  We analyse the data for the proton structure function $F_2$ over the entire
$Q^2$ domain, including especially low $Q^2$, in terms of perturbative and
non-perturbative QCD contributions. The small distance configurations are given
by perturbative QCD, while the large distance contributions are given by the
vector dominance model and, for the higher mass $q \bar{q}$ states, by the
additive quark approach. The interference between states of different
$q\bar{q}$ mass (in the perturbative contribution) is found to play a crucial
role in obtaining an excellent description of the data throughout the whole
$Q^2$ region, including photoproduction.
