  New observations in favour of a significant role of secular evolution are
reviewed: central star formation boosted in pseudo-bulge barred galaxies,
relations between bulge and disk, evidence for rejuvenated bulges. Numerical
simulations have shown that secular evolution can occur through a cycle of bar
formation and destruction, in which the gas plays a major role. Since bars are
weakened or destroyed in gaseous disks, the high frequency of bars observed
today requires external cold gas accretion, to replenish the disk and allow a
new bar formation. The rate of gas accretion from external filaments is
compatible with what is observed in cosmological simulations.
