atavism(redirected from atavistically)
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the appearance in organisms of features that are absent in their immediate ancestors but which did exist in very distant ancestors. An example of atavism could be the development of a caudal appendage in man, the appearance of two extra toes along the sides of the completely developed middle toe of the horse, or the development of pinnately compound leaves instead of phyllodes in the green wattle. Since to a certain degree individuals repeat the organizational features of their ancestors in the developmental process, a disturbance of normal development can lead to a situation in which the adult organism keeps for its entire life the ancestral features which normally appear in the embryo and usually disappear in the course of further development. An example of such atavism would be the appearance in man of a cervical fistula reminiscent of the gill slit of his mammalian ancestors, the fishes and amphibians. Polymastia in man (the formation of a larger than normal number of pairs of mammary glands), tridactylism in horses, and so forth, are also related to atavism. Atavism also includes the occurrence of features from distant ancestors in the regeneration of organs. In this instance, the organs regenerate with the features characteristic of the more ancient forms. Thus, in the regeneration of a lizard’s tail, the scale rings are sometimes formed in a more primitive shape. Upon regeneration of the anterior end of the body in certain annelids, head appendages are formed which are missing in the given form but which are inherent in its ancestors.
REFERENCEShimkevich, V. M. Biologicheskie osnovy zoologii. 5th ed., vols. 1–2. Moscow-Leningrad, 1923–25.
M. A. VORONTSOVA