  We study the effect of a massive central singularity on the structure of a
triaxial galaxy using N-body simulations. Starting from a single initial model,
we grow black holes with various final masses Mh and at various rates, ranging
from impulsive to adiabatic. In all cases, the galaxy achieves a final shape
that is nearly spherical at the center and close to axisymmetric throughout.
However, the rate of change of the galaxy's shape depends strongly on the ratio
Mh/Mg of black hole mass to galaxy mass. When Mh/Mg < 0.3%, the galaxy evolves
in shape on a timescale that exceeds 100 orbital periods, or roughly a galaxy
lifetime. When Mh/Mg > 2%, the galaxy becomes axisymmetric in little more than
a crossing time. We propose that the rapid evolution toward axisymmetric shapes
that occurs when Mh/Mg > 2% provides a negative feedback mechanism which limits
the mass of central black holes by cutting off their supply of fuel.
