  Using the largest sample of narrow emission line galaxies available so far,
we show that their spectral characteristics are correlated with different
physical parameters, like the chemical abundances, the morphologies, the masses
of the bulge and the mean stellar age of the stellar populations of the host
galaxies. It suggests that the spectral variations observed in standard
spectroscopic diagnostic diagrams are not due solely to variations of
ionization parameters or structures but reflect also the chemical evolution of
the galaxies, which in turn can be explained by different galaxy formation
processes.
