Material 1: From the Western Han Dynasty to the Northern Song Dynasty, what changes have taken place in the population ratio between the north and the south?

Political reasons: The main economic, political and cultural center of the Western Han Dynasty is Guanzhong area, which is the Weihe Plain. Now the western part of Shanxi Province and the eastern part of Shaanxi Province have been the birthplaces of the Han nationality since ancient times, and the population is naturally quite large, which has been the national population gathering place since the Zhou Dynasty. Pre-Zhao, pre-Qin, post-Qin, Western Wei, Northern Zhou, Sui and Tang Dynasties, the ancient capital of the Eleventh Dynasty, so the Han population was first concentrated in the north, mainly due to political factors. The pre-Qin period was called Baiyue, which was mainly a minority country such as Chu, Shu, Ba, Wu and Yue. After the Qin Dynasty destroyed the Six Kingdoms, the Han people began to move southward on a large scale, and began to cross the Yangtze River valley and develop the Lingnan area to the south, until the Han people were everywhere in Guangdong and Guangxi. After many wars in the north, especially the Yongjia Rebellion, the nomadic people in the northwest began to invade the Central Plains, forcing a large number of Han people to cross the river and move southward, forming the first large-scale migration tide in China. The second wave of migration was caused by the Song-Jin War in the Song Dynasty, which led to a large number of Han people settling down in the south. Until the Southern Song Dynasty, the population distribution of Han people changed greatly. The fundamental reason is that the rise of the nomadic regime in the northwest forced the political center in the north to move hard.

Economic reasons: During the Western Han Dynasty, after decades of Sino-Hungarian war, it finally gained the right to rule the lush areas in the western regions. During the Western Han Dynasty, it set up the Western Regions Metropolitan Government to manage the Western Regions as a trade channel with the western world. With the opening of the Silk Road, the Han Dynasty gained a lot of trade profits, which prompted a large number of people to gather in the northern region, thus prospering the northern economy and further consolidating Chang 'an's position as the national political and economic center until its demise due to Tibet at the end of the Tang Dynasty. The rise of Tubo forces hindered the commercial route to the west, and finally cut off the Central Plains dynasty, making it lose its jurisdiction over the western regions, thus affecting the development of commerce. Therefore, Guanzhong, as the economic center, gradually declined, and the economic decline forced a large number of people engaged in commercial handicrafts to migrate to the east and south. With the Yongjia Rebellion, the political center of the Central Plains dynasty moved south, and the economic center of the Central Plains dynasty was also difficult to move, which further promoted the development of the southern water transport industry. With the development of southern economy, a large number of commercial towns along the Yangtze River began to prosper. Economic prosperity will inevitably provide more people with more channels to make a living, and will inevitably attract a large number of northerners to move south. Since the Southern Song Dynasty, the population of the south has surpassed that of the north. The fundamental reason is that the national economic center of gravity is difficult to move because of the war.

Environmental reasons: mainly the development of agriculture can not be separated from environmental changes. There was no advanced agricultural production technology in ancient times, and the development of agriculture depended on climate change to a great extent. Therefore, when the climate is good, there will inevitably be bumper harvests and poor harvests, which will directly lead to famine. The existence of famine directly affects the long-term stability of the country. The people's food problem has always been a problem of maintaining national stability in the ancient Central Plains Dynasty. For example, ancient Egypt originated in the Nile Delta, because the upper reaches of the Nile brought a lot of fertile soil to converge into an alluvial plain at the mouth of the Nile, which promoted the growth of food crops. Therefore, Egypt's agriculture is very developed, and the prosperity of agriculture directly increases the population growth, which will inevitably produce brilliant civilization. Therefore, as one of the four ancient civilizations, ancient Egypt basically had a unique agricultural environment.

However, with the increase of population, it will inevitably lead to the heat island effect, and agriculture will inevitably bear more and more population pressure, and it will take some time to restore the fertility of local soil. When the fertility is exhausted and cannot be replenished, it will inevitably lead to a reduction in grain production, which will inevitably lead to poor harvest and famine. Moreover, due to the increase of population in Egypt, a large number of vegetation will be cut down as living materials, further causing serious soil erosion. In particular, the climate in North Africa is getting worse and worse, and the Sahara desert is gradually expanding eastward, which also affects the supply of the upper Nile to the lower Nile. Eventually, as the Nile cut off, it could not provide enough river water and conserve silt to supplement the irrigation in the agricultural production area of the Nile Delta, which contributed to the severity of famine and eventually led to the decline of the ancient Egyptian empire. The environmental problems faced by ancient China and ancient Egypt were the same, as were the agricultural centers and political and economic centers of the Central Plains Dynasty. It is concentrated in the Weihe Plain in the north, so the rise and fall of agriculture in the Weihe Plain (Guanzhong Plain) is the basis for the increase and decrease of population in the north. However, with the increase of population in Guanzhong area, a serious heat island effect has been formed, and the local soil fertility is declining. Coupled with the increase of population, the surrounding forests will inevitably be opened up as new population gathering places, and the vegetation will be destroyed. Serious soil erosion and sandstorms will begin to wreak havoc on a large scale.

Agricultural output began to decrease, and there was a problem in food supply. When the food was not enough to meet the needs of the local population, a large number of agricultural population began to migrate to the fertile and uncultivated southern region. Coupled with the invasion of northern nomads, a large number of farmland was used as grassland to raise livestock, so a large number of northern population migrated to the south during the Han and Song Dynasties.

The change of environment and climate has led to the lack of fertility of cultivated land in the north and poor harvest, and the cultivated land in the south has not been reclaimed, so the population in the south has increased greatly.

Social reasons: Since the feudal land ownership, private ownership of land has been seriously annexed, and a large number of officials and landlords began to embezzle farmers' land. Well-field system began to destroy the means of production and production. A large number of farmers lost their land and became landlords. The class contradiction between the upper landlords and the lower peasants began to intensify, coupled with the depletion of fertility of agricultural land in the north, land desertification and the spread of agricultural output, which caused serious population pressure. Famine inevitably triggered a large-scale peasant uprising, which led to the outbreak of war. A large number of landless farmers began to move to the south of the Yangtze River where private ownership of land was not serious. Because farmers were the main population in ancient China, a large number of non-agricultural population moved south, which inevitably led to a large number of non-agricultural population moving south. After the Song Dynasty, the grain output of the East China Plain in the south surpassed that of the Guanzhong Plain in the north, which led to the south being able to feed more people. This is the social reason why the population of South China surpassed that of North China in Song Dynasty.

Most importantly, the above four points and the rest of the national war problems are not serious and can be ignored. Mainly political, economic, environmental and social issues.