  The line strength indices Hb, Mg2, and <Fe>, together with their gradients
and dependence on the galaxy luminosity (mass), observed in elliptical galaxies
are customarily considered as reliable indicators of systematic differences in
age and abundances of Mg and Fe. In this paper, we address the question whether
or not the indices Mg2 and <Fe> (and their gradients) are real indicators of
chemical abundances and enhancement of these or other effects must be taken
into account. We show that this is not the case because the above indices are
severely affected by the unknown relative percentage of stars as function of
the metallicity. In order to cast the problem, first we provide basic
calibrations for the variations dHb, dMg2, and d<Fe> as a function of the age
Dlog(t) (in Gyr), metallicity Dlog(Z/Z_o), and D[Mg/Fe]. Second, we analyze the
implications of the gradients in Mg2 and <Fe> observed across these systems.
Finally, the above calibration is used to explore the variations from galaxy to
galaxy of the nuclear values of Hb, Mg2, and <Fe>. The differences dHb, dMg2,
and d<Fe> are converted to differences Dlog(t), Dlog(Z/Z_o), and D[Mg/Fe]. We
thoroughly examine the relationship Dlog(t)-Mv, Dlog(Z/Z_o)-Mv, and
D[Mg/Fe]-Mv, and advance the suggestion that the duration of the star forming
period gets longer or the age of the last episode of stellar activity gets
closer to the present at decreasing galaxy mass. This result is discussed in
the context of current theories of galaxy formation and evolution. i.e. merger
and isolation. In brief, we conclude that none of these can explain the results
of our analysis, and suggest that the kind of time and space dependent IMF
proposed by Padoan et al. (1997) and the associated models of elliptical
galaxies elaborated by Chiosi et al. (1997) should be at work.
