  We use hydrodynamic simulations to examine how the baryonic components of
galaxies are assembled, focusing on the relative importance of mergers and
smooth accretion in the formation of ~L_* systems. In our primary simulation,
which models a (50\hmpc)^3 comoving volume of a Lambda-dominated cold dark
matter universe, the space density of objects at our (64-particle) baryon mass
resolution threshold, M_c=5.4e10 M_sun, corresponds to that of observed
galaxies with L~L_*/4. Galaxies above this threshold gain most of their mass by
accretion rather than by mergers. At the redshift of peak mass growth, z~2,
accretion dominates over merging by about 4:1. The mean accretion rate per
galaxy declines from ~40 M_sun/yr at z=2 to ~10 M_sun/yr at z=0, while the
merging rate peaks later (z~1) and declines more slowly, so by z=0 the ratio is
about 2:1. We cannot distinguish truly smooth accretion from merging with
objects below our mass resolution threshold, but extrapolating our measured
mass spectrum of merging objects, dP/dM ~ M^a with a ~ -1, implies that
sub-resolution mergers would add relatively little mass. The global star
formation history in these simulations tracks the mass accretion rate rather
than the merger rate. At low redshift, destruction of galaxies by mergers is
approximately balanced by the growth of new systems, so the comoving space
density of resolved galaxies stays nearly constant despite significant mass
evolution at the galaxy-by-galaxy level. The predicted merger rate at z<~1
agrees with recent estimates from close pairs in the CFRS and CNOC2 redshift
surveys.
