David F. Slade's critical edition of Desvíos de la naturaleza: O tratado del origen de los monstruos by Pedro de Peralta Barnuevo and José de Rivilla Bonet y Pueyo is the first complete edition published since the text's original 1695 debut. At the center of the treatise is the case of conjoined twins born in Lima, Peru, in 1694. Although they died shortly after their birth, their appearance became a crucial locus of scientific, theological, historical, and philosophical discourse. Organized into ten chapters and an extended appendix that reviews other similar medical cases, the text is an early example of eighteenth-century criollo subjectivity. This edition includes a critical introduction that offers a literary history of the text and its place in a larger context of multidisciplinary works related to monstrous births. It makes the case for the hybrid authorship of the text between Peralta Barnuevo and Rivilla Bonet y Pueyo, marking the first of Peralta Barnuevo's many publications. The introductory essay further explores Desvíos de la naturaleza: O tratado del origen de los monstruos as an important contribution to criollo scientific knowledge, the study of monsters in the Ibero-American and broader Western world, as well as the implications for our current representations of disability. This edition is number 105 in the Ediciones críticas series from Juan de la Cuesta Hispanic Monographs.