  By using a combination of detailed experimental studies and simple
theoretical arguments, we identify a novel mechanism characterizing the hopping
transport in the Mott insulating phase of Ca$_{2-x}$Sr$_x$RuO$_4$ near the
metal-insulator transition. The hopping exponent $\alpha$ shows a systematic
evolution from a value of $\alpha=1/2$ deeper in the insulator to the
conventional Mott value $\alpha=1/3$ closer to the transition. This behavior,
which we argue to be a universal feature of disordered Mott systems close to
the metal-insulator transition, is shown to reflect the gradual emergence of
disorder-induced localized electronic states populating the Mott-Hubbard gap.
