  We present new results on the Fundamental Plane of galaxies in two rich
clusters, Cl1358+62 at $z=0.33$ and MS2053--04 at $z=0.58$, based on Keck and
HST observations. Our new data triple the sample of galaxies with measured
Fundamental Plane parameters at intermediate redshift. The early-type galaxies
in these clusters define very clear Fundamental Plane relations, confirming an
earlier result for Cl0024+16 at $z=0.39$. This large sample allows us to
estimate the scatter reliably. We find it to be low, at 0.067 dex in $\log
r_e$, or 17% in re, similar to that observed in comparable low redshift
clusters. This suggests that the structure of the older galaxies has changed
little since $z=0.58$.
  The $M/L_V$ ratios of early-type galaxies clearly evolve with redshift; the
evolution is consistent with $\Delta \log M/L_V \sim -0.3z$. The $M/L_V$ ratios
of two E+A galaxies in Cl1358+62 are also lower by a factor of $\sim 3$,
consistent with the hypothesis that they underwent a starburst 1 Gyr
previously. We conclude that the Fundamental Plane can therefore be used as a
sensitive diagnostic of the evolutionary history of galaxies. Our data, when
compared to the predictions of simple stellar population models, imply that the
oldest cluster galaxies formed at high redshift ($z > 2$). We infer a different
evolutionary history for the E+A galaxies, in which a large fraction of stars
formed at $z<1$. Larger samples spanning a larger redshift range are needed to
determine the influence of starbursts on the general cluster population.
