“…He concludes that there have been repeated periods of aridity. Petrov (1966) considers that climatic fluctuations in that part of the world may have been an underlying cause of the retreat of settlement. More recently, Chiao-Min Hsieh (1976) has pointed out that Chu K'O-Chen (1973) identified three distinct periods of drought as having afflicted China in the fourth, seventh and fifteenth centuries AD, the last at any rate resulting in important migratory movements.…”