  Measurements of the proton structure function $F_2$ for $0.6 < Q^2 < 17
{GeV}^2$ and $1.2 \times 10^{-5} < x <1.9 \times 10^{-3}$ from ZEUS 1995
shifted vertex data are presented. From ZEUS $F_2$ data the slopes $dF_2/d\ln
Q^2$ at fixed $x$ and $d\ln F_2/d\ln(1/x)$ for $x < 0.01$ at fixed $Q^2$ are
derived. For the latter E665 data are also used. The transition region in $Q^2$
is explored using the simplest non-perturbative models and NLO QCD. The data at
very low $Q^2$ $\leq 0.65 {GeV}^2$ are described successfully by a combination
of generalised vector meson dominance and Regge theory. From a NLO QCD fit to
ZEUS data the gluon density in the proton is extracted in the range $3\times
10^{-5} < x < 0.7$. Data from NMC and BCDMS constrain the fit at large $x$.
Assuming the NLO QCD description to be valid down to $Q^2\sim 1 {GeV}^2$, it is
found that the $q\bar{q}$ sea distribution is still rising at small $x$ and the
lowest $Q^2$ values whereas the gluon distribution is strongly suppressed.
