The most recent analyses of the distribution in redshift of the
Ly-
systems
(Hunstead et al. 1987;
Tytler 1987b)
show that the number of systems observed per unit
redshift is best represented by N(z)
(1 + z)
with
=
2.3 ± 0.4 for 1.5 
z
3.8.
The data on metal line systems are less consistent. A substantial fraction of
metal line systems are optically thick in the Lyman continuum. Over the
redshift
range 0.4 z
3.5 these systems yield
= 1.1 ± 0.5
(Bechtold et al. 1984).
Lanzetta et al. (1987)
find that the lower ionization systems which are
characterized by strong
MgII lines have =
2.4 ± 0.8 for 0.2 z
2.1. Interesting limits are not yet
published for the high ionization systems dominated by CIV but Sargent
on page 6 of
this volume presents results of his new CIV survey. One further result
can be deduced
for the metal line systems which have extremely strong damped
Ly- lines.
Wolfe et al. (1986),
Smith et al. (1986) and
Tytler (1987a)
have concluded that these systems occur at z
2.7 with about 5-6 times the
frequency expected from
21 cm observation of local galaxies. If the association with galaxies is
correct then these systems should have
2.3.
In summary it seems that with the exception of the systems which are optically
thick in the Lyman continuum, the
Ly- and metal line systems could
have similar
rates of evolution. It is important to check this possibility. If
confirmed, one should be suspicious of models in which the
Ly- systems and metal line
systems are totally unrelated entities, and particularly if the
Ly- are pressure
supported by the
intergalactic medium while the metal line systems are not.