This essay uses the Popular Science writing of nineteenth-century American zoologist William Keith Brooks to analyze modern approaches to evolution and sexual difference. Brooks’s popular writing invented a Darwinian model of sexual difference that has received little scholarly attention. Although the little it has received has criticized Brooks for reducing supposedly gendered cognitive processes to the physiology of sex cells, this essay argues that none of Brooks’s questionable conclusions actually follow from such a reduction. Brooks’s essays instead mobilized the notion of “tendency,” which created a class of characteristics that could exist even where they did not reliably appear. Brooks’s use of this notion developed out of his rhetorical appeal to functions, allowing us to put a long-standing debate within the philosophy of science over that term in dialogue with feminist critiques of functionalism. More specifically, Brooks viewed the generation of racial difference as the function and tendency of men and the preservation of those differences as the function and tendency of women—a definition that reworks historical accounts of the relationship between race and gender. Finally, Brooks’s notion of tendencies provided a template for the application of evolution to human psychology, making it possible to argue that the mind preserved notions of sex and race even as they became less reliably fixed in the body. This final notion of tendencies reveals surprising commonalities between the contemporary evolutionary psychology of gender, which has been the target of intense feminist criticism, and recent efforts to reinterpret the legacy of Darwinism for the humanities generally and for feminist theory in particular.