  We propose a sequence (the Blazar main sequence, BMS) that links the two main
components of the Blazar class, namely, the Flat Spectrum Radio Quasars and the
BL Lacertae objects, and yields all their distinctive features in a correlated
way. In this view, both type of sources are centered on a supermassive Kerr
hole close to maximal spin that produces a relativistic jet observed close to
the axis, and on the associated accretion disk that emits more isotropically.
But the FSRQs are energized by accretion at rates dm/dt ~ 1, and are dominated
by the disk components (thermal and electrodynamic jet-like component) which
provide outputs in excess of L ~ 10^46 erg/s. On the other hand, accretion
levels dm/dt << 1 are enough to energize the BL Lacs; here the radiation is
highly non-thermal and the power partly contributed by the rotational energy of
the central Kerr hole, with the latter and the disk together sustaining L ~
10^44 erg/s in the jet frame for several Gyrs. If so, we expect the BL Lacs to
show quite different evolutionary signatures from the FSRQs, and in particular,
number counts close to the Euclidean shape, or flatter if the sources make a
transition to the BL Lac from a FSRQ mode. In addition, for lower dm/dt along
the BMS, we expect the large scale electric fields to be less screened out, and
to accelerate fewer particles to higher energies with non-thermal radiations at
higher frequencies; so in moving from FSRQs to BL Lacs these non-thermal
radiations will peak at frequencies inversely correlated with the disk output.
For the BL Lacs such dependence implies increased scatter when one tries a
correlation with the total ouput. At its endpoint, the BMS suggests widespread
objects that are radiatively silent, but still efficient in accelerating cosmic
rays to ultra high energies.
