Throughout the nineteenth century evolution theory and the possibility of reconciling it with Christian doctrine was a matter for free discussion among Catholics, but suddenly towards the end of the century the attitude of the Roman doctrinal authorities hardened. Nothing public was done but theologians, upon hearing that their theses had been examined in Rome and judged to be untenable, began to withdraw their books from sale, make public retractions and cease to discuss the subject. What processes lay behind these events, and who were the main persons responsible for what was effectively a U-turn in the official church attitude of Pope Leo XIII's pontificate, are questions that have been unanswerable until now. However, in 1998 the archives of the Holy Office, now renamed the Congregation for the Doctrine of the Faith, were opened to scholars so that it has become possible to clarify these particular mysteries. This article does that for the first time, and the story it has to tell provides insights into the dynamics of cultural and intellectual interactions within the Catholic Church community and with the rest of the world. This Catholic story should be seen as something more than a struggle that took place within the ranks of the Catholic Church: it was an episode in the history of science and its cultural relations in the modern world.