  We show that two phenomena of superfluidity, superfluidity of weakly
interacting bosons and superconductivity of the BCS model, are unified using
the collective mode arising from the Berry connection for many-body wave
functions. The superfluidity is attributed to the presence of this mode, which
is stabilized by the interaction between particles that causes fluctuations of
the number of particles participating in it. It is suggested that the existence
of this collective mode and its stabilization is more fundamental to the
occurrence of superconductivity than the electron-pair formation.
