  The available experimental data on shell evolution indicate that the strength
of the spin-orbit (SO) single-particle potential may be enhanced in
neutron-rich nuclei. We observe that such a simple scheme destroys the Harmonic
Oscillator (HO) magic numbers N=8 and 20 and generates new SO magic numbers
like N=6, 14, 16, 32 and 34. The traditional SO magic numbers like N=28 and 50
and N=14 seen in $^{22}$O are eroded somehow in neutron-rich nuclei due to the
sensitivity of larger-$l$ orbitals to the depth of the central potential but
they are more robust than the HO magic numbers. The N=82 shell closure persists
in neutron-rich nuclei while HO shell closures like N=40 and 70 do not emerge.
Both mechanisms contribute to enhancing the N=56 and 90 gaps by splitting the
$1d_{5/2}$ and $0g_{7/2}$ and the $0h_{9/2}$ and $1f_{7/2}$ orbitals.
