It is commonly held that contradiction and ambivalence are typical of Aboriginal policies, particularly those of the late 19th and early 20th centuries. These contradictions, often witnessed between policy and its application, have been recognized as a competition between pragmatic factors and humanitarian concerns. However, as is evidenced by the ‘mix-race discourse’ of the laws and policies that make up Aboriginal policy in Canada, the US, and Australia, these contradictions can in part be explained by a post-Enlightenment science that debated the role and place of mixed-ancestry Natives. While mixed-ancestry Natives were the specific targets of law and policy that aimed to fix their identities in a ‘Native-Newcomer’ racial binary, officials were ambivalent and ambiguous when it came to how they fit into that binary. The question of whether they should be considered ‘Aboriginal’ and if they should therefore be assimilated or segregated remained one of the most enduring questions of Aboriginal policy in the century between 1850 and 1950. This dissertation considers these contradictions and how the role of mixed-ancestry Natives in Aboriginal policies can explain them. Instead of seeing those contradictions as anomalies or as illogical, I posit that they are a logical product of scientific debates over racial hybridity. Fundamentally, I argue that mixed-ancestry Natives were the targets of ambivalent policies that were shaped by debates among nineteenth-century scientists about the implications of racial mixing. These debates were reflected in the inconsistencies and apparent contradictions of the laws and practices that make up Aboriginal policy in Canada, the US, and Australia. In particular, these debates were reflected in the ambiguity and ambivalence of policies that tried to direct how Indigenous peoples of mixed-ancestry should be dealt with, defined, and categorized. The contradictions and ambiguities in law and policy reflect on a larger scale the tension between attempting to apply a hypothetical dichotomized racial hierarchy on the reality of a hybridized society. These tensions were a major influencing factor on the direction and development of Aboriginal policy in these three countries, and produced a consistent albeit ambivalent body of ‘mixed-race’ discourse.