This study aims to distinguish irregularities due to borrowing from those due to lexical diffusion and those due to expressivity. The method adopted proposes the comparison of virtual reconstructions as the basis for reconstruction. Virtual reconstructions are obtained by applying in reverse the phonological rules set up for the fundamental vocabulary to the cultural vocabulary. From that point it becomes possible to establish chronological stages for roots or words and assign an order to them. The method is illustrated by a study of names of fish in the Bantu languages of Gabon. We show migration currents from the east towards the west, and the comparison of virtual reconstructions reveals that the ichthyological culture is relatively recent and on the whole does not go back to the Proto-Bantu period.