  The evolution of the abundance of galaxy clusters is not a reliable measure
of Omega if there are features on scales of a few Mpc in the primordial power
spectrum. Conversely, if we know the cosmological model parameters from other
measurements, the cluster abundance evolution permits us to probe features in
the power spectrum that are in the nonlinear regime at the present epoch, and
hence difficult to discern directly from current epoch measurements.
  We have investigated the influence of an artificially introduced Gaussian
feature on an otherwise unperturbed SCDM power spectrum. Using these modified
spectra as an input to cosmological N-body simulations, we are able to show
that in terms of the cluster abundance evolution, a SCDM model displays
characteristics similar to an OCDM model. However, strong modifications would
also be visible at a redshift z=0 in the dark matter power spectrum whereas
minor alterations to the usual SCDM spectrum are washed away by non-linear
evolution effects. We therefore conclude that features with characteristics
such as discussed here might not be detectable using observations of the galaxy
power spectrum, the local cluster abundance or the large-scale velocity field
as measured by the velocity distribution of galaxy clusters. The only quantity
that shows a pronounced difference at the present epoch between our models
under investigation is the halo-halo correlation function which appears to be
strongly biased with respect to an unmodified SCDM model. This is due to a lack
of power on certain scales which subsequently modifies the relative amplitude
of high-and low-k waves. Apart from observations of the evolution of cluster
abundance, measurements of the Lyman-alpha forest at high redshift could put
constraints on possible features in the power spectrum, too.
