The Descent of Man

sexes, or a greater effect on one sex than on the
other, owing to some constitutional difference.
Individual differences between the members of
the same species are admitted by every one to
occur under a state of nature. Sudden and
strongly marked variations are rare; it is also
doubtful whether if beneficial they would often
be preserved through selection and transmitted
to succeeding generations. (35. 'Origin of Species'
fifth edit. 1869, p.104. I had always perceived,
that rare and strongly-marked deviations
of structure, deserving to be called monstrosities,
could seldom be preserved through natural
selection, and that the preservation of even
highly-beneficial variations would depend to a
certain extent on chance. I had also fully appreciated
the importance of mere individual differences,
and this led me to insist so strongly on
the importance of that unconscious form of
selection by man, which follows from the preservation
of the most valued individuals of