Hereditary reappearance of features, usually physical, absent from parents and recent ancestors. Such features may be characteristic of more distant ancestors, or they may constitute a return to characteristics not typical of the species or race at all, but found in allied species or races having a common evolutionary ancestry (as in the case of strongly pronounced canine teeth in humans).
Atavism does not necessarily represent a deterioration. It may be the re-emergence of valuable genetic characteristics of less-specialized progenitors. Many phenomena have been labelled atavisms which admit of other interpretations such as the consequences of arrested development due to malnutrition, a qualitative variation in some vestigial organ, or the predictable consequences of hybridization.