Charles Bonnet

1720-1793

Swiss Naturalist and Philosopher

Charles Bonnet, the eminent Swiss naturalist, is primarily remembered for his discovery of parthenogenesis, which is a form of reproduction without fertilization. His theories on embryological development and catastrophic evolution were also of great interest to eighteenth-century naturalists and philosophers.

Bonnet's wealthy family had immigrated to Switzerland to escape the persecution of Huguenots in their native France. Although Bonnet originally studied law and was elected to Geneva's town council, he was more interested in the natural sciences. Because of his family fortune, he was able to devote himself to the pursuit of his scientific and philosophical interests. Following the example of his teacher Réné Antoine Ferchault de Réaumur (1683-1757), Bonnet became interested in the life cycles of insects. This interest led to many significant contributions regarding insect metamorphosis and the publication of his Treatise on Entomology (1745). In addition to his studies of parthenogenesis in aphids, Bonnet discovered that caterpillars and butterflies breathe through pores, which he named stigmata. He also carried out investigations of the structures and functions of plant leaves. Unfortunately, a serious eye disease compelled him to give up research that required direct observation. Nevertheless, Bonnet made many other contributions to theoretical issues in natural science and to speculative issues in natural philosophy.

While studying the life cycle of aphids, Bonnet discovered that female aphids were able to reproduce without fertilization by the male. In his philosophical work Considerations on Organized Bodies (1762), Bonnet suggested that the eggs of each female organism contained the "germs" of all subsequent generations as an infinite series of preformed individuals.

One of the great debates in eighteenth-century natural science concerned the question of whether embryological development followed a preformationist or epigenetic model. According to preformationist theories, a miniature individual preexisted in either the egg or the sperm and began to grow as the result of the proper stimulus. Many preformationists believed that all the embryos that would eventually develop had been formed by God at the Creation. The theory of epigenesis, in contrast, was based on the belief that the embryo developed gradually from an undifferentiated mass by means of a series of steps during which new parts were added. Although Aristotle and the great seventeenth-century physiologist William Harvey had been advocates of epigenesis, many of the most distinguished eighteenth-century naturalists preferred preformationist theories. The increasing acceptance of the mechanical philosophy and microscopic observations during the late seventeenth century apparently favored preformationist theories. Preformationist theories, however, suggested that only one parent could serve as the source of the preformed individual. Some preformationists argued that God had implanted or encapsulated the "germ" of all human beings in the ovaries of Eve. Bonnet's studies of parthenogenesis supported ovist preformationism and allowed him to develop a sophisticated version of encapsulation theory. Thus, the pious Bonnet was encouraged to discover compelling evidence of harmony between natural philosophy and religious doctrine.

Bonnet found that female aphids that hatched during the summer gave birth to live offspring without fertilization. The new generation of males and females mated in the fall and the females laid eggs. By carefully segregating young females to avoid the possibility of contact with males, Bonnet was able to raise 30 generations of virgin aphids. According to Bonnet, the discovery of parthenogenesis proved that the ovaries of the first female of every species contained the miniature precursors for all future members of that species. In most species, the male seemed to provide the stimulus for growth of these preformed individuals. Once development was initiated, the unfolding and growth of the preformed individual could proceed as a purely mechanical process. Despite the difficulties posed by preformationist theory, Bonnet's work was still admired by many nineteenth-century naturalists.

Preformationism seemed to support the conclusion that God had created essentially immortal and immutable species. In response to reports that some species had become extinct in the distant past, Bonnet argued in his Philosophical Revival (1769) that periodically the Earth experienced universal catastrophes that destroyed almost all living beings. Following such catastrophes, the survivors advanced to a higher level on the evolutionary scale. Thus, combining his biological work and his philosophical speculations, Bonnet became the first natural philosopher to use the term "evolution" in a biological context. Further philosophical inquiries led to his Essay on Psychology (1754) and Analytical Essay on the Powers of the Soul (1760), which have been called pioneering works in physiological psychology.

LOIS N. MAGNER