The title of this essay, comes from the Sherlock Holmes mystery entitled "Silver Blaze," which refers the "curious incident" as to the absence of an expected reaction. In this article, the author discusses an essay that will examine such an absent reaction, or at least a muted one: the limited impact of early intelligence testing on European debates about and practice of secondary coeducation. When such testing began in the early twentieth century, many leading educators, physicians, and psychologists believed that there were significant differences in mental abilities between the sexes that made secondary coeducation difficult if not impossible. In this essay, the author also focuses on gender differences and inequality in education. (Contains 53 footnotes.)