From Preformation to Epigenesis: Late-Eighteenth-Century Biology

BETWEEN THE SEVENTEENTH CENTURY and the second half of the eighteenth, opinions about the manner in which life was generated, formed, and nourished underwent several shifts, both in Europe more widely and in Germany in particular. These have been well documented by historians of science, and it is not my object to reconstruct the minute specifics of each eighteenth-century scientist's argument, far less to evaluate them in terms of their correctness relative to modern models of evolution and genetics. Rather, I trace the general outlines of the controversy and highlight particular aspects—both in the content of the arguments and the manner in which they are made—that show how emphasis shifted in this period from a static to a dynamic model of development. Although I will focus on texts by authors who were proponents of epigenesis, I also give a brief characterization of some preformationist arguments to provide necessary context for the later epigenetic writings.

Broadly speaking, then, these changes in scientific theory can be viewed in the debate about whether generation is best explained by preformation (frequently called “evolution,” a term that has very different connotations for modern readers) or by epigenesis. On the preformation model, all life was formed by God at the creation of the world and lies dormant, waiting to be activated at the proper moment. Either the male sperm or the female egg (both theories had their advocates in this period, though ovism was somewhat more common) contained the offspring in a complete but tiny and partially transparent state, and sexual reproduction served merely to activate its process of growth, which involved only getting larger and becoming more opaque. According to the epigenetic model, however, the germs of offspring are unformed, undifferentiated, literally unorganized—that is, not possessing organs—and their development involves not just an increase in their size but a formation of their parts as well. Each of these theories had its particular challenges (if a child existed preformed inside its mother's ovaries, why did children often resemble a mixture of both their parents? or if offspring were initially unformed, how could the growing body possibly “know” whether a given area was to become a nose or a knee?) and each had wide-ranging implications for religious beliefs and cultural practices.