Aberrations and variations within kinds of creatures required explanation to Western medievals, who took the Genesis creation narratives together with Aristotelian species to imply that change was limited to within species; consequently, species were presumed static. Medieval philosophers often explained variation—including “new” kinds like mules—as due to problems in procreation/gestation (following Aristotle) or by sin. I argue that Aquinas's explanation of variation in women, people with disabilities, and mules suggests that Aquinas cannot be taken to entirely reject the possibility of (...) new kinds, and parallels in his explanation of the existence of women and the possible existence of new kinds provides warrant for a re‐evaluation of his understanding of the notion of the natures or essences shared by kinds. Sin—individual or original—is an inadequate explanation for variation, and the argumentative parallels between Aquinas's treatment of women and mules challenge presumptions about what medievals mean by “static kinds” at all, revealing space for evolutionary thought. (shrink)