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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,* 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.

http://www.infidels.org/library/historical/charles_darwin/descent_of_man/chapter_06.html