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.