  The unification of X-ray and radio selected BL Lacs has been an outstanding
problem in the blazar research in the past years. Recent investigations have
shown that the gap between the two classes can be filled with intermediate
objects and that apparently all differences can be explained by mutual shifts
of the peak frequencies of the synchrotron and inverse Compton component of the
emission. We study the consequences of this scheme using a new sample of X-ray
selected BL Lac objects comprising 104 objects with z<0.9 and a mean redshift
z=0.34. 77 BL Lacs, of which the redshift could be determined for 64 (83%)
objects, form a complete sample. The new data could not confirm our earlier
result, drawn from a subsample, that the negative evolution vanishes below a
synchrotron peak frequency log (peak-frequency) = 16.5. The complete sample
shows negative evolution at the 2 sigma level (<V_e/V_a> = 0.42 +- 0.04). We
conclude that the observed properties of the HRX BL Lac sample show typical
behaviour for X-ray selected BL Lacs. They support an evolutionary model, in
which flat-spectrum radio quasars (FSRQ) with high energetic jets evolve
towards low frequency peaked (mostly radio-selected) BL Lac objects and later
on to high frequency peaked (mostly X-ray selected) BL Lacs.
