  The paper deals with the conditions required to form at least two stellar
generations in globular clusters under the constraints generated by feedback
from massive stars as well as radiative cooling and the metallicity of the
primordial clouds. Our calculations are based on two main constraints to the
star formation efficiency of the first stellar generation (1G) $\epsilon_{1G}$.
First, $\epsilon_{1G}$ is restricted to warrant that stellar winds and
supernovae do not disrupt the leftover gas out of which a second generation
(2G) would form. Second, $\epsilon_{1G}$ is also limited such that the
metallicity enhancement caused by trapped supernovae is, in agreement with the
observations, not larger than $\sim$ 0.1 dex. Several central parameters define
the globular clusters end result: the mass and radius of the primordial clouds,
their metallicity and $\epsilon_{1G}$. The parameter space composed by models
which fulfilled all constraints, is here shown to coincide remarkably well with
the scattered observed anti-correlation between the fraction of first
generation stars ($f_{\textrm{1G}}$) and total cluster mass. Our models also
discern, in agreement with the data, between single and multiple population
clusters in a metallicity versus mass (or radius) plane. Hence, our results
suggest that the presence of multiple stellar populations is closely linked to
the ability of proto-globular clusters to retain a fraction of leftover gas.
