ATAVISM. – The reappearance of morphological, physiological, or psychological hereditary traits from ancestors, sometimes very distant, which have remained latent for many generations; in this way, the return is so unexpected that it gives the impression of a completely new character. This seemingly strange fact is in full accord with the laws governing the phenomena of heredity (see HEREDITY), particularly of the alternate form (Mendelian type). This shows how so-called recessive hereditary traits, unlike dominant ones, if present in only one parent, remain latent for an indeterminate number of generations, manifesting in offspring only when the determining elements (genes) are present in the hereditary vectors (chromomeres) of both parents (see EUGENICS); this explains the easier reappearance of morbid atavistic traits in the offspring of consanguineous unions, especially since these traits generally behave as recessives. The term atavism is also used in common language to indicate the tendency of a species to preserve its own individual hereditary characteristics, despite any contingent causes (environment, crossbreeding, etc.) that might tend to modify them.
Depending on the distance of the ancestor in whom the atavistic trait first appeared, one speaks of familial, historical, or prehistoric atavism. A "bestial atavism," invoked by some to explain the appearance of non-human traits (harelip, polymastia, branchial fissures, etc.), necessarily involves the acceptance of a complete evolutionism, which, at the present state of science, does not present truly probative arguments (Cotronei, p. 427).