  We use N-body simulations to model the 12 Gyr evolution of a suite of star
clusters with identical initial stellar mass functions over a range of initial
cluster masses, sizes, and orbits. Our models reproduce the distribution of
present-day global stellar mass functions that is observed in the Milky Way
globular cluster population. We find that the slope of a star cluster's stellar
mass function is strongly correlated with the fraction of mass that the cluster
has lost, independent of the cluster's initial mass, and nearly independent of
its orbit and initial size. Thus, the mass function - initial mass relation can
be used to determine a Galactic cluster's initial total stellar mass, if the
initial stellar mass function is known. We apply the mass function - initial
mass relation presented here to determine the initial stellar masses of 33
Galactic globular clusters, assuming an universal Kroupa initial mass function.
Our study suggests that globular clusters had initial masses that were on
average a factor of 4.5 times larger than their present day mass, with three
clusters showing evidence for being 10 times more massive at birth.
