The agricultural revolution changed the traditional system of agriculture which had existed in Lowland Scotland. Thousands of cottars and tenant farmers migrated from farms and smallholdings to the new industrial centres of Glasgow, Edinburgh and northern England. Particularly after the end of the boom created by the Revolutionary and Napoleonic Wars (1790–1815), Highland landlords needed cash to maintain their position in London society. Whereas rents had often been paid in kind, the landlords turned to money rents. They evicted the farmers who had occupied run rig arable land and shared grazing. Their holdings were let to large scale sheep farmers, who could afford substantially higher rents. The eviction of tenants went against "", the principle that clan members had an inalienable right to rent land in the clan territory. Particularly in the north and west Highlands, estates offered alternative accommodation in newly established crofting communities, with the intention that the resettled tenants worked in fishing or the kelp industry. These evictions were the first phase of the Highland Clearances. The total population of the Highlands continued to rise throughout the clearances. The result was a continuous exodus from the land—to the cities, and further afield to England, Canada, America and Australia. The potato famine of the 1840s, caused by potato blight, which devastated the population of Ireland, reached the Highlands in 1846. The overcrowded crofting communities were highly dependent on the potato. Although 150,000 people faced disaster, they were rescued by an effective emergency relief system that stands in dramatic contrast to the failures of relief in Ireland and prevented a major demographic crisis.