  It has been suggested that a continuous low star formation rate has been the
dominant regime in IZw 18 and in dwarf galaxies for the lifetime of these
objects (Legrand et al. 1999). Here, we discuss and model various star-forming
histories for IZw 18. Particularly, we show that if the metallicity observed in
IZw 18 results from starburst events only, the observed colors constrain the
fraction of the metals ejected from the galaxy to be less than 50-70 %. We
demonstrate that the continuous star formation scenario reproduces the observed
parameters of IZw 18. A continuous star formation rate (SFR) of about 10E-4
Msol/yr during 14 Gyr reproduces precisely the observed abundances. This SFR is
comparable with the lowest SFR observed in low surface brightness galaxies (van
Zee et al. 1997). Generalized to all galaxies, the low continuous SFR scenario
accounts for various facts: the presence of star formation in quiescent dwarfs
and LSBG, the metallicity increase with time in the most underabundant DLA
systems, and the metal content extrapolations to the outskirts of spiral
galaxies. Also the apparent absence of galaxies with a metallicity lower than
IZw 18, the apparent absence of HI clouds without optical counterparts, and the
homogeneity of abundances in dwarfs galaxies are natural outcomes of the
scenario. This implies that, even if starbursts are strong and important events
in the life of galaxies, their more subdued but continuous star formation
regime cannot be ignored when accounting for their chemical evolution.
