Darwin chose an apt architectural image when he wrote J. D. Hooker that 'the “principle of Divergence” …with “Natural Selection” is the key-stone of my Book' (Correspondence 7: 102). In the Origin, the fifteen-page section on divergence is placed strategically at the end of Chapter 4 on natural selection, where it distributes the weight between the core theory and the evidence for descent. Darwin portrays adaptation and the origin of species as emerging out of the entangled plenitude of mutual relations mediated by natural selection. The principle of divergence united this ecological vision with Darwin’s complementary view that evolutionary history can be read in the irregular branching of the taxonomic tree of life. However, there is an irony in the historical fate of the principle. Much of twentieth-century evolutionary biology rejected Darwin’s explanation of 'speciation' as muddled (Mayr 1942, 1992; Sulloway 1979; Coyne and Orr 2004). Yet the profound depth of ecological relationships and the very diversity of life that Darwin evoked through the principle can be understood as one of the Origin’s most enduring contributions. Moreover, the standing of contemporary approaches to speciation that, like Darwin's, emphasize ecological factors - but now often supplemented by moderate isolation of various kinds - while remaining controversial, is perhaps higher than it has ever been.