"As the Constitution of the United States embodies a federal union of political sovereignties whose separate existence is older than that of the Union itself, the simplest principle of analysis indicates the fact, that, in order fully to grasp the nature of the composite whole, it is first necessary to comprehend the nature of the units out of whose aggregation it arose. Any exhaustive investigation into the structure of our federal system must necessarily begin with the historical origin of the states that compose it."