Wallace Line to the west of the Philippines. Wallace's studies in Indonesia demonstrated the emerging theory of evolution, at about the same time as Joseph Dalton Hooker and Asa Gray published essays also supporting Darwin's hypothesis. Understanding of the biogeography of the region centers on the relationship of ancient sea levels to the continental shelves. Wallace's Line is visible geographically when the continental shelf contours are examined; it can be seen as a deep-water channel that marks the southeastern edge of the Sunda Shelf which links Borneo, Bali, Java, and Sumatra underwater to the mainland of southeastern Asia. Australia is likewise connected
Mainland Southeast Asia Thailand, Laos, Cambodia and Vietnam). To the south it forms the Malay Peninsula, on which are located Southern Thailand and Peninsular Malaysia; the latter is variably considered part of Mainland Southeast Asia or separately as part of Maritime Southeast Asia. Mainland Southeast Asia contrasts with Maritime Southeast Asia, mainly through the division of largely land-based lifestyles in Indochina and the sea-based lifestyles of the Malay and Philippine archipelagos, as well as the dividing line between the Austroasiatic, Tai–Kadai, Sino-Tibetan languages (spoken in Mainland Southeast Asia) and Austronesian languages (spoken in Maritime Southeast Asia). The countries of mainland Southeast Asia received