  We have measured the column density distribution function, f(N), at z=0 using
21-cm HI emission from galaxies selected from a blind HI survey. f(N) is found
to be smaller and flatter at z=0 than indicated by high-redshift measurements
of Damped Lyman-alpha (DLA) systems, consistent with the predictions of
hierarchical galaxy formation. The derived DLA number density per unit
redshift, dn/dz =0.058, is in moderate agreement with values calculated from
low-redshift QSO absorption line studies. We use two different methods to
determine the types of galaxies which contribute most to the DLA cross-section:
comparing the power law slope of f(N) to theoretical predictions and analysing
contributions to dn/dz. We find that comparison of the power law slope cannot
rule out spiral discs as the dominant galaxy type responsible for DLA systems.
Analysis of dn/dz however, is much more discriminating. We find that galaxies
with log M_HI < 9.0 make up 34% of dn/dz; Irregular and Magellanic types
contribute 25%; galaxies with surface brightness > 24 mag arcsec^{-2} account
for 22% and sub-L* galaxies contribute 45% to dn/dz. We conclude that a large
range of galaxy types give rise to DLA systems, not just large spiral galaxies
as previously speculated.
