  Perhaps the most noteworthy of recent findings in extragalactic globular
cluster (GC) research are the multimodal GC metallicity distributions seen in
massive early-type galaxies. We explore the origin of these distinct GC
populations, the implications for galaxy formation and evolution, and identify
several new properties of GC systems. First, when we separate the metal-rich
and metal-poor subpopulations, in galaxies with bimodal GC metallicity
distributions, we find that the mean metallicity of the metal-rich GCs
correlates well with parent galaxy luminosity but the mean metallicity of the
metal-poor ones does not. This indicates that the metal-rich GCs are closely
coupled to the galaxy and share a common chemical enrichment history with the
galaxy field stars. The mean metallicity of the metal-poor population is
largely independent of the galaxy luminosity. Second, the slope of the GC
system radial surface density varies considerably in early-type galaxies.
However the galaxies with relatively populous GC systems for their luminosity
(called high specific frequency, S_N) only have shallow, extended radial
distributions. A characteristic of high S_N galaxies is that their GCs are
preferentially located in the outer galaxy regions relative to the underlying
starlight. Third, we find that the ratio of metal-rich to metal-poor GCs
correlates with S_N. In other words, high S_N galaxies have proportionately
more metal-poor GCs, per unit galaxy light, than low S_N galaxies. Fourth, we
find steeper metallicity gradients in high S_N galaxies. This is due to the
greater number of metal-poor GCs at large galactocentric radii. We critically
review current ideas for the origin of GCs in giant elliptical (gE) and cD
galaxies and conclude that the gaseous merger model of Ashman & Zepf (1992) is
