Two dozen long period pulsars are separated from the swarm of ordinary
pulsars by an obvious gap in the P versus Sd diagram (where Sd = log
P + 21.0), with a plausible upper boundary for ordinary pulsars. Possi
ble pulsar evolutionary tracks are discussed to explain the diagram in
terms of previously suggested scenarios of magnetic field decay. The
(P-Sd) diagram is difficult to understand if there is no magnetic fiel
d decay during the active life of pulsars. However, if the magnetic fi
elds of neutron stars decay exponentially, almost all slowly rotating
pulsars must have been injected with a very long initial spin period o
f about 2 seconds, which seems impossible. Based on qualitative analys
es, it is concluded that magnetic fields of neutron stars decay as a p
ower-law, with a time scale related to the initial field strengths. Th
e plausible boundary and the gap are suggested to naturally divide pul
sars with distinct magnetic ''genes'', ie. pulsars which were born fro
m strongly magnetized progenitors - such as Bp stars, and pulsars born
from normal massive stars. The possibility remains open that a fracti
on of slowly rotating pulsars were injected with long initial spin per
iods, while others would have a classical pulsar evolution history. It
is suggested that PSR B1849+00 was born in the supernova remnant Kes-
79 with an initial period of about 2 seconds.