
Letter to J.D. Hooker, 29 March 1863
In The Correspondence of Charles Darwin, volume 11, 1863; Frederick Burkhardt, Duncan Porter, Sheila Ann Dean, Jonathan R. Topham, Sarah Wilmot, editors; Cambridge University Press, September 1999, page 278
Sometimes paraphrased as “One might as well speculate about the origin of matter.”
Other letters, notebooks, journal articles, recollected statements