Wallace's Line - An imaginary line separating the Malay Archipelago into two regions, each with its character istic plants and animals. For example to the east of the line, the marsupials and monotremes are the only native mammals; to the west only placentals occur. This suggested to Wallace that the islands to the west were once joined to Asia and those to the east to Australia. The line that bears his name passes between Borneo and the Celebes in the north and between the islands of bali and Lombok in south.