  We present a series of simple, largely analytical models to compute the
effects of disruption on the mass function of star clusters. Our calculations
include evaporation by two-body relaxation and gravitational shocks and mass
loss by stellar evolution. We find that, for a wide variety of initial
conditions, the mass function develops a turnover or peak and that, after 12
Gyr, this is remarkably close to the observed peak for globular clusters, at
M_p = 2 10^5 solar masses. Below the peak, the evolution is dominated by
two-body relaxation, and the mass function always develops a tail of the form
psi(M) = const, reflecting that the masses of tidally limited clusters decrease
linearly with time just before they are destroyed. This also agrees well with
the empirical mass function of globular clusters in the Milky Way. Above the
peak, the evolution is dominated by stellar evolution at early times and
gravitational shocks at late times. These processes shift the mass function to
lower masses while nearly preserving its shape. The radial variation of the
mass function within a galaxy depends on the initial position-velocity
distribution of the clusters. We find that some radial anisotropy in the
initial velocity distribution, especially when this increases outward, is
needed to account for the observed near-uniformity of the mass functions of
globular clusters. This may be consistent with the observed near-isotropy of
the present velocity distributions because clusters on elongated orbits are
preferentially destroyed. These results are based on models with static,
spherical galactic potentials. We point out that there would be even more
radial mixing of the orbits and hence more uniformity of the mass function if
the galactic potentials were time-dependent and/or non-spherical.
