The Descent of Man

a reflex action, yet it is not improbable that there
is a certain amount of interference between
the development of free intelligence and of instinct,—which
latter implies some inherited
modification of the brain. Little is known about
the functions of the brain, but we can perceive
that as the intellectual powers become highly
developed, the various parts of the brain must
be connected by very intricate channels of the
freest intercommunication; and as a consequence
each separate part would perhaps tend
to be less well fitted to answer to particular
sensations or associations in a definite and inherited—that
is instinctive—manner. There
seems even to exist some relation between a
low degree of intelligence and a strong tendency
to the formation of fixed, though not
inherited habits; for as a sagacious physician
remarked to me, persons who are slightly imbecile
tend to act in everything by routine or
habit; and they are rendered much happier if
this is encouraged.