  It is suggested that there have been at least two physically distinct epochs
of massive cluster formation. The first generation of globular clusters may
have formed as halo gas was compressed by shocks driven inward by ionization
fronts generated during cosmic reionization at z ~ 6. On the other hand a
second generation of massive clusters might have been formed at later times by
compression, and subsequent collapse, of giant molecular clouds. In some cases
this compression may have been triggered by heating of the interstellar medium
via collisions between gas-rich disk galaxies. It is also suggested that the
present specific globular cluster frequency in galaxies was mainly determined
by the peak rate of star creation, with elevated peak rates of star formation
per unit area resulting in high present specific globular cluster frequencies.
