story. Every shallow shelf was dry land 20,000 years ago. The gold line runs exactly where the teal stops.
Bali and Lombok look like twins. Same volcanic arc, same climate, and very different ancestry.
Borneo and Sulawesi sit face to face across the Makassar Strait, yet belong to different worlds.
Four lines, one zone. Wallace, Huxley, Weber and Lydekker bracket Wallacea, a borderland rather than a wall.