If Congress sees fit to impose a capitation, or other direct tax, it must be laid in proportion to the census; if Congress determines to impose duties, imposts, and excises, they must be uniform throughout the United States. These are not strictly limitations of power. They are rules prescribing the mode in which it shall be exercised. ... This review shows that personal property, contracts, occupations, and the like have never been regarded by Congress as proper subjects of direct tax.