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The time gap between the supposed time of the Xia and the first written references to it have meant that the historicity of the Xia dynasty itself and the traditional narrative of its history are at best uncertain. The Skeptical School of early Chinese history, started by Gu Jiegang in the 1920s, was the first group of scholars within China to systematically question the traditional story of its early history. By critically examining the development of the narrative of early Chinese history throughout history, Gu concluded "the later the time, the longer the legendary period of earlier history... early Chinese history is a tale told and retold for generations, during which new elements were added to the front end".