This paper examines how the 19th-century British naturalist Alfred Russel Wallace used biogeographical mapping practices to draw a boundary line between Malay and Papuan groups in the colonial East Indies in the 1850s. Instead of looking for a continuous gradient of variation between Malays and Papuans, Wallace chose to look for a sharp discontinuity between them. While Wallace's "human biogeography" paralleled his similar project to map plant and animal distributions in the same region, he invoked distinctive "mental and moral" features as more decisive than physical ones. By following Wallace in the field, we can see his field mapping practices in action -- how he conquered the problem of local particularity in the case of human variation. His experiences on the periphery of expanding European empires, far from metropolitan centers, shaped Wallace's observations in the field. Taking his cues from colonial racial categories and his experiences collaborating with local people in the field, Wallace constructed the boundary line between the Malay and Papuan races during several years of work in the field criss-crossing the archipelago as a scientific collector. This effort to map a boundary line in the field was a bold example of using the practices of survey science to raise the status of field work by combining fact gathering with higher-level generalizing, although the response back in the metropole was less than enthusiastic. Upon his return to Britain in the 1860s, Wallace found that appreciation for observational facts he had gathered in the field was not accompanied by agreement with his theoretical interpretations and methods for doing human biogeography.