"Tree Thinking": The Rhetoric of Tree Diagrams in Biological Thought

Abstract

Tree-like visualizations have played a central role in taxonomic and evolutionary biology for centuries, and the idea of a “tree of life” has been a pervasive notion not only in biology but also in religion, philosophy, and literature for much longer. The tree of life is a central figure in Darwin’s Origin of Species in both verbal and visual forms. As one of the most powerful and pervasive images in biological thought, what conceptual and communicative work has it enabled? How have the visual qualities and elements of the tree form interacted with biological thinking over time? This paper examines the pre-Darwinian history of tree images, the significance of Darwin’s use of such images, and the development of tree diagrams after Darwin. This history shows evidence of four separate traditions of visualization: cosmological, logicalphilosophical, genealogical, and materialist. Visual traditions serve as rhetorical contexts that provide enthymematic backing, or what Perelman calls “objects of agreement,” for interpretation of tree diagrams. They produce polysemic warrants for arguments in different fields. The combination of the genealogical tradition with the cosmological and the logical changed the framework for thinking about the natural world and made Darwin’s theory of evolution possible; the later materialist tradition represents the “modernization” of biology as a science.