  We have formed ``composite spectra'' by combining the integrated-light
spectra of individual galaxies in 8 intermediate-redshift and 12 low-redshift
clusters of galaxies. Because these composite spectra have much higher
signal-to-noise ratios than individual galaxy spectra, they are particularly
useful in quantifying general trends in star formation for galaxy populations
in distant clusters, z > 0.3. By measuring diagnostic features that represent
stellar populations of very different ages, a grand-composite spectrum can
reflect the fractions of those populations as accurately as if excellent
spectral measurements were available for each galaxy.
  Measuring the equivalent widths of spectral features in composite spectra is
especially well-suited for comparing cosmic variance of star formation in
clusters at a given redshift, or comparing clusters over a range of redshifts.
When we do this we find that [O II] emission and especially Balmer absorption
is strong in each of our intermediate-redshift clusters, and completely
separable from a sample of 12 present-epoch clusters, where these features are
weak. Specifically, we show by comparing to the H-delta strengths of
present-epoch populations of continuously star-forming galaxies that the
higher-redshift samples must contain a much higher fraction of starburst
galaxies than are found today in any environment.
