While the revenues of the United States have usually been derived almost entirely from duties, imposts, and excises, as to which the rule of uniformity is applicable, the federal government is not limited to such sources for the raising of revenue. It is within the scope of its taxing power to levy per capita taxes, or direct taxes on property, but such taxes are required to be apportioned among the states in accordance with the population, for it was provided in the federal constitution as originally adopted that, "Representatives and direct taxes shall be apportioned among the several states which may be included within this union, according to their respective numbers, which shall be determined by adding to the whole number of free persons, including those bound to servitude for a term of years, and excluding Indians not taxed, three-fifths of all other persons" (Art. I, § 2, ¶ 3), and that, "No capitation or other direct tax shall be laid, unless in proportion to the census or enumeration hereinbefore directed to be taken" (Art. I, § 9, ¶ 4). The "census or enumeration" referred to in the latter provision has reference to the enumeration provided for in connection with the first provision which furnished a basis for determining the ratio of representation among the states The apportionment of representation has been changed (Amend. XIV, § 2), so that representatives are now "apportioned among the several states according to their respective numbers, counting the whole number of persons in each state, excluding Indians not taxed"; but it still remains true that direct taxes, and also capitation taxes, must be apportioned among the states on the same basis as representatives, that is, on the basis of enumeration of the population. Therefore, if the federal government should desire to raise revenue by a capitation tax, it would not be possible for it to do so by exacting, for instance, a fixed sum from each male person over twenty-one years of age within the limits of its jurisdiction; but it must determine how much each person shall pay by .apportioning the aggregate amount to be raised among the several states in accordance with their respective populations, as ascertained by the census, and then determine how much each person of the class of persons required to pay the capitation tax and residing in any particular state must pay in order to make up the total amount of tax apportioned to that state. Likewise, if the United States should desire to raise revenue by a direct tax on real property, it could not tax all the lands within the jurisdiction of the United States on the basis of value, nor on the basis of extent, that is, for instance, at so much per acre; but it must apportion among the several states in accordance with their population the total amount of the revenue to be raised by taxes upon real property and must then provide the rate at which real property in each of the several states shall be taxed in order to raise in each state the sum apportioned to that state.

The inconvenience and difficulty which would attend the levy of capitation and direct taxes in accordance with this method are so great that no successful effort has ever been made or probably ever will be made to raise revenue in this manner, although several times attempted. The object of these provisions, however, was evidently not to render such taxes impossible, but to preserve some proportionate relation between representation and taxation of this character. The discussion attending the incorporation of these provisions into the federal constitution seems to indicate a purpose to attach to the advantage which the states in which slavery existed were given by way of additional representation on account of including in their enumeration three-fifths of the slave population, a corresponding burden by way of imposition of a higher proportion of capitation and direct taxes. At the time of the adoption of the federal constitution, the elective franchise in states where slavery existed was limited to free white persons, and the result of giving such states representation on the basis of an enumeration of population which included three-fifths of the slave population was that the voters in those states had a larger proportional representation in Congress than had the voters in states not including any slave population. As capitation and direct taxes have not generally been resorted to as a source of revenue, the states which had the slave population did not actually suffer any detriment to correspond with the increased representation thus given to them; and since the amendment of the constitution abolishing slavery and apportioning representation in accordance with actual population, there seems to be no sufficient reason for perpetuating the requirement that capitation and direct taxes shall be apportioned among the states in accordance with population, rather than imposed on all persons and property within the limits of the United States in accordance with the rule of uniformity; but until the constitution shall be amended in this respect, the difficulties attending the levy of capitation and other direct taxes must remain.

Within comparatively recent years there has been a notable illustration of the practical effect of the provision requiring apportionment of direct taxes. In 1894 Congress provided for a tax on incomes, that is, a tax on individuals graduated on the basis of the income of each. This provision seems to have been adopted in order to supply anticipated deficiencies in import duties to result from a radical change of tariff schedules. But it was held by the Supreme Court of the United States in the Income Tax Case that a tax on incomes was in effect a tax on the ownership of the property from which the income was derived; and that whether this property was realty or personalty, such a tax was a direct tax and could only be levied on the basis of apportionment, and as the statute did not provide for apportionment it was invalid. The decision was, not that it was beyond the power of Congress to impose a tax upon incomes, but that such a tax must be distributed by apportionment among the states, and not determined simply on the plan of uniformity throughout the United States.