Wallace noticed on expeditions to the Amazon and the Malay archipelego that mammals in Southeast Asia are more advanced than their Australian cousins. His suggestion was that the two continents had split before the better adapted mammals had evolved in Asia. The isolated Australian marsupials were able to thrive, whilst those in Asia were driven to extinction by competition from more advanced mammals. This led to his theory of natural selection, which he presented to the Linnean Society in 1858 with Charles Darwin. This volume reprints those papers presented to the Linnean Society.