  We review the measurements of magnetic fields of OBA stars. Based on these
data we confirm that magnetic fields are distributed according to a lognormal
law with a mean log(B)=-0.5 (B in kG) with a standard deviation sigma=0.5. The
shape of the magnetic field distribution is similar to that for neutron stars.
This finding is in favor of the hypothesis that the magnetic field of a neutron
star is determined mainly by the magnetic field of its predecessor, the massive
OB star. Further, we model the evolution of an ensemble of magnetic massive
stars in the Galaxy. We use our own population synthesis code to obtain the
distribution of stellar radii, ages, masses, temperatures, effective magnetic
fields and magnetic fluxes from the pre-main sequence (PMS) via zero age main
sequence (ZAMS) up to the terminal age main sequence (TAMS) stages. A
comparison of the obtained in our model magnetic field distribution (MFD) with
that obtained from the recent measurements of the stellar magnetic field allows
us to conclude that the evolution of magnetic fields of massive stars is slow
if not absent. The shape of the real MFD shows no indications of the magnetic
desert proposed previously. Based on this finding we argue that the observed
fraction of magnetic stars is determined by physical conditions at the PMS
stage of stellar evolution.
