  We compare observed clustering of quasars and galaxies as a function of
redshift, mass, luminosity, & color/morphology, to constrain models of quasar
fueling and spheroid-BH co-evolution. High redshift quasars are shown to be
drawn from progenitors of local early-type galaxies, with the characteristic
quasar luminosity L* reflecting a characteristic mass of 'active' BH/host
populations at each redshift. Evolving observed high-z quasar clustering to z=0
predicts a trend of clustering in 'quasar remnants' as a function of stellar
mass identical to that observed for early-types. However, quasar clustering
does not simply reflect observed early (or late)-type populations; at each
redshift, quasars cluster as an 'intermediate' population. Comparing with the
age of elliptical stellar populations reveals that this 'intermediate'
population represents those ellipticals undergoing or terminating their final
significant star formation at each epoch. Assuming that quasar triggering is
associated with the formation/termination epoch of ellipticals predicts quasar
clustering at all observed redshifts without any model dependence or
assumptions about quasar light curves, lifetimes, or accretion rates. This is
not true for disks or quasar halos: i.e. quasars do not generically trace star
formation, disks, or halo assembly. Quasar clustering at all z is consistent
with a constant halo mass ~4x10^{12} M_sun, similar to local 'group scales.'
The observations support a scenario in which major mergers trigger quasar
activity and dominate bright, high-z quasar populations. We show that
measurements of quasar clustering versus luminosity at z~1 can be used to
constrain different lower-luminosity AGN fueling mechanisms, and that high-z
clustering is sensitive to whether or not systems 'shut down' growth at z>3.
