  The secular evolution process, which slowly transforms the morphology of a
galaxy over its lifetime, could naturally account for observed properties of
the great majority of physical galaxies if both stellar and gaseous accretion
processes are taken into account. As an emerging paradigm for galaxy evolution,
its dynamical foundation had been established in the past few years, and its
observational consequences are yet to be fully explored. The secular evolution
picture provides a coherent framework for understanding the extraordinary
regularity and the systematic variation of galaxy properties along the Hubble
sequence.
