Atavism

He was a magnificent atavism, a man so purely primitive that he was of the type that came into the world before the development of the moral nature. He was not immoral, but merely unmoral.

Source: wiktionary

Hence on false premises was built up that belief in spirits or invisible beings outside ourselves, which by some curious atavism was re-emerging in modern days among the less educated strata of mankind.

Source: wiktionary

She had lost the power to care about his faults. Strange, sweet, poisonous indifference! She was drugged. And she knew it. Would she ever wake out of her dark, warm coma? She shuddered, and hoped not. Mrs Tuke would say atavism. Atavism! The word recurred curiously.

Source: wiktionary

Upon the death of Theodoric in 526, Ibidus retired from public life to compose his celebrated work (whose pure Ciceronian style is as remarkable a case of classic atavism as is the verse of Claudius Claudianus, who flourished a century before Ibidus); but he was later recalled to scenes of pomp to act as court rhetorician for Theodatus, nephew of Theodoric.

Source: wiktionary