  We explore the viability of the unification of BL Lacs and FR I radio
galaxies by comparing the core emission of radio galaxies with those of BL Lacs
of similar extended radio power, taking advantage of the newly measured optical
nuclear luminosity of FR I sources. The spectral properties of complete samples
are also studied in the radio-optical luminosity plane: starting from the
Spectral Energy Distribution (SED) of BL Lacs, we calculate the predicted
luminosity of FR I nuclei in the frame of a simple one--zone model, by properly
taking into account the relativistic transformations. We find that the bulk
Lorentz factors required by the spread in the observed luminosities in all
bands are significantly smaller than those implied by other, both observational
and theoretical, considerations. This discrepancy is also reflected in the fact
that FR I nuclei are over-luminous by a factor of 10-10^4, with respect to the
predictions, both in the radio and in the optical band.
  In order to reconcile these results with the unification scheme, velocity
structures in the jet are suggested, where a fast spine is surrounded by a slow
(but still relativistic) layer so that the emission at different angles is
dominated by different velocity components: the fast one dominates the emission
in BL Lacs while the slow layer dominates the emission in misaligned objects.
Furthermore for the lowest luminosity BL Lacs it has to be also postulated that
their beaming factor in the radio band is lower than in the optical (and
X-ray), as would result from deceleration of the jet.
  The self-consistency of the unification model therefore requires that both
intrinsic differences in the SED and different beaming properties play a
substantial role in characterizing the phenomenology of these sources.
