of the dynasty in the Xinhai Revolution up until 1930. In the early 20th century, Britain sent an expedition force to Tibet and forced Tibetans to sign a treaty. The Qing court responded by asserting Chinese sovereignty over Tibet, resulting in the 1906 Anglo-Chinese Convention signed between Britain and China. The British agreed not to annex Tibetan territory or to interfere in the administration of Tibet, while China engaged not to permit any other foreign state to interfere with the territory or internal administration of Tibet. Furthermore, similar to Xinjiang which was converted into a province earlier, the Qing government also turned Manchuria into three provinces in the early 20th century, officially known as the "Three Northeast Provinces", and established the post of Viceroy of the Three Northeast Provinces to oversee these provinces, making the total number of regional viceroys to nine. Society Population growth and mobility The population grew in numbers, density, and mobility. The population grew from roughly 150 million in 1700, about what it had been a century before, then doubled over the next century, and reached a height of 450 million on the eve of the Taiping Rebellion in 1850. The spread of New World crops, such as maize, peanuts, sweet potatoes, and potatoes decreased the number of deaths from malnutrition. Diseases such as smallpox were brought under control by an increase in inoculations. In addition, infant deaths were decreased due to improvements in birthing techniques performed by doctors and midwives and an increase in medical books available to the public. Government campaigns decreased the incidence of infanticide. In Europe population growth in this period was greatest in the cities, but in China growth in cities and the lower Yangzi was low. The greatest growth was in the borderlands and the highlands, where farmers could clear large tracts of marshlands and forests. The population was also remarkably mobile, perhaps more so than at any time in Chinese history. Indeed, the Qing government did far more to encourage mobility than to discourage it. Millions of Han Chinese migrated to Yunnan and Guizhou in the 18th century, and also to Taiwan. After the conquests of the 1750s and 1760s, the court organized agricultural colonies in Xinjiang. Migration might be permanent, for resettlement, or the migrants (in theory at least) might regard the move as a temporary sojourn. The latter included an increasingly large and mobile workforce. Local-origin-based merchant