Existing data on near-barrier fusion and capture excitation functions for about 50 medium and heavy nucleus--nucleus systems have been analysed using a phenomenological model, in which fusion barriers are assumed to have Gaussian distributions. Systematics of the barrier-distribution parameters, the mean barrier and its variance, are presented. Deduced values of the variance parameter show an important role of nuclear structure effects, which we propose to account for by relating values of the variance parameter with fusion energy thresholds calculated with the fusion adiabatic potential.

PACS numbers: 25.70.Jj