
volume I, chapter VI: "On the Affinities and Genealogy of Man", pages 200-201 http://darwin-online.org.uk/content/frameset?pageseq=213&itemID=F937.1&viewtype=image
The sentence "At some future period … the savage races" is often quoted out of context to suggest that Darwin desired this outcome, whereas in fact Darwin simply held that it would occur.
The Descent of Man (1871)