Alfred Russel Wallace (1823-1913) is one of the most famous 19th-century naturalists. He shared his research with Charles Darwin and independently formulated the theory of natural selection. His most extensive fieldwork was carried out in the Amazon River Basin and in the Malay Archipelago, where his observations led him to propose an imaginary line running between the region''s islands. Today, this line is still known as Wallace''s Line and is the boundary between the organisms of the Australian and Asian regions. In later years, Wallace became more of a social activist.