ATAVISMO. – Reappearance of morphological, physiological, and psychic hereditary traits of more or less distant, often extremely remote, ancestors, which have remained hidden for a long series of generations; in this way, the return is so unexpected that it gives the impression of the emergence of a completely new trait. The fact, strange in appearance, is fully consistent with the heredity (v.), particularly those of alternate form (Mendelian type). The latter shows us how the so-called recessive hereditary traits, unlike dominant ones, if present in only one parent, remain latent for an indeterminate number of generations, appearing in the offspring only if the determining elements (genids) are present in the carriers of heredity (chromomeres) of both parents (v. EUGENICA); this explains the easier reappearance of morbid atavistic traits in the offspring of consanguineous unions, all the more since these generally behave as recessive traits. The term a. is also used in common language to indicate the tendency of the species to preserve its own individual hereditary characteristics, against every contingent cause (environment, crossbreeding, etc.) that would tend to modify them.
According to the distance of the ancestor in whom the atavistic trait was present, one speaks of familial, historical, or prehistoric a. An animal a., invoked by some to explain the appearance of non-human traits (harelip, polymastia, branchial clefts, etc.), necessarily entails acceptance of an integral evolutionism which, in the present state of science, offers no truly probative arguments (Cotronei, p. 427).