Book excerpt: Two striking discoveries made 1740 a turning point in the history of 18th-century biology. Charles Bonnet established that aphids could reproduce without male fertilization. Shortly afterwards Abraham Trembley proved that a tiny aquatic animal, the fresh water polyp, or hydra, could regenerate from cuttings like some plants. The discovery of the polyp was important because of the disturbing metaphysical issues that it raised. In their letters written during the decade of the 1740s to Reaumur, the great French Academician, both Trembley & Bonnet referred to the polyp as an enigma. Not only did it seem to present a new mode of animal reproduction, previously unsuspected, but it called into question the prevailing mechanistic view of animal biology & brought into focus the problem of animal soul. Drawing on some of the most illuminating letters from the private archives of the Trembley family, this study focuses on the discovery of the polyp, using the correspondence of Bonnet & Trembley to understand their common Genevan background & their possible differences in approach from that of Reaumur.