This article suggests that the spaces of British settler colonialism and metropolitan science were interconnected, underexamined, grounds upon which both ethnography and colonial governance developed. Focusing on the governmental and ethnographic activities of Sir George Grey during the mid-19th century it argues that the origins of ethnography and the specifically humanitarian governance of spaces invaded by settlers were co-constituted. Although anthropologists have long recognised the complicity of ethnography in modern colonialism, the relationship runs far deeper and extends far more broadly, than has been appreciated in even the most incisive critiques. That relationship was also located in violent settler colonial spaces that have been relatively neglected in the anthropological historiography. The article concludes that Grey's governmental practices, and his representations of them, established the terms upon which cultural genocide, with its logic of elimination, could be posited as a humane alternative to racial extermination. It shows that Grey's promotion of amalgamation, articulated as a preferably cultural and social extinction over a physical one, went on to influence the highest levels of colonial administration. On behalf of the British Empire as a whole, Grey thus helped to reconcile settler colonialism with humanitarian governance.