It will be seen that the QSO absorption lines have many potential uses
in cosmology.
We have real prospects that they can be used to study the gross
properties of the
interstellar gas in galaxies back to the time of their formation. The
counts of absorption
lines as a function of redshift will eventually provide an important
cosmological
test. Observations of correlated absorption, both in redshift and in
nearby QSOs on
the plane of the sky, are leading to important information on the
evolution of the
large-scale distribution of galaxies in space. Finally, the enigmatic
Ly-
clouds offer
a unique opportunity to directly study the evolution of the
intergalactic medium and
the meta-galactic ionizing radiation flux and to provide information on
primordial abundances of the elements.
Acknowledgements. I thank my co-workers, A. Boksenberg and C. Steidel, for allowing me to quote the results of our joint work in advance of publication. I also thank D. Tytler for providing me with details of his unpublished work. The work described in this paper was supported in part by the National Science Foundation under Grant AST84-16704.