  Stellar chemical element ratios have well-defined systematic trends as a
function of abundance, with an excellent correlation of these trends with
stellar populations defined kinematically. This is remarkable, and has
significant implications for Galactic evolution. The source function, the
stellar Initial Mass Function, must be nearly invariant with time, place and
metallicity. Each forming star must see a well-mixed mass-averaged IMF yield,
implying low star formation rates, with most star formation in at most a few
regions of similar evolutionary history. These well-established results are
difficult to reconcile with standard hierachical formation models, which
assemble many stellar units: galaxy evolution seems to have been dominated by
gas assembly, with subsequent star formation. Recent results, and some new
ones, on the Galactic bulge, stellar halo and thick disk t hat justify this
conclusion are presented.
