  We consider 400 H II galaxies in which the oxygen abundances were obtained by
electron temperature based methods. We split the sample in three metallicity
bins. In each bin, a narrow sequence is found not only in pure emission line
ratio diagrams but also in terms of Hbeta equivalent width, indicating the
existence of an evolutionary sequence. Our diagrams show unambiguously the
existence of an evolution on a timescale of a few Myr. We compare the observed
sequences with photoionization models of appropriate metallicity. In order to
understand the evolution of H II galaxies one needs to consider the evolution
of the gas as well as that of the stars. The observations require EW(Hbeta) to
decrease more rapidly than predicted by the passive evolution of a starburst. A
photoionized adiabatic expanding bubble with a covering factor decreasing with
time reproduces most diagrams. However, the question of the heating of H II
galaxies and the origin of the nebular He II 4686 emission are not settled. We
find evidence for self-enrichment in nitrogen on a time scale of several Myr.
