  Neutral intergalactic clouds are so greatly out numbered by galaxies that
their integral HI content is negligible in comparison to that contained in
optically luminous galaxies. In fact, no HI cloud that is not associated with a
galaxy or grouping of galaxies has yet been identified. This points to a causal
relationship that relies on gravitational potentials that bind galaxies also
being responsible for confining HI clouds to sufficient density that they can
become self-shielding to the ionizing background radiation. Unconfined clouds
of low density become ionized, but confined clouds find themselves vulnerable
to instability and collapse, leading to star formation.
