Evolution of precollective nuclei and a tripartite classification of nuclear structure

Precollective nuclei, that is, those with E(4₁⁺)/E(2₁⁺)2.0, are shown to evolve structurally in a very simple and universal way, in which E(4₁⁺) is linear against E(2₁⁺) with a slope of unity. A clue to this behavior is provided in terms of a seniority v=0 pair addition mode. Combined with recent studies of vibrator and rotor nuclei, this ``seniority'' regime gives a new tripartite global characterization of the evolution of nuclear structure.