ATAVISM. - The reappearance of morphological, physiological, or psychic hereditary characters from more or less distant ancestors, often very distant ones, which had remained hidden for a long series of generations; in this way the return is so unexpected that one has the impression of the appearance of a completely new character. The fact, strange in appearance, is in full agreement with the laws that regulate the phenomena of heredity (v.), in particular, of the alternating form (Mendelian type). This shows us how so-called recessive hereditary characters, unlike dominant ones, if present in only one parent, remain latent for an indeterminate number of generations, manifesting themselves in the offspring only if the determining elements (genids) are present in the carriers of heredity (chromomeres) of both parents (v. EUGENICS); this explains the easier reappearance of morbid atavistic characters in the offspring of consanguineous unions, all the more so since these generally behave as recessives. The term a. is used in common language also to indicate the tendency of the species to preserve its own individual hereditary characteristics, against any contingent cause (environment, crosses, etc.) that would tend to modify them.
According to the distance of the ancestor in which the atavistic character was present, one speaks of familial or historical or prehistoric a. A bestial a., invoked by some to explain the appearance of non-human characters (cleft lip, polymastia, branchial clefts, etc.) certainly involves the acceptance of an integral evolutionism which, in the current state of science, does not present truly probative arguments (Cotronei, p. 427).
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Giuseppe de Ninno