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处在当今时代,从事各门学科的研究工作者都面临着知识结构的更新问题。这一问题对于史学工作者也同样存在。侧如,有人曾经指出,近五千年来,“出现过四个寒冷时期,其最低温度大体在公元前一○○○年、公元四○○年、一二○○年和一七○○年。这几个时期恰与北方游牧民族几次南下中原吻合。”这并非偶然的巧合,因为“在寒冷期内,我国宜农区与宜牧区的分界——长城线被实际推到了黄河以南,
In today’s era, researchers engaged in various disciplines face the problem of updating the structure of knowledge. This problem also exists for historians. On the other hand, it has been pointed out that in the past 5,000 years, “there have been four cold periods, with the lowest temperatures generally in BC -1000, 2000, 2000 and 1 700 In recent years, these nomads coincided with the fact that nomadic peoples in the north have been able to go southward to Central Plains for several times. ”This is no accidental coincidence because" in the cold period, the dividing line between China’s Yilan and Yi-pastoral areas - the Great Wall is actually pushed to the Yellow River south,