Atavism

Atavism (Lat. atāvus, 'ancestor'), or REVERSION, is a term applied to the frequent appearance of ancestral, but not parental characteristics in an animal or plant. It is a commonplace that a boy 'takes after his grandfather,' and all organisms exhibit the same tendency to reproduce often remote ancestral characters. An occasional horse exhibits the long-lost stripes of the wild form; a blue pigeon, like the primitive Columba livia, sometimes turns up unexpectedly in a pure breed; or a cultivated flower reverts to the simpler and more normal type of the original wild plant. Even in detailed anatomical structure, a comparatively trivial character lost for many generations may suddenly reappear. Darwin has shown that reversion is greatly favoured by the disturbance to the organisation which results from crossing, while in other cases a return to more natural or primitive conditions of life brings about the ancestral variation. The possibilities of the variation are at once insured and limited by the summing up of the past history in the constitution of the germ, but the direct conditions which determine the particular modification are in many cases very obscure. See DARWINIAN SYSTEM, GENERATIONS, HEREDITY, INSANITY, VARIATION, and Darwin's Animals and Plants under Domestication.