As a matter of practical expediency; the power of federal taxation for the raising of public revenue is exercised almost entirely by the levy and collection of duties, imposts, and excises. These are merely different forms of taxes. " Duties and imposts" cover levies on imports and exports of commodities; and the power to levy these, in its discretion, is restricted only by two clauses, the one included in the general provision already quoted that they shall be uniform; and the other in the clause providing that, "No tax or duty shall be laid on articles exported from any state" (Art. I, § 9, ¶ 5). In the exercise of the power to levy and collect duties and imposts, import duties are levied which are either (1) specific, that is, in accordance with quantity, or (2) ad valorem, that is, in accordance with value, upon very many classes of goods brought from foreign countries; and from this source a large portion of the revenue of the United States government is derived. The extent to which this taxing power shall be carried, and the methods in which it shall be exercised, are peculiarly within the discretion of Congress, and as the states cannot levy taxes on imports or exports, as already indicated, this is a source of revenue available only to the United States.

The term "excises" applies to taxes laid upon the manufacture, sale, or consumption of commodities within the country, upon the privilege of pursuing occupations, upon corporate privileges, and the like. The power of levying and collecting such taxes is exercised in a variety of ways. Thus manufacturers of alcohol and various forms of liquors containing alcohol, and manufacturers of tobacco in various forms, are required to . pay taxes on their manufactured products; sellers of such products, whether at wholesale or retail, are required to pay a license fee for the privilege of carrying on their business; license fees are also required for the privilege of conducting other forms of business or engaging in specified occupations; and when specific emergencies have necessitated special revenues, stamp duties have been imposed on various forms of commercial transactions. Some corporations chartered by the federal government, such as national banks, are required to pay taxes for the privilege of exercising their franchise rights; licenses are exacted from those who employ vessels in foreign commerce under the jurisdiction of the United States, or who engage in the coast trade, or in navigating the navigable waters; those who make use of the postal facilities afforded by the federal government are required to make compensation therefor as Congress in its discretion shall deem expedient; and in a variety of other ways persons who enjoy specific benefits from the operation of the federal government are compelled to contribute to its support. There seems to be no specific limitation on the power of the United States to levy excise taxes except that already indicated, viz., excises like duties and imposts must be uniform throughout the United States.

Since the acquisition by the United States of Porto Rico and the Philippines, serious difficulties have arisen in determining whether the requirement that "all duties, imposts and excises shall be uniform throughout the United States " (Art. I, § 8, ¶ 1), prevents the levying of duties on goods brought from such territorial possessions into the ports of the United States. It has been settled in the case of De Lima v. Bidzvell and the other insular cases, that since the final acquisition of such territorial possessions by treaty with Spain, they are not foreign territory, and that the ordinary tariff duties on goods brought into the United States from foreign countries are not applicable. This being so, the question is whether special provisions as to tariff duties on goods brought from those islands into the United States are valid under the clause just quoted as to uniformity. The view which seems to be generally accepted is that the uniformity provided for is uniformity as among the states, that is, throughout the territory of the states which originally constituted the United States, and those states which have since been admitted to the Union; and therefore that the requirement of uniformity does not apply to duties on goods exported from the insular possessions into the United States; and accordingly a tariff duty on exports from such insular possessions has been sustained. It would seem clear that the clause prohibiting the levying of duties by the United States on goods exported from any state (Art. I, § 9, ¶ 5) is not applicable to duties on exports from these insular possessions. In fact, the power of Congress to levy duties on imports and exports to and from such possessions in order to maintain territorial governments, and not to raise revenue for the support of the general government, is not derived from the taxing clause of the federal constitution but by implication from the power to "make all needful rules and regulations respecting the territory or other property belonging to the United States" (Art. IV, § 3, ¶ 2), and is not subject to the rule that duties, imposts, and excises shall be uniform.