  We examine the common belief that the $N_pN_n$ scheme is manifested as a
direct consequence of the valence proton-neutron interaction which has proven
to be a dominant factor in developing collectivity in nuclei. We show that the
simplification of the $N_pN_n$-plot of the lowest $2^+$ excitation energy is
introduced merely because the excitation energy always decreases when the
valence nucleon number becomes larger.
