The Wealth of Nations by Part 3 Chapter 2 Page 20

Slavery continued to take place almost universally for several centuries afterwards, till it was gradually abolished by the joint operation of the two interests above mentioned, that of the proprietor on the one hand, and that of the sovereign on the other. A villain enfranchised, and at the same time allowed to continue in possession of the land, having no stock of his own, could cultivate it only by means of what the landlord advanced to him, and must, therefore, have been what the French called a metayer.

It could never, however, be the interest even of this last species of cultivators to lay out, in the further improvement of the land, any part of the little stock which they might save from their own share of the produce, because the lord, who laid out nothing, was to get one half of whatever it produced.