  At low $x \cong Q^2/W^2 << 1$, in deep inelastic scattering, the photon
fluctuates into a $q \bar q$ vector state that interacts via two gluons with
the proton. The energy dependence is determined by the saturation scale, in our
approach given by $\Lambda^2_{sat} (W^2) \sim (W^2)^{C_2}$. Imposing DGLAP
evolution, we find $C_2^{theory} = 0.27$ in agreement with the
model-independent analysis of the HERA data. Different values of the exponent
$C_2$ are correlated with different ratios of the longitudinal to transverse
structure function. This stresses the need for experiments to separate
longitudinal and transverse contributions in deep inelastic scattering.
