  The observed redshift distribution of Ly-$\alpha$ lines and metal systems is
examined in order to discriminate and to trace the evolution of structure
elements observed in the galaxy distribution, at small redshifts, and to test
the theoretical description of structure evolution. We show that the expected
evolution of filamentary component of structure describes quite well the
redshift distribution of metal systems and stronger Ly-$\alpha$ lines with
$\log(N_{HI})\geq$14, at $z\leq$ 3. The redshift distribution of weaker
Ly-$\alpha$ lines can be attributed to the population of poorer structure
elements (Zel'dovich pancakes), which were formed at high redshifts from the
invisible DM and non luminous baryonic matter, and at lower redshifts they
mainly merged and dispersed.
