The historian Hugh Clark proposed a three-stage model of broad political trends during this time period. The first stage (880–910) consists of the period between the Huang Chao Rebellion and the formal end of the Tang dynasty, which saw chaotic fighting between warlords who controlled approximately one or two prefectures each. The second stage (910–950) saw the various warlords stabilise and gain enough legitimacy to proclaim new dynasties. The third stage (950–979) saw the forceful reunification of China by the Later Zhou dynasty and its successor the Song dynasty, and the demilitarisation of the provinces. Southern China, divided into several independent dynastic kingdoms, was more stable than the North which saw constant regime change. Consequently the Southern kingdoms were able to embark on trade, land reclamation and infrastructure projects, laying the groundwork for the Song dynasty economic boom. This economic shift to the south also led to a vast southward migration. Several Northern dynasties originated in the northeast, and centralisation of the north led to a migration of provincial elites into the capital, particularly northeasterners, creating a new metropolitan culture.