  We demonstrate a significant (> 5-sigma) correlation between the mean color
of metal-poor globular cluster (GC) systems and parent galaxy luminosity. A
Bayesian Markov Chain Monte Carlo method is introduced to find the mean color,
and is easily generalizable to quantify multimodality in other astronomical
datasets. We derive a GC color--galaxy luminosity relation of the form Z ~ L^
(0.15 +/- 0.03). When combined with evidence against a single primordial GC
metallicity--galaxy luminosity relation for protogalactic fragments, the
existence of such a correlation is evidence against both accretion and major
merger scenarios as an explanation of the entire metal-poor GC systems of
luminous galaxies. However, our relation arises naturally in an in situ picture
of GC formation, and is consistent with the truncation of metal-poor GC
formation by reionization. A further implication is that the ages of metal-poor
GCs in dwarf galaxies constrain the main epoch of galaxy formation in
hierarchical models. If the ages of old metal-poor GCs in Local Group dwarfs (>
11 Gyr) are typical of those in dwarfs elsewhere, then the bulk of galaxy
assembly (at least in clusters and groups) must have occurred at z > 2.5,
contrary to the predictions of some structure formation models.
