鉴于历史时期美洲作物的巨大影响，一般认为，美洲作物之于清代人口增长有巨大作用。在大数据时代，美洲作物不仅是农史、历史地理、物质文化史等的重要命题，亦是经济史研究中量化历史研究的第一等题目。已有计量推导证明美洲作物，尤其是玉米，对清代人口增长贡献率超过了20%，看似符合史学研究的先验性结论，亦说明传统中国是典型的马尔萨斯社会。我们利用理论与实证的方式以美洲作物经济地理为中心的研究再出发，却得出了不一样的结论：

一是玉米、番薯虽然传入时间较早，但发挥功用时间较迟，除了番薯在明末的福建、广东尚有可圈可点之处，其余基本都不入流，直至乾隆中期之后南方山区开始推广、在道光年间完成推广。换言之，18世纪中期到19世纪中期是二者在南方山区推广最快的阶段，之后才作为主要粮食作物发挥了巨大功用，在南方平原地带，则一直建树不多；最终在南方形成了西部山区玉米种植带和东南丘陵番薯种植带，虽有交汇，但分庭抗礼，边界在湖广、广西。

二是北方玉米、番薯推广更晚，光绪以降的清末民国时期才有较大发展，最终奠定了一般粮食作物的地位，然仍并无二者在南方山区之地位。玉米胜于番薯，尤其在春麦区番薯几无踪迹，玉米在北方山区值得一书，在平原也有所发展，在总产量上得以超越南方。

三是乾隆以降，尤推乾隆帝，对番薯大加劝种，嘉庆以来，多见官方禁种玉米，这些虽有效果，但收效不大，番薯由于未融入北方当地的种植制度，多是昙花一现，灾后即撤，玉米暗合了棚民开山的需求，屡禁不止，愈演愈烈，归根到底，这些都是农民自发选择，不是国家权力所能管控的。

要之，美洲作物推广作为技术革新之一，是由人口增长决定的，不能倒因为果。实际上美洲粮食作物价值凸显的时间是在19世纪中期之后，且主要在山区缓解人口压力。玉米恐怕并非“可以解释（清代）人口增长的23%”，我们初步研究发现至迟在19世纪中期，玉米、番薯提供人均粮食占有量43.83斤、供养2473万—2798万人。至少太平天国（人口峰值）之前的人口压力并非源自美洲作物，即美洲作物不是刺激人口增长的主要因素，就全国而言美洲作物发挥更大功用的时间是在近代以来，已经错过了人口激增的阶段。依然是传统粮食作物在中国人口增长的问题上居功至伟。在美洲作物的问题上，并非传统史学，而是计量史学把虚假的相关性看成因果关系。这也证明，博赛拉普的反马尔萨斯理论，人口压力决定食物生产，更加符合传统中国国情。

关键词：美洲作物；玉米；番薯；本土化；量化历史

Abstract

In view of the great influence of American crops in the historical period,it is generally believed that the American crops have a great effect on the growth of the population in the Qing Dynasty.In the big data age,the American cropsare not only an important topic of Agricultural history,historical geography,material culture's history,etc,but also the first question in quantitative historical studies.It has been proved that American crops,especially maize,have contributed more than 20% to the population growth in the Qing Dynasty,which seems to be in line with the transcendental conclusions of history,and has been proved that traditional China is a typical Malthus society.We use theoretical and empirical methods to study again,but draw different conclusions.

Firstly,maize and sweet potato have been introduced early,but the impact time was late.Besides the sweet potato in Fujian and Guangdong which is important in the end of the Ming Dynasty,they were all unimportance in everywhere.They began to spread in southern mountainous areas till the middle of Qianlong period,and are popularized and completed in the southern mountain area in the Daoguang period.In a word,maize and sweet potato are popularized very fast in southern mountainous areas from the mid eighth Century to the mid nineteenth Century,after that,they played a great role as a major food crop,but they have not been flourishing in the southern plain.Finally,the western mountain maize planting areas and the southeast hilly sweet potato planting areas were formed in the south China.Although there was a confluence,the border was in Hunan Hubei and Guangxi.

Secondly,maize and sweet potato are extended later in the North China,which had a great development after the Guangxu period,and finally laid the status of the general grain crops,but can't compare with the position of the Maize and sweet potato in the South Mountain Area.Of course,maize is better than the sweet potato in the North,especially in the spring wheat area.Maize is worth mentioned in the northern mountainous area,and has also developed in the plains which is surpassed the south in total output.

Thirdly,since the Qianlong period,especially Emperor Hongli,the sweet potato has been greatly encouraged.Official banned maize after Jiaqing period that were little effective.Because of its failure to integrate into the northern cropping system,sweet potato is a flash in the pan,after the disaster is withdrawn,Maize coincides with the needs of the shed people to reclaim the mountains,despite repeated bans can't be stopped which growed more and more.In the final analysis,these are farmers' spontaneous choices,not controlled by state power.

So,as one of technical innovation American crops are determined by the population growth,can not reverse cause and effect.In fact,the value of American crops highlighted in the middle of the 19th Century,and largely alleviated population pressure in the mountains.Maize,I'm afraid,is not “an explanation for 23% of the population growth in the Qing Dynasty”.The author's preliminary study found that maize and sweet potatoes provided 43.83 Jin per capita grain and feeded 2473～2798 million in the middle of the nineteenth Century.At least the population pressure before the Taiping Heavenly Kingdom（Population peak）can not due to American crops,that is to say,American crops are not the main factors to stimulate population growth.The time for American crops to play a greater role in the country had missed the stage of population growth which was in fact in modern times.Traditional grain crops are highly valued in China's population growth.On the issue of American crops,it is not traditional historiography,but quanti tative history have made the false relevance regarded as a causal relationship.It is also proved that bossaap's anti Malthus theory that population pressure determine food production,which is more in line with the traditional Chinese national conditions.

Key words:American crops; Maize; Sweet potato; Localization;Quantitative history