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” (Leibner, Malebranche, Spencer, Haeckel, Malpighi, Haller, Stammerdan), 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 “enfolding of germs,” according to which every individual would actually contain all the germs that can develop from it and from its descendants. The theory of absolute preformation, now universally abandoned, must be distinguished from relative preformation, which, while denying the actual differentiation of organs in the germ, nevertheless admits in it a determinate structuring corresponding to the various organs that will gradually be actualized in embryonic development.
The controversies between epigenism and preformationism were especially heated in the 17th-18th centuries. Modern biology in general, through “germinal localizations,” recognizes a fundamental organization in the germ, thus approaching an intermediate position between preformation and e. in the strict sense (v. anche EMANOLOLIA).