  We discuss the implications of the recent discovery that Starburst Nucleus
Galaxies (SBNGs) have lower oxygen abundances than "normal galaxies" of the
same morphological type. Our interpretation of this result is that SBNGs are
young galaxies, still in the process of formation. We consider several
alternatives, but none of them provides a viable explanation. This new result
has important consequences for our understanding of galaxy evolution, as it
confirms the scenario of hierarchical formation of galaxies, and explains
recent observations of the Hubble Space Telescope.
