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It is often assumed a priori that the Aborigines in Taiwan were forced out by the newly arrived Han settlers. While the contests for land between the highland Hakka and the Aborigines were quite frequent, often with deadly results, it was far more common for the Qing era new immigrants to acquire land through purchases.
The above [source: http://www.darc.ntu.edu.tw] is an executed purchase agreement between a plaines aboriginal seller by the name of 利加力龜達 and two "honest [according to the document]" Han-buyers 郭然 and 石普. The transaction took place in the 11th Month of 嘉慶九年 (1804) for a parcel of land located in 金包里 (now 金山, north of Danshui).
The agreement detailed the boundaries of this piece of land and the purchase price of 40 silver dollars. And the reason for selling was because the owner was no longer capable of tilling the land which he had inherited from his grandfather. The permanent nature of this sale was also stipulated in the contract. It was signed with a thumb print by the seller and also by his witness, a relative named 順生. The official seal of approval was stamped by 翁麗力, the village manager/interpreter, a Han-Chinese, and the commission was also specified in the agreement. The aboriginal seller, most likely an illiterate, had also approved the deal with a palm print.
This was usually how the Aborigines parted with their land ownership during the Qing period that had actually continued into the early Japanese colonial [the Meiji] era. Many such purchase agreements have survived and some archived. The transactions appeared fair and square, at least on paper. Although, despite the lack of evidence, argument to the contrary, i.e., the Han-Taiwanese must have short-changed the ignorant/innocent Aborigines in some way, persists to this day. Naturally, this is not to say that there had never been any instances when the Aborigines were cheated by the Han-people [or vice versa for that matter] for 400 years is a long time. In fact, in early years, there were reports that some Han-men married Aboriginal women for their properties. At least one aspect appears clear that the Aborigines were not forced at knife-/gun-point to give up their land. Now, with more primary documents becoming available, a better understanding should result.
To complete the post: there were other ways for the Aborigines to lose their land ownership. During the Japanese era, those who leased their fields to Han-tenant farmers lost the land title to the government - which abolished the "big bad landlords slave-driving the little tenants" system, allegedly. The land was actually confiscated for developmental purposes, e.g., for growing sugarcane crops and building of sugar processing factories. And often through the eminent domain, whole tribes were relocated to remote areas. This vast governmental land holding was taken over by the KMT gov't in 1945, never returned to the original owners/tribes. And starting in 1947, the 公地放領 [Distribution of the Public Land] component of the 耕者有其田 [Tillers Own Their Own Land Reform] policy has made the situation from bad to worse. This has been a major source of discontent ever since.