It is impossible to deny that the early rule of international law was that the head of state, either directly or through his agents, was alone competent to make treaties, which were binding upon his successors. This was natural at a time when no type of international agreement was known other than the treaty in solemn form to which monarchs were parties. Today, new types of agreement have come into being, to which the parties are not heads of states but either the state itself (as in the Treaty of Versailles, 1919) or governments or departments of state. In all these cases, and even in cases where the parties are formally the heads of states, the unit now considered to be bound is the state, through its organs. This substitution of states for monarchs as the subjects of the law of nations, at any rate in the matter of treaties, has been brought about very largely by the French and American Revolutions of the eighteenth century, and by the development of the notion of the state as an international person. The question of the competence to make treaties binding on states, who may by their laws have limited that competence, has therefore become one of great interest in modern theory.