Theories of Generation

Dissertation sur la génération, les animalcules spermatiques, et ceux d'infusions : avec des observations microscopiques sur le sperme et sur différentes infusions / par le Baron de Gleichen ; ouvrage traduit de l'allemand, orné de trente-quatre planches, dont plusieurs enluminées.

Wilhelm Friedrich von Gleichen-Russwurm (1717-1783). Dissertation sur la generation, les animalcules spermatiques, et ceux d'infusions. Paris: De l'imprimerie de Digeon, An VII [1799] Rare Folio QL 966 .G5514

Wilhelm Friedrich von Gleichen-Russwurm was a biologist who worked primarily on plants, but in this publication, he compares the sperm cells of various animals.

“We know that a fertilized egg contains some preformed elements—namely, the genes and a certain number of different regions of cytoplasm—and we know that during development these interact in epigenetic processes to produce final adult characters and features that are not individually represented in the egg. "
Conrad Waddington

Today, the fertilization process is common knowledge: sperm and egg cells carrying genes from each parent combine to create their offspring.  However, before scientists gained an understanding of cells, genes, and DNA, people had no mechanism for understanding conception.  Aristotle, considered a medical authority for thousands of years, thought that a man's contribution to an embryo was its form or spiritual essence, while the woman only contributed the raw material, its flesh.  Theories derived from Aristotle were known as epigenesis because they involved the shaping of raw material, part by part, into a human being.

By the seventeenth century, scientists were beginning to question these ancient ideas.  Using microscopes, some scientists noted that structures present in adult organisms were also present in their embryos.  They hypothesized that embryos were pre-formed, miniature versions of their adult selves with pre-determined anatomy and behavior.  Some scientists held that the pre-formed embryo was contained in the mother’s egg; others that it was in the father’s sperm.  This family of hypotheses came to be known as preformationism.

Preformationism directly influenced ideas about heredity and human nature. Several scientists suggested that all humans had been contained, unchanged and pre-formed, in the eggs of Eve.  This new science was even used to bolster religious doctrine.  Jan Swammerdam, an entomologist and one of the founders of preformationism, wrote, “Thus original sin is explained, for all men were contained in the organs of Adam and Eve.” For preformationists, everything about an individual was pre-determined, not just before birth, but at the beginning of time. 

Preformationism remained the dominant theory of generation for most of the eighteenth century, but gradually, scientific thought shifted and new discoveries were made, including cell theory and modern genetics.  Conrad Waddington, the biologist who coined the term epigenetics from the root word epigenesis, noted that both epigenesis and preformation are involved in embryonic development. Modern epigenetics thus builds on these two strains of thought to consider the interaction between that which is fixed and that which is fluid in our everyday lives.

Preformation and Ovism

The works on display in this section are by early scientists who worked on demystifying the processes of generation.