
Darwin's first published expression of the concept of natural selection.
"On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection" Journal of the Proceedings of the Linnean Society of London: Zoology (read 1 July 1853; published 20 August 1858) volume 3, pages 45-62, at page 51 http://darwin-online.org.uk/content/frameset?pageseq=7&itemID=F350&viewtype=image
Other letters, notebooks, journal articles, recollected statements