The tree and the table: Darwin, Mendeleev and the meaning of 'theory'.

Darwin and Mendeleev revolutionized their respective disciplines by organizing diverse facts into simple, pictorial representations-a tree and a table. Each representation provides a foundation for a scientific theory for two reasons. First, a successful theory unites diverse phenomena under a single explanatory framework. Second, it does so in a way that defines paths for future inquiry that extends its reach and tests its limits. For Mendeleev, this meant creating a table that accommodated the current understanding of the elements but also contained blanks that predicted the discovery of previously unknown elements. More importantly, the structure of the table helped shape future research to define the structure of matter. For Darwin, envisioning life as a tree meant defining the rules that govern the origin of adaptations, species and shape the constantly shifting diversity of life. At the same time, his theory inspired research into the laws of inheritance and created diverse new areas of research, like behaviour, sexual selection and biogeography. The shared property of Darwin and Mendeleev's contributions was to provide a unifying rational explanation for natural phenomena. This article is part of the theme issue 'Mendeleev and the periodic table'.