Retrogressive development: transcendental anatomy and teratology in nineteenth-century Britain.

In 1855 the leading British transcendental anatomist Robert Knox proposed a theory of retrogressive development according to which the human embryo could give rise to ancestral types or races and the animal embryo to other species within the same family. Unlike monsters attributed to the older theory of arrested development, new forms produced by retrogression were neither imperfect nor equivalent to a stage in the embryo's development. Instead, Knox postulated that embryos contained all possible specific forms in potentia. Retrogressive development could account for examples of atavism or racial throwbacks, and formed part of Knox's theory of rapid (saltatory) species change. Knox's evolutionary theorizing was soon eclipsed by the better presented and more socially acceptable Darwinian gradualism, but the concept of retrogressive development remained influential in anthropology and the social sciences, and Knox's work can be seen as the scientific basis for theories of physical, mental and cultural degeneracy.