The role of bars
  Secular evolution and fueling is driven by bars in spiral galaxies, and the related dynamical processes are reviewed. It is shown that gravity torques dominate over viscous torques, and produce gas infall to the center. In this infall, the bar wave accepts the angular momentum, which produces its destruction. In the end, a central mass concentration is build, which also contributes to this destruction. While gas can be stalled at ILR for a while, secondary bars then take over the fueling. In a galaxy life-time, several bar episodes can successively develop. Through external gas accretion, disks are replenished by intermittence, between two bar phases. While the general trend for a galaxy is to evolve towards early-types, its morphology can oscillate, and turn back transiently to later-types. 
Francoise Combes