  Recent progress in the understanding of the role of bars and gravitational
instabilities in galaxy disks is reviewed. It has been proposed that bars can
produce mass transfer towards the center, and progressively metamorphose
late-type galaxies in early-types, along the Hubble sequence. Through this mass
transfer, bars are self-destroyed, and can act only during a certain
"duty-cycle" in the galaxy life. After sufficient gas infall, another bar-phase
can spontaneously occur. This recurrent evolution is strongly dependent on
environment.
  A scenario is proposed, based on N-body simulations time-scales of the
bar-life events, to explain the observed bar frequency, gas mass fraction,
bulge and possible black hole mass growth, in a typical spiral.
