Congressional Record–House, March 27 …Page 2580
…..The sixteenth amendment authorizes the taxation of income “from whatever source derived” – thus taking in investment income – “without apportionment among the several States.” The Supreme Court has held that the sixteenth amendment did not extend the taxing power of the United States to new or excepted subjects but merely removed the necessity which might otherwise exist for an apportionment among the States of taxes laid on income whether it be derived from one source or another. So the amendment made it possible to bring investment income within the scope of a general income-tax law, but did not change the character of the tax. It is still fundamentally an excise or duty with respect to the privilege of carrying on any activity or owning any property which produces income.
The income tax is, therefore, not a tax on income as such. It is an excise tax with respect to certain activities and privileges which is measured by reference to the income which they produce. The income is not the subject of the tax: it is the basis for determining the amount of tax.