Unveiling Darwin's Theory of Evolution Through the Epistemological Study of His Diagram

The expression of scientific knowledge relies on a set of non-linguistic representations: ``pictorial representations'', such as encyclopaedic illustrations (photographs, paintings, figurative drawings, etc.), ``schematic representations'', such as textbook illustrations (for instance the drawing of a cell in a biology textbook) and finally ``diagrammatic representations'' [1, Chap. 7], such as those found in subjects like physics which are used to simplify calculatory expressions (Feynman diagrams, Penrose diagrams, etc.). Diagrammatic representations are of particular interest because they play a crucial role, which is not only pedagogical and heuristic, but also epistemic. This paper will endeavour to contribute to the study of the epistemic role of diagrammatic representations through a notable and yet little-noticed case-study: Darwin's diagram. Introduced in the middle of the fourth chapter of his On the Origin of Species (1859), this diagram could at first be mistaken for a family tree. Through an analysis of all the elements of the diagram (continuous lines, dotted lines, letters, Roman numerals, etc.) and Darwin's own comments, this paper will show how the explanatory hypotheses of a major scientific theory can be best understood thanks to this tool of epistemic representation.