  In recent years, a growing zoo of compact stellar systems (CSSs) have been
found whose physical properties (mass, size, velocity dispersion) place them
between classical globular clusters (GCs) and true galaxies, leading to debates
about their nature. Here we present results using a so far underutilised
discriminant, their stellar population properties. Based on new spectroscopy
from 8-10m telescopes, we derive ages, metallicities, and [\alpha/Fe] of 29
CSSs. These range from GCs with sizes of merely a few parsec to compact
ellipticals larger than M32. Together with a literature compilation, this
provides a panoramic view of the stellar population characteristics of
early-type systems. We find that the CSSs are predominantly more metal rich
than typical galaxies at the same stellar mass. At high mass, the compact
ellipticals (cEs) depart from the mass-metallicity relation of massive
early-type galaxies, which forms a continuous sequence with dwarf galaxies. At
lower mass, the metallicity distribution of ultra-compact dwarfs (UCDs) changes
at a few times $10^7$ M$_{\odot}$, which roughly coincides with the mass where
luminosity function arguments previously suggested the GC population ends. The
highest metallicities in CSSs are paralleled only by those of dwarf galaxy
nuclei and the central parts of massive early types. These findings can be
interpreted as CSSs previously being more massive and undergoing tidal
interactions to obtain their current mass and compact size. Such an
interpretation is supported by CSSs with direct evidence for tidal stripping,
and by an examination of the CSS internal escape velocities.
