Reversion was caused by excision.
Excision sometimes picks up a piece of the bacterial chromosome and moves it to a new place.
Is genetics today simply different from Mendelian genetics of the 1930's, or does it actually provide a more accurate description of hereditary phenomena. McClintock provided rigorous formal description of phenomena, but provided no mechanistic explanation Supervenience Two systems that have identical properties at a lower hiearchical level, e.g., chemical, will also have identical properties at a higher hierarchical level, e.g., heredity.