Thomas Henry Huxley recalled that after he had read Darwin’s Origin of Species, he had exclaimed to himself: “How extremely stupid not to have thought of that!” (Huxley 1900, 1: 183). It is a famous but puzzling remark. In his contribution to Francis Darwin’s Life and Letters of Charles Darwin, Huxley rehearsed the history of his engagement with the idea of transmutation of species. He mentioned the views of Robert Grant, an advocate of Lamarck, and Robert Chambers, the anonymous author of Vestiges of the Natural History of Creation (1844), which advanced a crude idea of transmutation. He also recounted his rejection of Agassiz’s belief that species were progressively replaced by the divine hand. He neglected altogether his friend Herbert Spencer’s early Lamarckian ideas about species development, which were also part of the long history of his encounters with the theory of descent. None of these sources moved him to adopt any version of the transmutation hypothesis.

Huxley was clear about what finally led him to abandon his longstanding belief in species stability: