The local chemical inhomogneity of the interstellar medium at a given time is an important factor in models of galactic chemical evolution. It can affect both the G-dwarf metallicity problem (Tinsley 1975, Talbot and Arnett 1973) and the correlation of the abundances of different elements (Tinsley 1976). Observational evidence of abundance gradients in our own Galaxy, and someothergalaxies (summarised by Peimbert 1975, van den Bergh 1975), implies that in homogeneities over a large length scale must be created and survive during galactic evolution. Brief consideration of the mixing of the Galactic disk (e.g. Edmunds 1975, 1976) suggests
THE UNIVERSAL RELATION OF GALACTIC CHEMICAL EVOLUTION: THE ORIGIN OF THE MASS-METALLICITY RELATION
