ATAVISM, sometimes called reversion, the term given in biology to the reproduction in a living person or animal of charac teristics of an ancestor more remote than its parents (see HEREDITY) . Loosely used, it connotes a reversion to an earlier type. Individuals reproduce unexpectedly the traits of earlier ancestors, and ethnologists and criminologists frequently explain by "atavism" the occurrence of degenerate species of man ; but the subject is complicated by other possible explanations of such phenomena. Many cases of atavism or reversion have been satisfactorily explained as due to recombination of Mendelian factors (see HEREDITY) ; and others are undoubtedly the result of arrested development (see EMBRYOLOGY).