"A major driving force behind Darwin's research was his own disdain for slavery, prejudice and human suffering.""Disdain", really? I guess that comes from things Darwin said about slavery. But it doesn't follow from his books...
The first edition is available from Project Gutenberg. The original title is more informative: "THE PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE".
In "Descent of Man", he says:
"At some future period, not very distant as measured by centuries, the civilised races of man will almost certainly exterminate, and replace, the savage races throughout the world. At the same time the anthropomorphous apes, as Professor Schaaffhausen has remarked (18. 'Anthropological Review,' April 1867, p. 236.), will no doubt be exterminated. The break between man and his nearest allies will then be wider, for it will intervene between man in a more civilised state, as we may hope, even than the Caucasian, and some ape as low as a baboon, instead of as now between the negro or Australian and the gorilla."