Therefore, here, too, the struggle among themselves arises less from inner aversion than from hunger and love. In both cases, Nature looks on calmly, with satisfaction, in fact. In the struggle for daily bread all those who are weak and sickly or less determined succumb, while the struggle of the males for the female grants the right or opportunity to propagate only to the healthiest. And struggle is always a means for improving a species' health and power of resistance and, therefore, a cause of its higher development.
"At some future period, not very distant as measured by centuries, the civilized races of man will almost certainly exterminate, and replace the savage races throughout the world... "
Aaaaaand, here’s the context:
“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 Lemuridæ—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, 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.” - Darwin, Descent of Man
Which basically says as civilization continues to expand, it tends to eliminate or influence the “missing links” from which civilization came. He was using this as one explanation for the absence of living intermediate forms in his day. Life as competition, imagine that.