Well, to parse it out a little bit for you, at one point in The Descent of Man, Darwin discusses why there appears to be a sort of gap in coverage of the spectrum of all types of monkeys/apes. (Edited this sentence for clarity twice.)
The great break in the organic chain between man and his nearest allies, which cannot be bridged over by any extinct or living species, has often been advanced as a grave objection to the belief that man is descended from some lower form; but this objection will not appear of much weight to those who, from general reasons, believe in the general principle of evolution. Breaks often occur in all parts of the series, some being wide, sharp and defined, others less so in various degrees; as between the orang and its nearest allies—between the Tarsius and the other Lemuridae—between the elephant, and in a more striking manner between the Ornithorhynchus or Echidna, and all other mammals. But these breaks depend merely on the number of related forms which have become extinct. 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.
With respect to the absence of fossil remains, serving to connect man with his ape-like progenitors, no one will lay much stress on this fact who reads Sir C. Lyell’s discussion (19. ‘Elements of Geology,’ 1865, pp. 583- 585. ‘Antiquity of Man,’ 1863, p. 145.), where he shews that in all the vertebrate classes the discovery of fossil remains has been a very slow and fortuitous process. Nor should it be forgotten that those regions which are the most likely to afford remains connecting man with some extinct ape- like creature, have not as yet been searched by geologists.
Places like Answers in Genesis and Harun Yayha quote mine one part of it of it:
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.
They then read this as meaning Darwin hoped that the civilized races would exterminate the other races. Some say that it means that Darwin thought that baboons and Australians are more related, and on and on. Needless to say, I read it as Darwin getting a bit excited in the H G Wells style and imagining what might happen in the future - that man in a more civilized state (he hopes) than Caucasians might wipe out other races and the great apes, leaving a large, hard to understand gap in the relationship between remaining species. (Edited this one too for clarity.)
