  The consequences are explored of an observationally established relation of
the star formation rate (SFR) of star-forming galaxies with their stellar mass
(M) and cosmic time (t), such that SFR is proportional to M x t^{-2.5}. It is
shown, that small systematic differences in SFR dramatically amplify in the
course of time: galaxies with above average SFR run into quasi-exponential mass
and SFR growth, while galaxies with below average SFR avoid such exponential
growth and evolve with moderate mass increase. It is argued that galaxies
following the first path would enormously overgrow if keeping to form stars all
the way to the present, hence should quench star formation and turn passive. By
the same token, those instead avoiding the quasi-exponential growth may keep to
form stars up to the present. Thus, it is conjectured that this divergent
behaviour can help understanding the origin of the dichotomy between passive,
spheroidal galaxies, and star-forming, disk galaxies.
