Bonnet’s first published work appeared in 1745, entitled Traité d’insectologie, in which were collected his various discoveries regarding insects, along with a preface on the development of germs and the scale of organized beings. Botany, particularly the leaves of plants, next attracted his attention; and after several years of diligent study, rendered irksome by the increasing weakness of his eyesight, he published in 1754 one of the most original and interesting of his works, Recherches sur l’usage des feuilles dans les plantes; in which among other things he advances many considerations tending to show (as was later done by Francis Darwin) that plants are endowed with powers of sensation and discernment. Bonnet also studied photosynthesis in plants and noted the emission of bubbles by a submerged illuminated leaf (but see Jan Ingenhousz, who is also given credit for this observation). This very visible production of oxygen by an illuminated leaf is still used regularly in school laboratories as a way of investigating rates of photosynthesis.
Affected by his observation of the aphid, Bonnet argued, in Considérations sur les corps organisés (1762; “Considerations on Organized Bodies”), that each female organism contains within its germ cells (i.e., eggs) an infinite series of preformed individuals (Theory of Preformation), leading to an immortality and immutability of species. In his Contemplation de la nature (Amsterdam, 1764–1765; translated into Italian, German, English and Dutch), one of his most popular and delightful works, he sets forth, in eloquent language, the theory that all the beings in nature form a gradual scale rising from lowest to highest (scala natura), without any break in its continuity.
In order to explain the fossil findings of extinct species, Bonnet, in his work La Palingénésie philosophique (1769; “The Philosophical Revival”), advocated the view that Earth is periodically struck by global disasters. In these disasters most organisms die and the survivors climb the scala natura to reach new heights. According to this, mankind, the peak of evolution, would develop into angels after the next disaster, when plants would become animals, animals would become intelligent beings, and minerals would become plants. This disaster theory to explain evolution strongly influenced Erasmus Darwin (1731-1802), Charles Darwin’s (1809-1882) grandfather. This makes Charles Bonnet one of the first biologists to use the term evolution in a biological context. However, he was stuck in the preformation theory, which implied the immutability of species and precluded biological evolution.