Mediating Objects. Scientific and Public Functions of Models in Nineteenth-Century Biology David Ludwig Final Version forthcoming in History and Philosophy of the Life Sciences (2013) A bstract - The aimof this article is to examine the scientific and public functions of two- and three-dimensional models in the context of three episodes fromnineteenth-century biology. I argue that these models incorporate both data and theory by presenting theoretical assumptions in the light of concrete data or organizing data through theoretical assumptions. Despite their diverse roles in scientific practice, they all can be characterized as mediators between data and theory. Furthermore, I argue that these different mediating functions often reflect their different audiences that included specialized scientists, students, and the general public. In this sense, models in nineteenth-century biology can be understood as mediators between theory, data, and their diverse audiences. K eywords - Models as mediators, material models, history of Embryology, Ernst Haeckel, WilhelmHis Models are ubiquitous in scientific practice and of crucial importance in a large variety of disciplines. 1 Despite their abundance, models are not clearly defined entities and their overwhelming diversity – consider theoretical models in quantumphysics compared to anatomical teaching models – undermines the prospects of an all-encompassing definition (Leonelli 2007). One way of reacting to this situation is to distinguish between different kinds models such as theoretical, two-dimensional, or three-dimensional models. Many current debates drawthis distinction at least implicitly by focusing solely on theoretical models. Especially philo- sophers followIan Hacking in considering a model “something you hold in your head rather than in your hands” (Hacking 1983, 219). 1 It is clear that theoretical models often differ considerably fromtwo- and threedimensional models and it seems that we can avoid many definitional problems by clearly distinguishing between them. 1For discussions of theoretical models in philosophy of science, see Bailer-Jones (2009) as well as Frigg and Hartmann (2006). Discussions of material models are more common in the history of science, see Dirks and Knobloch (2008) and especially Hopwood and de Chadarevian (2004). Natascha Myers proposes an interesting account that combines theoretical and material models “in the embodied imagination of the modeler” (Myers 2007, 63). 1