We develop the idea (Harris & Pudritz 1994, ApJ, 429, 177) that, like currently forming star clusters and associations, globular clusters (mean mass ≃3×105 M ⊙) were born in the ‘cores’ of much larger (∼108 – 109 M ⊙) star-forming complexes which we call ‘supergiant molecular clouds,’ or SGMCs. The number N(m) of protoclusters at mass m is then determined by a steady-state balance between their growth by core-core collisions, and their self-destruction via the side effects of star formation. This mass spectrum is ultimately passed on to the globular cluster system (GCS) itself, by virtue of the very high star-formation efficiency required to produce a bound stellar cluster from a gaseous core.