Definition of "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.

1904, Jack London, chapter 10, in The Sea-Wolf (Macmillan’s Standard Library), New York, N.Y.: Grosset & Dunlap

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.

1925 July – 1926 May, A[rthur] Conan Doyle, “(please specify the chapter number)”, in The Land of Mist (eBook no. 0601351h.html), Australia: Project Gutenberg Australia, published April 2019

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.

1938, Howard Phillips Lovecraft, Ibid

I have even read in a book of criminology that the tramp is an atavism, a throw-back to the nomadic stage of humanity.

1933 January 9, George Orwell [pseudonym; Eric Arthur Blair], chapter XXXVI, in Down and Out in Paris and London, London: Victor Gollancz […]

"...he traces the roots of objectless imperialism to three sources, each an atavism. Modern imperialism, according to Schumpeter, resulted from the combined impact of a "war machine", warlike instincts, and export monopolism".

1986, Michael Doyle, “Liberalism and World Politics”, in American Political Science Review, volume 80, number 4