The evolution of single galactic field pulsars on a P-˙P diagram is considered. An unavoidable increase in the mean value of braking indices n averaged over many years, for pulsars with characteristic times in the range 4.5 < log < 5.5, of up to 5 and more is shown. Some candidates for such pulsars are suggested. The increase in deviation between the real lifetimes t, of pulsars and their characteristic times , with increase in , is explained. The important result of this work is that the origin of single pulsars with a relatively low magnetic field B < 3 × 1011 G and with > 107 years is established. The problem is solved by assuming that the birth of pulsars can be happen at the mentioned values, too. Such births occur, probably, as a result of disruption of binary systems after the second supernova explosion in high-mass X-ray binary systems. In these binaries, mass accretion on the surface of X-ray pulsars leads to a decrease in the magnetic field from its initial value B ≈ 1012-1013 G to B < 3 × 1011 G .