Epigenesis means to grow upon, as is opposed to preformation.
One of the most important issues in the premodern biology of the 18th century was the struggle between preformationist and epigenetic theories of development. The preformationist view was that the adult organism was contained, already formed in miniature, in the sperm, and that development was the growth and solidification of this miniature being. Preformationists assumed that the germs of all living beings were preformed and had been since the Creation. Preformationism sought to maintain and secure--against the irritation posed by the complexity of organic phenomena--the claim for a thorough and rational determination of the material world.
The Deistic theory of preformism assumed that God had determined the organizational forms of all organisms at the creation of the world. After that, the mechanical laws of matter did the rest. Thus preformism could be a mechanistictheory in which the divine watchmaker conceives a plan of the object which guides its production. (This is the Aristotelian causa formalis ) According to Descartes, "if one knew in detail all the parts of the seed of a particular species of animal, for instance, Man, one could deduce from that alone for reasons entirely mathematical and certain, the whole figure and conformation of each of its parts."
For preformationists, the egg, or rather the germ, supposedly housed a homunculus, a tiny version of the adult, each part of which expanded into the corresponding part of the adult. The main theories of how germs were stored were panspermism, in which the germs were disseminated everywhere (advocated by Claude Perrault and Charles Bonnet) and the emboitement or encasement of homunculi in the sexual organs, either in the female --ovism -- (Malebranche) or in the male -- animalculism. (Leibniz) Charles Bonnet described this encapsulation as mise en abime.
According to Helmut Müller-Sievers, one practical application of the theories of ovism and animalculism was to be found in the choice of arranged marriages rather than romantic love. Whether the male or the female sex was the determinant, the character of only one partner was important in preformation.The other could be selected on the basis of external and practical considerations. Sex and love could be separated. In "epigenetic" romantic love, on the other hand, the mixing of the male and female opened a new realm of complication.
The early meaning of the term evolution was linked to preformation to describe embryological change as "nothing else but a gradual and natural Evolution and Growth of the parts." For example, The great swiss anatomist Albrecht von Haller used evolution in this sense describing an unfolding or "evolutio" of the embryo, in which no part came into being that had not essentially existed before. (see R.J. Richards, "Evolution," in Keller and Lloyd, Keywords in Evolutionary Biology.) (see also D'Arcy Thompson, On Growth and Form, unabridged ed. p.82 ) Thus nothing new emerges in the preformationist realm of forms, and nature is bereft of any productive energy.
The theory of epigenesis , on the other hand, sought to address the creative dimensions of nature in time, particulary in the development of the embryo. Adherents of the theory of epigenesis recognized that the organism was not yet formed in the fertilized egg, but that it arose as a consequence of profound changes in shape and form during the course of embryogenesis. Aristotle had been the first to ask whether all the parts of the embryo come into existence together, or if they appear in succession. According to Aristotle, organisms generate themselves successively under the guidance of a formative drive. The generation of each organism was the result of a male formal cause (conveyed by the semen) and a female material cause (the menstrual blood.) (see form/matter ) Modern science would reject all causes other that efficient cause. Yet ethical disputes concerning abortion still revolve around the issues of when the human embryo should be considered a person.
The theory of epigenesis proposed by Aristotle, Harvey, and a few Italians proposed that "the parts of animals are successively generated out of fluid according to certain laws." In the seventeenth and eighteenth centuries, adherents of epigenesis included Descartes (? see above), while preformists included Malebranche. Descartes attempted to supplant Aristotelian epigenesis with a mechanistic doctrine of generation, in which the origin and morphogenesis of living beings could be derived from the motion and divisibility of matter alone. But the obvious failure of Descartes' attempt to give a reasonable account of how physical forces could lead to the complex structures of the embryo, threatened to ruin the entire Cartesian edifice. (see J. Roger, Les Sciences de la vie dans la pensée francaise du XVIIIe siècle.)
At the close of the seventeenth century, preformism had already been undermined by developments in geological and cosmological theory, which implied that life had not always been possible; by changes in the conception of biological species--no longer defined in terms of similarity of form, but in common descent and ability to propagate (the sterile mule posed a problem for preformism); and by the limits to the divisibility of matter posited by atomism (how many generations could be encased?) Although, according to Müller-Sievers, "the obliteration of preformism by epigenesis is a purely textual event." For " no microscope or telescope will ever show epigenesis."
Still, the discovery of a large number of new facts made three theoretical issues stand out:
The question of the laws which govern regeneration,
The question of the laws regulating the embryology of the organism,
and the question of the origin of the germ.
In the mid eighteenth century, Abraham Tremblay's discovery of regeneration (called "reproduction") in the fresh-water polyp, the hydra, was advanced as a central explanatory problem of biological theory, leading to the concept of the organism as a self-reproducing entity. The hydra's plasticity and its ambiguous status between animal and vegetal raised questions as to whether the organism should be understood as a colony of living units, (the way the tree was understood) or as a global and interdependent whole. The hydra seemed to fill the gap between the vegetable and animal kingdoms and thus reinforce the claims of the "great chain of being." For theorists like Charles Bonnet, the polyp was the "zoophyte" predicated by Leibniz as the missing link that would soon be found between plants and animals.
Tremblay's illustrations of the polyp
