The preformation theory or preformation theory ( Latin prae 'before' and formatio 'design', 'education') is a developmental theory that was advocated by the Greek philosopher Anaxagoras in antiquity and reappeared in modern times in the 17th century and then up to was prevalent into the 19th century. Another name that was used at the time was evolution , although “evolution” (in the sense of development as a pure growth process) had a completely different meaning than it does today . The preformists assumed that the entire organism in the sperm ( animalkulism ) or in the egg ( ovism or ovulism ) was preformed (“preformed”) and only had to develop and grow. This contrasted with the theory of epigenesis , according to which the structures and organs of an organism only develop in the course of individual development . Epigenesis had been the prevailing opinion since antiquity and up into the 17th century, but was then replaced by preformist ideas in the course of the Enlightenment as no longer plausible. It was not until the early 19th century that it was able to establish itself again as a view that is still valid today.
Living things were considered indivisible individuals in the 17th century. The idea of reproduction that is taken for granted today did not arise until the end of the 18th century. Until then, the “procreation” of a living being was viewed as an act of creation that basically required divine intervention. A distinction was made between spontaneous generation , through which "lower" animals (such as worms, insects, snakes and mice) appeared to emerge from dead matter, from "semen generation" in humans and higher animals, which required a womb. For the latter, Giuseppe degli Aromatari (1587–1660) first formulated the idea in 1625 that an organism could already be “preformed” (preformed) before it was conceived and only had to develop. Aromatari relied on studies of plants in whose seeds and bulbs he saw the daughter plants already formed, and speculatively transferred this to animals and humans. According to this view, divine intervention had already taken place during the creation of the world , in which all previous and future generations were created nested according to the principle of Russian Matryoshka dolls. This corresponded to the zeitgeist , which sought to overcome the metaphysical teachings of scholasticism and antiquity ( Aristotle ) and pursued the ideal of a mechanistic explanation of nature, and therefore quickly found acceptance. Based on the idea that the pre-formed organisms only had to be "unwound" (Latin: evolvere ), in the 17th and 18th centuries, instead of preformation or preformism, one spoke of evolution , which means that "evolution theory" had a completely different meaning than today .
Further historical development
The theory of preformation received strong support through microscopic examinations. Marcello Malpighi examined the development of the embryo in the hen's egg and interpreted this as the growth of already preformed structures. After the discovery of the sperm (1677), depictions such as that of Nicolas Hartsoeker emerged , who drew the entire embryo as a homunculus into a sperm head. Animalkulists such as Hartsoeker, Gottfried Wilhelm Leibniz and Antoni van Leeuwenhoek postulated that all future generations are preformed in the sperm and that the female organism only provides the basis of nutrition. Conversely, ovists such as Malpighi, Jan Swammerdam and Antonio Vallisneri (1661–1730) believed that the future organism was modeled in the egg, and they attributed only a stimulating function to the sperm.
There was only a few contradictions to the doctrine of preformation in the 17th century; to mention are William Harvey and René Descartes . Harvey had studied the development of the chicken embryo even before Malpighi and described it as the gradual differentiation of organs from homogeneous matter ( epigenesis ). However, his presentation was discredited by the fact that he postulated a metaphysical form-forming principle in the Aristotelian tradition. It was similar with Descartes, who spoke of “animal spirits” ( esprits animaux ) in this context . The discussion was almost exclusively about whether future generations were formed in the egg or in the sperm.
In the 18th century, too, there was almost no alternative to preformation theory. In the meantime the belief in spontaneous generation had been overcome, and it was considered impossible that complex living beings could emerge naturally from homogeneous matter. Furthermore, the creation doctrine of the Old Testament was the basis of such considerations. Therefore, it seemed inevitable to move the creation of all (also future begotten) organisms at the beginning of the world, and in 1710 coined Nicolas Andry the concept of nesting (fr. Emboitement ). To support this view, much empirical "evidence" has been added, such as the report by Charles Bonnet (1740) that female aphids can reproduce for several generations without males ( parthenogenesis ), or the description of the spherical alga Volvox , in which several generations can be seen nested. Bonnet's investigation also decided the dispute between the Ovists and Animalkulists in favor of the former.
Objections to the theory of preformations, such as those put forward in the 18th century by Étienne Geoffroy Saint-Hilaire , Pierre Louis Moreau de Maupertuis , René-Antoine Ferchault de Réaumur , John Turberville Needham and Georges-Louis Leclerc de Buffon , were mostly purely speculative and were rejected as unscientific and factually incorrect. Réaumur, who studied the development of insects and noticed differences in the construction of caterpillars and pupae, came to the conclusion: “If you try to get a clear idea of the first formation of some organic bodies, you soon feel that our ability to judge and the amount of knowledge available to us never leads us to this realization; we have to start with the development and growth of the already trained beings without daring to go any further. ” One exception was Caspar Friedrich Wolff , who showed Harvey in more detail than before that the chicken embryo develops from undifferentiated tissue and is by no means pre-formed . Wolff's criticism was also rejected by the leading representatives of the theory of preformations.
The theory of preformations was not overcome until the early 19th century, mainly due to further embryological work by Christian Heinrich Pander (1817) and Karl Ernst von Baer (1828), in which they recognized the importance of the germ layers.
- Änne Bäumer : Preformation / Epigenesis. In: Änne Bäumer (Ed.): History of Biology. Volume 3, Frankfurt am Main et al. 1996, pp. 49-72.
- Abba E. Gaissinovitch: Observations and hypotheses about procreation and germ development. In: Ilse Jahn (ed.): History of Biology , 3rd edition, Nikol, Hamburg 1998, chap. 6.4, pp. 259-270. (Adopted from the earlier editions with changed headings.)
- Jane Maienschein: Epigenesis and Preformationism . Stanford Encyclopedia of Philosophy 2008.
- François Jacob : The logic of the living. From spontaneous generation to the genetic code. Fischer, Frankfurt am Main 1972, ISBN 3-10-035601-2 , p. 27 f.
- François Jacob: The logic of the living. From spontaneous generation to the genetic code. Fischer, Frankfurt am Main 1972, ISBN 3-10-035601-2 , p. 32 f.
- Ilse Jahn , Rolf Löther, Konrad Senglaub (eds.): History of Biology. Theories, methods, institutions, short biographies. 2nd, revised edition. VEB Fischer, Jena 1985, p. 219.
- Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. Theories, methods, institutions, short biographies. 2nd, revised edition. VEB Fischer, Jena 1985, pp. 218-220.
- Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. Theories, methods, institutions, short biographies. 2nd, revised edition. VEB Fischer, Jena 1985, pp. 230-249.
- François Jacob: The logic of the living. From spontaneous generation to the genetic code. Fischer, Frankfurt am Main 1972, ISBN 3-10-035601-2 , p. 72.
- Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. Theories, methods, institutions, short biographies. 2nd, revised edition. VEB Fischer, Jena 1985, p. 245.