Preformationism
[1] Preformationists suggested 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, which is typically defined by religious beliefs.
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).
[6] The historical ideas of preformationism and epigenesis, and the rivalry between them, are obviated by the contemporary understanding of the genetic code and its molecular basis together with developmental biology and epigenetics.
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.
[4] 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.
[8] 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.
Thereafter, Giuseppe degli Aromatari and then Marcello Malpighi and Jan Swammerdam made observations using microscopes in the late 17th century, and interpreted their findings to develop the preformationist theory.
This image, depicting what historians now refer to as the homunculus, has become iconic of the theory of preformationism, and appears in almost every textbook concerning the history of embryological science.
According to Malebranche, "an infinite series of plants and animals were contained within the seed or the egg, but only naturalists with sufficient skill and experience could detect their presence."
Leibniz supported a theory called panspermism that the wasted sperm might actually be scattered (for example, by the wind) and generate life wherever they found a suitable host.
Jean Astruc, noting that parents of both sexes seemed to influence the characteristics of their offspring, suggested that the animalcule came from the sperm and was then shaped as it passed into the egg.
It competed with spontaneous generation and epigenesis, but those two theories were often rejected on the grounds that inert matter could not produce life without God's intervention.
Some animals' regenerative capabilities challenged preformationism, and Abraham Trembley's studies of the hydra convinced various authorities to reject their former views.
(Magner 173)[4] When John Dalton's atomic theory of matter superseded Descartes' philosophy of infinite divisibility at the beginning of the 19th century, preformationism was struck a further blow.
It is explained to mean 'partium super-exorientium additamentum', 'the additament of parts budding one out of another'.theory of epigenesis: the theory that the germ is brought into existence (by successive accretions), and not merely developed, in the process of reproduction.
