In the history of biology, preformationism (or preformism) is the idea that organisms develop from miniature versions of themselves.Instead of assembly from parts, preformationists believe that the form of living things exist, in real terms, prior to their development. It suggests that all organisms were created at the same time, and that succeeding generations grow from homunculi, or animalcules, that have existed since the beginning of creation.Epigenesis, then, in this context, is the denial of preformationism: the idea that, in some sense, the form of living things comes into existence. As opposed to “”strict”” preformationism, it is the notion that “”each embryo or organism is gradually produced from an undifferentiated mass by a series of steps and stages during which new parts are added.”” (Magner 2002, p. 154) This word is still used, on the other hand, in a more modern sense, to refer to those aspects of the generation of form during ontogeny that are not strictly genetic, or, in other words,epigenetic.The historical ideas of preformationism and epigenesis, and the rivalry between them, are obviated by our contemporary understanding of the genetic code and its molecular basis together with developmental biology and epigenetics.
Pythagoras is one of the earliest thinkers credited with ideas about the origin of form in the biological production of offspring. It is said that he originated “”spermism””, the doctrine that fathers contribute the essential characteristics of their offspring while mothers contribute only a material substrate. Aristotleaccepted and elaborated this idea, and his writings are the vector that transmitted it to later Europeans. Aristotle purported to analyse ontogeny in terms of the material, formal, efficient, and teleological causes (as they are usually named by later anglophone philosophy) – a view that, though more complex than some subsequent ones, is essentially more epigenetic than preformationist. Later, European physicians such as Galen, Realdo Colombo and Girolamo Fabrici would build upon Aristotle’s theories, which were prevalent well into the 17th century.In 1651, William Harvey published On the Generation of Animals (Exercitationes de Generatione Animalium), a seminal work on embryology that contradicted many of Aristotle’s fundamental ideas on the matter. Harvey famously asserted, for example, that ex ovo omnia—all animals come from eggs. Because of this assertion in particular, Harvey is often credited with being the father of ovist preformationism. However, Harvey’s ideas about the process of development were fundamentally epigenesist. As gametes (male sperm and female ova) were too small to be seen under the best magnification at the time, Harvey’s account of fertilization was theoretical rather than descriptive. Although he once postulated a “”spiritous substance”” that exerted its effect on the female body, he later rejected it as superfluous and thus unscientific. He guessed instead that fertilization occurred through a mysterious transference by contact, or contagion. Harvey’s epigenesis, more mechanistic and less vitalist than the Aristotelian version, was, thus, more compatible with the natural philosophy of the time. Still, the idea that unorganized matter could ultimately self-organize into life challenged dominant Christian theology of the time as well as the mechanisticframework of Cartesianism. Because of technological limitations, there was no available mechanical explanation for epigenesis. The groundbreaking scientific insights provided by Galileo and Descartes seemed instead to support preformationism. It was simpler and more convenient to postulate preformed miniature organisms that expanded in accordance with mechanical laws. So convincing was this explanation that some naturalists claimed to actually see miniature preformed animals (animalcules) in eggs and miniature plants in seeds. In the case of humans, the term homunculus was used.”