The Wealth of Nations Book 3 Chapter 4 Page 21

the full value of the farm, a greater surplus, or what is the same thing, the price of a greater surplus, was obtained for the proprietor, which the merchants and manufacturers soon furnished him with a method of spending upon his own person in the same manner as he had done the rest.

The same cause continuing to operate, he was desirous to raise his rents above what his lands, in the actual state of their improvement, could afford. His tenants could agree to this upon one condition only, that they should be secured in their possession for such a term of years as might give them time to recover with profit whatever they should lay out in the further improvement of the land. The expensive vanity of the landlord made him willing to accept of this condition; and hence the origin of long leases.