We consider the roles of two widely invoked mechanisms for the metallicity-luminosity correlation among late-type galaxies : higher astration level and decreasing efficiency of heavy-element loss with increasing luminosity . We find that both mechanisms contribute about equally to the range in oxygen abundance between low ( logL _ { B } \sim 8 ) and high ( logL _ { B } \sim 10.5 ) luminosity galaxies . We also consider the transition from spirals to irregulars , finding that both the oxygen abundance deficiency ( indicating the degree of mass exchange between a galaxy and its environment ) and the gas fraction vary smoothly along the sequence . Thus we find no ” irregular versus spiral dichotomy ” .