  In this paper: 1. The assumption of the dependence of Doppler factor on the
emission frequency (Fan et al. 1993) is used to explain the observational
differences between the radio-selected BL Lac objects(RBLs) and the
X-Ray-Selected BL Lac objects (XBLs): a) Hubble relation; b) different
multiwavelength correlations; c) different regions in the effective spectral
index diagram; d) different polarization. The results suggest that RBLs and
XBLs are the same.
  2. From the analysis of the relation between infrared magnitude and redshift,
it is proposed that the parent population of BL Lac objects should be FRI radio
galaxies and FRII(G) radio galaxies showing the optical spectra of a galaxy.
  3. From the superluminal motion, the assumption (Fan et al. 1993) is
confirmed.
  4. Based on the relation between polarization and Doppler factor (Fan et al.
1997, A&A, 327, 947), it is proposed that the ratio of the beamed luminosity to
the unbeamed luminosity in the source frame of OVVs/HPQs is smaller than that
of BL Lac objects.
