Perçues jusqu’au XIXe siècle comme des masses gélatineuses aux propriétés urticantes, les méduses ont été classées dans le groupe des zoophytes, organismes intermédiaires entre les animaux et les végétaux. Les savants s’en sont alors servis pour démontrer la continuité entre les êtres vivants végétaux et animaux et elles sont devenues des arguments essentiels dans l’établissement de l’échelle des êtres. A partir du XIXe siècle, on découvre, en particulier grâce à la microscopie, la complexité de leur anatomie, ce qui permet enfin de les caractériser de manière positive. Au milieu du XIXe siècle, on met en évidence les différentes phases de leur cycle de vie et l’organisation en colonie de certaines méduses. Les méduses deviennent alors l’un des principaux centres d’intérêt des zoologistes et des embryologistes qui cherchent à trouver la signification évolutive des différentes étapes de leur développement. Le terme de méduse apparaît alors pour désigner un de leurs stades de vie. En parallèle de cette histoire conceptuelle, les différentes étapes de cette classification ont été marquées par des difficultés d’étude qui ont fait que les méduses constituaient un des rares groupes non définis par des caractères positifs à la fin du XVIIIe siècle. En effet, si les expéditions scientifiques ont permis de récupérer de nombreux échantillons, bien vite s’est posé le problème de leur conservation. Face aux difficultés de maintenir leur morphologie et leurs couleurs après leur prélèvement, les images naturalistes de méduses sont donc devenues des objets d’étude essentiels dans la connaissance et la classification de ces organismes. Ces images donnent un reflet de l’évolution conceptuelle et technique, qui a accompagnée la classification des méduses / Perceived until the 19th century as gelatinous masses with stinging properties, jellyfish were classified in the group of zoophytes, intermediate organisms between animals and plants. Scientists then used it to demonstrate continuity between plant and animal living beings and became essential arguments in establishing the scale of beings. From the nineteenth century, we discover, particularly through microscopy, the complexity of their anatomy, which finally allows to characterize them positively. In the mid-nineteenth century, we highlight the different phases of their life cycle and the colony organization of some jellyfish. Jellyfish become one of the main interests of zoologists and embryologists who seek to find the evolutionary significance of the different stages of their development. The term jellyfish then appears to designate one of their life stages. In parallel with this conceptual history, the various stages of this classification were marked by study difficulties which made jellyfish one of the few groups not defined by positive characters at the end of the eighteenth century. Indeed, if the scientific expeditions allowed to recover many samples, quickly the problem of their conservation was posed. Faced with the difficulties of maintaining their morphology and their colors after their collection, the naturalistic images of jellyfish have thus become essential objects of study in the knowledge and classification of these organisms. These images give a reflection of the conceptual and technical evolution, which accompanied the classification of jellyfish

Many histories of natural history see it as a descriptive science, as a clear forerunner to modern studies of classification, ecology and allied sciences. But this thesis argues that the story of unproblematic progression from eighteenth-century natural history to nineteenth-century and modern natural history is a myth. Eighteenth-century natural history was a distinct blend of practices and theories that no longer exists, though many individual elements of it have survived. The natural history that I discuss was not solely about collecting, displaying, naming and grouping objects. Though these activities played an important part in natural history (and in many histories of natural history) this thesis focuses on some other key elements of natural history that are too often neglected: elements such as experimenting, theorising, hypothesising, seeking causes, and explaining. Usually these activities are linked to natural philosophy rather than natural history, but I show how they were used by naturalists and, by extension, create a new way of understanding how eighteenth-century natural history, natural philosophy and other sciences were related. The first chapter is about the end of eighteenth-century natural history and looks at the role of the Linnean Society of London. It argues that this society tried to homogenise British natural history through the promotion of the Linnean sexual system of plant classification and through the suppression of the kinds of experimental and theoretical work described in this thesis. To understand that experimental and theoretical work, and to see what British natural history really entailed in this period, three central chapters focus on specific case studies. The second chapter shows how English-based naturalists such as John Ellis (1710-1776) approached the problem of distinguishing plants from animals, and especially about how they used chemical experiments to decide whether things such as coral and corallines should be placed in the animal or plant kingdom. The third chapter discusses sensitive plants and the overlaps between natural history and natural philosophy. It draws on case studies of naturalists who investigated things like plant motion and apparent plant sensitivity with different observational and experimental methods, and tried to explain them using various mechanical and vitalist explanations. The fourth chapter focuses on the controversy over whether plants (like animals) can be male or female and shows the theoretical and experimental tools that naturalists used to address this issue. Together, these chapters give a very detailed insight into the everyday practices and theories used by eighteenth-century naturalists and show the variety of activities that made up the field. The next two chapters focus on the identity and interactions of naturalists and show how they created a distinctive science: the fifth chapter is about how someone in England could go about becoming an authority on natural history in the late eighteenth century; and the final chapter looks outwards from Britain and examines how British natural history influenced, and was influenced by, European natural history; it uses correspondence to examine how British naturalists communicated with their overseas counterparts and what each party gained from those exchanges.