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 mechanistic theory 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 polypCaspar Friedrich Wolff (1733 - 1794) is usually looked upon as the father of epigenetic descriptive embryology. In his Theoria Generationis, (1759) Wolff promised to explain the emergence of the organism not as a gradual unfolding (or evolution) of preformed germs, but as an actual production of something new. In his History of Vitalism, Hans Driesch emphasized the vitalistic aspect of Wolff's theories, particularly the construction of a vital force which Wolff calls vis essentialis , an organizing, formative, and therefore ultimately spiritual force. For Wolff, this force directs the epigenesis of the embryo as well as directing the conservation of the mature body. Driesch points to the the need for such a force to buttress his contention that "All believers in epigenesis are Vitalists." Hans Driesch, History of Vitalism, p.39) (see vitalism)
The Count de Buffon proceeded from the assumption that the living body appropriates through nourishment small, irreducible particles, molécules organiques which are transformed into components of the various organs by the moule interieure. Once the growth of the body is completed, these formed but no longer needed molecules are stored in the seminal fluid. In Buffon's epigenetic theory, the generation of a new organism consists in the equal mixing of male and female seminal fluid, in which the formed molecules "recognize" one another, congregate through a force of attraction, and build the corresponding organs in the embryo.
Albrecht von Haller defended preformation in part from anti-Enlightenment and religious motives. Insisting on the one-sidedness of the progentive act, he broke with the thousand-year old medical belief that female orgasm was necessary for the generation of new life. While Haller departed from purely mechanistic models by recognizing living forces, these forces were not formative. His "irritability," (cf "excitable media in Goodwin) was a quality of muscles, and his "sensibility" a quality of nerves, both experimentally substantiated by the first large-scale experiments on living animals.
Johann Friedrich Blumenbach, a former student of Haller's established epigenesis as the undisputed model of thought in the life sciences. In 1781, he published Über den Bildungstrieb and das Zeugungsgeschäfte (The Formative Drive and its relation to the business of Procreation.) Blumenbach cut up freshwater polyps and established that they would strive to replace the removed parts by themselves, but significantly in a smaller size than before their unfortunate encounter with the natural scientist. This regeneration, as well as the healing process of human injuries demonstrated to Blumenbach that an organic force strove to develop all available organic material into its original form and functional ability. Blumenbach called this "Bildungstrieb ," or formative drive. (It is important to note the enormous connotative dimension of the word Bildung in German, which, starting with Luther's translation of the bible, was understood as the process of formation of a national language) But like Newton, Blumenbach was willing to accept the "occult quality" of the term and not to try to define it. The formative drive, he pointed out, "like names applied to every other kind of vital power, of itself, explains nothing: it serves merely to designate a peculiar power formed by the combination of the mechanical principle with that which is susceptible of modification."
While parallels are often drawn between the projects of Immanuel Kant and of Copernicus, Kant was more directly concerned with Blumenbach's accounts of epigenesis, referring to the concept directly in the Critique of Judgement (eg. section 81). Kant adopted Blumenbach's Bildungtrieb as a scientific analogue to his concept of purposive organization, or Zweckmässigkeit . According to Helmut Müller-Sievers It was the combination of law-guided development and free, spontaneous origin which attracted Kant to Blumenbach's concept. The epigenetists had attempted to solve the riddle of generation by positing the possibility of natural originality. Thus, if the conditions of the the possibility of cognition of nature would be secured, for Kant, by positing an isomorphism between the cognizing faculty and the cognized natural product, the concept of epigenisis would allow Kant to develop the system of pure reason in such a manner that it contained it own foundations -- such that the categories of reason could contain, on the side of understanding, the grounds of the possibility of all experience in general. (Helmut Müller-Sievers, Self-Generation. Biology, Philosophy, and Literature Around 1800, p.) According to Müller-Sievers, the concept of epigenesis allows philosophical and literary discourses to account for their own origin without recourse to extraneous causes. The metaphysical basis of epigenesis is the assumption that subjective forces share in the formation of living organisms. Epigenesis is thus the condition of any claim to absoluteness and is the very discourse that brings the distinction between natural and artificial relations into play in the first place.
According to Gilles Deleuze, both preformism and epigenesis conceive of the organism as a fold.
In current biological usage, epigenesis refers primarily to the origin of new structures during embryonic development.(see genotype / phenotype)Mosaic development is the name given today to the parts of the embryo which give rise to one part of an adult. To the extent that its causes are internal, it is contrasted to inductive development, whose cause is an external stimulus. Experimental embryology has found that similar results are possible from either sort of cause. Contemporary biology is marked by a tension between genetics and embryology. Geneticists, following Weismann, stress the power and agency of genes, while embryologists study the "surplus economy of the soma," the time and space in which the rest of the organism exerts its effects. (see Fox Keller, Refiguring Life)
Recently, a number of biologists have criticized the deterministic claims of genetics, which seems to claim that the "blueprint" for the organism is to be found in the genes. R. C . Lewontin has pointed out that the term development as Entwicklung is an unfolding in stages and revelation of an already immanent structure. (The corresponding Italian term is svillupo and the Spanish term desarollo. ) He compares the biological usage to the term to development of an exposed film to make a photographic image. "The image is already immanent in the exposed film, and the process of development simply makes this latent image apparent." (The Triple Helix, p. 5) Lewontin has criticized these notions of development as not open to outside contingency. (cf. closed / open systems) He points out that the development of most organisms is a consequence of a unique interaction between their internal and external milieus, and that, furthermore, organisms structure and change their environments. In fact, Lewontin goes so far as to say that in the history of biology, "It is really preformationism that has triumphed." (rather than epigenesis) "There is no essential difference, but only one of mechanical detail, between the view that the organism is already formed in the fertilized egg and the view that the complete blueprint of the organism and all the information necessary to specify it is contained there." (The Triple Helix, p.6) (see organicism for an aesthetic equivalent)
The British Biologist Conrad Hal Waddington was the leading advocate of his time of the need to integrate the findings of genetics with those of embryology and called upon cybernetics to support his acccounts. Conrad Waddington and R.B. Goldschmidt attempted to formulate an "organismic" conception of gene action . Waddington's concept of the epigenetic landscape is a mode of visualizing this process, in which development is like a ball rolling downhill. This Landscape has hills, valleys, and basins (cf basin of attraction)
Waddington coined the term homeorhesis (similar flow) to describe a system which returns to a trajectory, and the term chreod (or creode)--(necessary path) to describe the trajectory itself, a canalized pathway of change along the epigenetic landscape. "Canalization" was the term Waddington employed to refer to the process by which developmental reactions "are adjusted so as to bring about one end result regardless of minor variations in conditions during the reaction" The pathway is regulated -- the system tends to return to it after disturbance-- as a result of the metabolic complexity of " open systems of autocatalytic reactions." (For Rupert Sheldrake the chreod exists within a morphic field. )Waddington tacitly invoked the work on self-steering mechanisms out of which cybernetics had grown when he pointed out that "The path followed by a homing missle, which finds its way to a stationary target, is a creode." (Strategy of the Genes, quoted by Fox Keller) The idea of canalized pathways are also a way of visualizing history, in that a location can be described by the path that lead there. (see also bifurcation.) Researchers of complex systems imagine the surface of the epigenetic landscape deformed by movement upon it more like an elastic sheet. The landscape is modified by any development as the development depends on the landscape. They coevolve. Gerry Webster describes Waddington's epigenetic landscape as a temporalized version of William Bateson's concept of "Positions of Organic Stability," described in his Materials for the Study of Variation, (1894), itself derived from F. Galton's Natural Inheritance, of 1889. The comparison of this theoretical landscape with erosion is highly interesting. The ways in which rivers meander, for example are a clear form of self-organization in a landscape of possiblity. (see self-organized criticality)
This idea of landscape has also been used to describe problem spaces in there is no obvious way to find an optimum solution. These are fitness landscapes in which the solutions are peaks and the algorithms try to climb them. When the search for a solution is visualized as seeking the lowest point, the rolling ball, as it were, can get stuck in in small valleys and miss the more important ones. Programmers trying to devise algorithms for exploring these problem landscapes combine rules for sub-optimization (i.e. roll downhill in relation to immediate surroundings) with some "jolts" to get out of small basins.