EPIGENESIS. - Biological theory (from Gr. ἐπὶ σοφία) is the event, and generation), according to which the organic differentiation of the new individual in generation occurs 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 individual already perfectly differentiated. In its absolute form the theory of preformation is connected with that of the "conglobation of germs," according to which every individual would actually contain all the germs that can develop from it and its descendants. The theory of absolute preformation, now universally abandoned, must be distinguished from relative preformation, which, while denying in the germ the actual differentiation of organs, yet admits in it a determined structuring corresponding to the various organs that will gradually be realized in embryonic development.
The controversies between epigenesis and preformationism were especially heated 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 in the strict sense and epigenesis (see also EMBARATO, 1903).