  We use a unified analytical formulation for the multiplicity functions of
clusters and galaxies which is free from the cloud-in-cloud problem encountered
in earlier approaches and well adapted to the description of the non-linear
clustering features. It is especially suited to simultaneously describe rich
clusters, groups and galaxies, consistently with the hierarchical picture of
gravitational clustering and their evolution in time. Using a simple model for
the X-ray luminosity (taking into account entropy considerations), we obtain
the X-ray luminosity distribution of groups and clusters. Then, using the same
formalism we derive the galaxy and quasar multiplicity functions. In
particular, we show that the use of the standard Press-Schechter prescription
leads to erroneous conclusions at low redshifts while our approach provides a
reasonable agreement with observations in a natural fashion because it is able
to distinguish galactic halos from groups or clusters. Thus, we obtain a global
and consistent picture of the X-ray emissions from all structures. In
particular, we show that future observations (e.g., from AXAF) could provide
interesting information on galaxy evolution. Indeed, they will constrain the
importance of a possible hot diffuse gaseous phase in galactic halos and they
could reveal massive galaxies which are just being formed, through the X-ray
emission of their cooling gas.
