  We present a model for chemical evolution of damped Ly alpha systems
considering production of metals by SNe II and infall associated with
hierarchical structure formation. The growth of metallicity in these systems is
a reflection of the competition between astration and infall. The apparent late
turn-on of these systems is due to the late cut-off of infall. The wide range
in [Fe/H] at a given redshift is explained by the range of the times for onset
of star formation and the range of the times for infall cessation in different
systems. The observed lower bound of [Fe/H] = -3 follows from the very rapid
initial rise of [Fe/H] subsequent to onset of star formation. To reach [Fe/H] =
-3 from a metal-free initial state requires only about 30 Myr so that the
probability of observing lower [Fe/H] values is very small.
