  It is found that a previously thought-to-be well established result of
density wave theory, that there is no interaction between a quasi-stationary
spiral density wave and the basic state (i.e. the axisymmetric part) of the
galactic disk, is in fact false. When solved as a nonlinear and globally
self-consistent problem, the presence of non-axisymmetric patterns such as
spirals or bars is shown to lead to significant interaction of the basic state
and the density wave, with the net result being the simultaneous acquirement of
a quasi-steady wave amplitude and the secular redistribution of disk matter.
The secular operation of this dynamical mechanism leads to the evolution of the
Hubble type of a galaxy from late to early.
