The Wealth of Nations by Part 5 Chapter 2 Page 125

poll-taxes in the southern provinces of North America, and in the West Indian Islands annual taxes of so much a head upon every negro, are properly taxes upon the profits of a certain species of stock employed in agriculture. As the planters are, the greater part of them, both farmers and landlords, the final payment of the tax falls upon them in their quality of landlords without any retribution.

Taxes of so much a head upon the bondmen employed in cultivation seem anciently to have been common all over Europe.

There subsists at present a tax of this kind in the empire of Russia. It is probably upon this account that poll-taxes of all kinds have often been represented as badges of slavery. Every tax, however, is to the person who pays it a badge, not of slavery, but of liberty.