A conception of the idiotic mind was used to substantiate late 19th-century theories of mental evolution. A new school of animal/comparative psychologists attempted from the 1870s to demonstrate that evolution was a mental as well as a physical process. This intellectual enterprise necessitated the closure, or narrowing, of the ‘consciousness gap’ between human and animal species. A concept of a quasi-non-conscious human mind, set against conscious intention and ability in higher animals, provided an explanatory framework for the human–animal continuum and (...) the evolution of consciousness. The article addresses a significant lacuna in the historiographies of intellectual disability, animal science, and evolutionary psychology, where the application of a conception of human idiocy to advance theories of consciousness evolution has not hitherto been explored. These ideas retain contemporary resonance in ethology and cognitive psychology, and in the theory of ‘speciesism’, outlined by Peter Singer in Animal Liberation (1975), which claims that equal consideration of interests is not arbitrarily restricted to members of the human species, and advocates euthanasia of intellectually disabled human infants. Speciesism remains at the core of animal rights activism today. The article also explores the influence of the idea of the semi-evolved idiot mind in late-Victorian anthropology and neuroscience. These ideas operated in a separate intellectual sphere to eugenic thought. They were (and remain) deeply influential, and were at the heart of the idea of the moral idiot or imbecile, targeted in the 1913 Mental Deficiency Act, as well as in 20th-century animal and human consciousness theory. (shrink)