The Wallace Line is an imaginary line that separates the faunal regions of Asia and Australia. It runs between Australia and the Asian islands and the mainland. The line marks the point where there is a difference in species on either side of the line.
The Wallace Line was drawn in 1859 by the British naturalist Alfred Russel Wallace. The English biologist T.H. Huxley named it.
The Wallace Line separates the biogeographical realms of Asia and 'Wallacea', a transitional zone between Asia and Australia also called the Malay Archipelago.
Species west of the line have a predominately Asiatic origin and those to the east show a mixture of Asian and Australian origins. For example, to the west of the line, you'll find species including elephants, tigers, rhinos, and orangutans, but on the eastern side, these are nowhere to be seen. South and east of the line marsupials and monotremes are found, unlike the placental mammals found north and west of it.
The primary reason animals do not cross the Wallace Line is simply the ocean's depth and width. Animals have little incentive to try to cross any fairly large body of ocean.