  In search for a synthetic understanding, a scenario for the evolution of the
star formation rate and the chemical abundances in galaxies is proposed,
combining gas infall from galactic halos, outflow of gas by supernova
explosions, and an oscillatory star formation process. The oscillatory star
formation model is a consequence of the modelling of the fractional masses
changes of the hot, warm and cold components of the interstellar medium. The
observed periods of oscillation vary in the range $(0.1-3.0)\times10^{7}$\,yr
depending on various parameters existing from giant to dwarf galaxies. The
evolution of metallicity varies in giant and dwarf galaxies and depends on the
outflow process. Observed abundances in dwarf galaxies can be reproduced under
fast outflow together with slow evaporation of cold gases into hot gas whereas
slow outflow and fast evaporation is preferred for giant galaxies. The
variation of metallicities in dwarf galaxies supports the fact that low rate of
SNII production in dwarf galaxies is responsible for variation in metallicity
in dwarf galaxies of similar masses as suggested by various authors.
