Only recently, with the rise of critical animal studies, have readings of Mary Shelley’s Frankenstein begun to do full justice to the hybrid nature of Frankenstein’s Creature, constructed (as Victor tells us) from materials found in the “slaughter-house” as well as the “dissecting room.” Yet even animal-studies scholars view the Creature’s brain as “human,” in the absence of any supporting evidence from Shelley’s text. Here, Alan Richardson traces the Creature’s horrific effect to dual anxieties that came to ferment during the early nineteenth century, both of them amply documented in the brain science of Shelley’s era and in published reactions to it. First, the line between human and animal was becoming notably porous, in natural history, in comparative anatomy and physiology, and even in such areas as the controversy over vaccination. Second, a new discourse of instinctive and innate mental tendencies had come to compete with both creationist and tabula rasa accounts of the human mind—a development that further eroded the border between human and animal. Frankenstein’s Creature, a literally monstrous hybrid, both embodies these anxieties and exaggerates them, as a fully material and yet rational humanoid entity with body parts, and perhaps neural organs and instincts, traceable to animals.