  X-ray selected surveys of clusters of galaxies can distinguish between
hierarchical models of the growth of structure in the Universe. Here we present
the first results from an X-ray selected, flux and surface brightness limited
deep survey for high redshift clusters and groups of galaxies based on ROSAT
PSPC pointed data. The log(N)-log(S) relation of all clusters in this survey is
consistent with that from most previous surveys but occupies a flux range not
previously covered. At high redshifts (z>0.3) the cluster luminosities are in
the range 4e43 to 2e44 erg/s, the luminosities of poor clusters. The number of
high redshift, low luminosity clusters is consistent with no evolution of the
X-ray luminosity function between redshifts of z=0.4 and z=0, and places a
limit of a factor of <1.7 (at 90% confidence) on the amplitude of any pure
negative density evolution of clusters of these luminosities, in contrast with
the factor of approx 3 (corresponding to number density evolution proportional
to (1+z)^{-2.5}) found in the EMSS survey at similar redshifts but higher
luminosities. Taken together, these results support hierarchical models in
which there is mild negative evolution of the most luminous clusters at high
redshift but little or no evolution of the less luminous but more common,
optically poor clusters. Models involving preheating of the X-ray gas at an
early epoch fit the observations, at least for Omega_0=1.
