  The star formation history of the universe shows strong evolution with
cosmological epoch. Although we know mergers between galaxies can cause
luminous bursts of star formation, the relative importance of such mergers to
the global star formation rate (SFR) is unknown. We present a simple analytic
formula for the rate at which halos merge to form higher-mass systems, derived
from Press-Schechter theory and confirmed by numerical simulations (for high
halo masses). A comparison of the evolution in halo formation rate with the
observed evolution in the global SFR indicates that the latter is largely
driven by halo mergers at z>1. Recent numerical simulations by Kolatt et al.
(1999) and Knebe & Muller (1999) show how merging systems are strongly biased
tracers of mass fluctuations, thereby explaining the strong clustering observed
for Lyman-break galaxies without any need to assume that Lyman-break galaxies
are associated only with the most massive systems at z~3.
