The central problem of biological evolution is the nature of mutation, but hitherto the occurrence of this has been wholly refractory and impossible to influence by artificial means, although a control of it might obviously place the process of evolution in our hands.

'The Recent Findings in Heredity' (unpublished lecture, 1916, Lilly Library), 3. Quoted in Elof Axel Carlson, Genes, Radiation, and Society: The Life and Work of H. J. Muller (1981)