“In examining the division of powers, as established by the Federal Constitution, remarking on the one hand the portion of sovereignty which has been reserved to the several States, and on the other, the share of power which has been given to the Union, it is evident that the Federal legislators entertained very clear and accurate notions respecting the centralization of government. The United States form not only a republic, but a confederation; yet the national authority is more centralized there than it was in several of the absolute monarchies of Europe....”