The Mark of the Beast historically and critically examines the dire effects of the process of animalization on both humans and animals. Roberts provides a general account of the theoretical division between humans and animals begun largely in the work of Aristotle and continued in that of Descartes and Kant. Following the philosophical provenance of the idea of animality, Roberts explores the practical and "scientific" uses of this idea, focusing largely on what Stephen J. Gould terms the "biodeterministic tradition" by evaluating the primarily ninteenth century theories of atavism, craniology, recapitulation, and so on, while also exploring the use of medical and psychological techniques of animalization.