Considerably more theoretical and observational work is needed to further
refine and test grand schemes of this type, including more detailed
simulations of
Kerr black hole magnetospheres and a better understanding of the
structure and strength of the magnetic field near rotating (and non-rotating)
holes. Furthermore, any complete grand scheme must explain, from first
principles, how the fundamental features of optical quasar spectra
(``nonthermal'' continuum, broad and narrow lines) are produced and how
accreting black holes should look at near-Eddington and super-Eddington
accretion rates.
On the observational side, good methods of estimating the central accretion
rate and black hole spin are needed to locate each source in
(MH, 
,
J)-space. As has been done in the past for stars
and galaxies, when relations between fundamental parameters and observable
properties are understood, an overall picture of how seemingly disparate
objects are related begins to emerge.
Acknowledgements
The author is grateful to S. Koide for discussions and for permission to use Figure 2. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA.