  We analyze LEP and SLC data from the 1995 Summer Conferences as well as from
low energy neutral current experiments for signals of new physics. The reasons
for doing this are twofold, first to explain the deviations from the standard
model observed in $R_b$ and $R_c$ and second to constrain non-standard
contributions to couplings of the $Z^0$ boson to all fermions and to the
oblique parameters. We do so by comparing the data with the Standard Model as
well as with a number of test hypotheses concerning the nature of the new
physics. These include non-standard $Zb\bar{b}$-, $Zc\bar{c}$- and
$Zs\bar{s}$-couplings as well as the couplings of the $Z^0$ to fermions of the
entire first, second and third generations and universal corrections to all up-
and down-type quark couplings (as can arise \eg\ in $Z'$ mixing models). We
find that non-standard $Zb\bar{b}$ couplings are both necessary and sufficient
to explain the data and in particular the $R_b$ anomaly. It is not possible to
explain $R_b$, $R_c$ and a value of the strong coupling constant consistent
with low energy determinations invoking only non-standard $Zb\bar{b}$- and
$Zc\bar{c}$-couplings. To do so one has to have also new physics contributions
to the $Zs\bar{s}$ or universal corrections to all $Zq\bar{q}$ couplings.
