
volume II, chapter II: "The Growth of the 'Origin of Species' — 1843-1856", page 23 http://darwin-online.org.uk/content/frameset?pageseq=39&itemID=F1452.2&viewtype=image; letter http://www.darwinproject.ac.uk/letter/entry-729 to J.D. Hooker (11 January 1844)
The Life and Letters of Charles Darwin (1887)