"An income tax is neither a property tax nor a tax on occupations of common right, but is an excise tax...The legislature may declare as 'privileged' and tax as such for state revenue, those pursuits not matters of common right, but it has no power to declare as a 'privilege' and tax for revenue purposes, occupations that are of common right."

Simms v. Ahrens, 271 SW 720 (1925)