EPIGENESIS. - Biological theory (from the Greek ἐπὶ "upon" + γένεσις "generation"), according to which the organic differentiation of the new individual in generation takes place by virtue of a gradual and successive formative development from the structural indeterminacy of the germ.
Already accepted by Aristotle and St. Thomas, it was defended by Wolff against the theory of "preformation" (Leibniz, Malebranche, Spencer, Haeckel, Malpighi, Haller, Swammerdam), which admitted in the germ the actual pre-existence, in extremely small proportions, of the already perfectly differentiated individual. In its absolute form the theory of preformation is connected with that of the "encapsulation of germs," according to which in every individual there would be actually present all the germs that can develop from it and from its descendants. The theory of absolute preformation, today universally abandoned, must be distinguished from relative preformation, which, while denying in the germ the actual differentiation of the organs, nevertheless admits in it a determinate structuring, corresponding to the different organs that will gradually be brought into being in embryonic development.
The controversies between epigenesis and preformationism were agitated especially in the 17th-18th centuries. Modern biology in general, with "germinal localizations," recognizes in the germ a fundamental organization, thus approaching an intermediate position between preformation and epigenesis in the strict sense (v. also EMANOLOLIA).
Bibliography3 works
Ugo Viglino