Trans histories have primarily focused on trans people, but what happens if you make trans history a method, not a subject? This talk proposes that examining the history of sex classification through a trans lens shows that no forms of sex and gender, including what might be considered cisgender or not-trans, pre-exist in clear-cut categories without constant management. Though scientific research has repeatedly dredged up bodies that could threaten the idea of binary and stable sex, norms of self-evident maleness and femaleness have endured because the ways that countless forms of life fail to fit those categories—and the work that goes into making them fit—have been rendered invisible. Using examples from nineteenth-century animal research, I excavate the often contradictory logics of racial hierarchizing and scientific expertise-building that have made sex classification a site not of binary consensus, but persistent uncertainty. In doing so, I show that the power to sort bodies by sex emerged not from solidified, agreed-upon parameters, or inherent bodily forms, but out of a mobile and malleable understanding of sex that enabled researchers to reclassify bodies and reconstitute sex itself at every turn.