Part of the only figure in the Origin of Species. Darwin first uses it to represent the divergence of variants within a species, showing successively more difference in a single lineage (a1 through a10) and splitting into multiple lineages (m, s, i, and so forth), some of which will become new species. Later, he expands the tree metaphor, explaining that "limbs divided into great branches ... were themselves once, when the tree was small, budding twigs; and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.
Source: Doolittle, W.F. Phylogenetic classification and the universal tree. Science 284, 2124-2128 (June 25, 1999).