  We develop a hybrid galaxy formation model which uses outputs from an N-body
simulation to follow the merger histories (or ``merger trees'') of dark matter
halos and treats baryonic processes, such as the cooling of gas within halos
and subsequent star formation, using the semi-analytic model of Cole et al. We
compare this hybrid model to an otherwise identical model which utilises merger
tree realisations generated by a Monte-Carlo algorithm and find that, apart
from the limited mass resolution imposed by the N-body particle mass, the only
significant differences between the models are due to the known discrepancy
between the distribution of halo progenitor masses predicted by extended Press
Schechter theory and that found in N-body simulations. We investigate the
effect of limited mass resolution on the hybrid model by comparing to a purely
semi-analytic model with greatly improved mass resolution. We find that the
mass resolution of the simulation we use, which has a particle mass of
1.4x10^10 Msolar/h, is insufficient to produce a reasonable luminosity function
for galaxies with magnitudes in the b_J band fainter than -17.
