In order, therefore, that the clauses cited from Article I of the Constitution may have proper force and effect save only as modified by the Amendment, and that the latter also may have proper effect, it is essential to distinguish between what is and what is not 'income' as the term is there used; and to apply the distinction as cases arise according to truth and substance without regard to form. "Congress by any definition it may adopt cannot conclude the matter, since it cannot by legislation alter the Constitution, from which it derives its power to legislate, and within whose limitations alone that power can be lawfully exercised" added