The Wallace line is an observed dividing line between Australasia and Asia – directly between Borneo and Sulawesi, for example.
To the east, the Sulawesi side, you’ll find the animals native to Australia. Wallabies, cockatoos, emu, etc.
To the west, on the Borneo/Bali/Java side, you’ll find Asian fauna. Monkeys, deer, thrushes.
Remarkably, in places the islands on either side of the line are relatively few miles apart – Bali and Lombok, for example – yet the fauna still display this rigid distinction.
This phenomenon was observed and mapped by Alfred Wallace in the mid-1800s, and subsequently became controversial, according to Simon Winchester in Krakatoa, “because of technical arguments among the world’s zoographers.”
And what is this zoological stuff doing in a book about a volcano?
Krakatoa exploded in 1883 with such violence that, Winchester says,
Airborn debris…lowered the planet’s temperature; it set barometers and tide meters flailing wildly thousands of miles away; it panicked American firefightes into battling what they thought were raging infernos, but that were in truth violent sunsets caused by the roiling clouds of Krakatoan dust.
That explosion was of course a result of the geological shifting under the surface of the Earth – continental drift, plate tectonics – things that were unknown until the early 1900s. Wallace’s observation provided interesting data points in the development of plate tectonics theory because of the line’s implication that all of the island chain of Australasia, for instance, was once a conjoined land mass.