  Unifying hierarchies of integrable equations are discussed. They are
constructed via generalized Hirota identity. It is shown that the Combescure
transformations, known for a long time for the Darboux system and having a
simple geometrical meaning, are in fact the symmetry transformations of
generalized integrable hierarchies. Generalized equation written in terms of
invariants of Combescure transformations are the usual integrable equations and
their modified partners. The KP-mKP, DS-mDS hierarchies and Darboux system are
considered.
