by treaty or from custom, need not establish rights and obligations for the other Powers, Still, by comparing the treaties that the Powers of Europe have made among themselves, one with another, we are led to certain principles that have been adopted almost universally by Powers that have made treaties on the same subject. Similarly with regard to custom: when a custom has been respected by the majority of the Powers of Europe, especially of the great Powers, its adoption by other Powers becomes easy, if it is at all applicable to them. As much cannot be said of express conventions. Still, it is often the case that a treaty made by two or more Powers serves as a model for treaties of the same kind to be made by other Powers. What is done by one Power in virtue of treaty is observed in others as custom. It may be that in certain matters the rule is founded on treaty for some States, and on custom for others.[1]
The importance assigned by Martens to treaties as an assured, though in itself imperfect, foundation of rights and obligations is the feature of his work that most emphatically commends it to the student of history. He alludes with special approval to those of his precursors, like Leibnitz,[2] who have based their knowledge and reasoning upon treaties and other public acts. His habit of mind and point of view are shown very clearly and strikingly in the classes of books that he holds to be necessary for those who study the positive Law of Nations; and his citation of works[3] still has considerable independent value. The following classes of books are, he thought, necessary: collections of treaties; collections of other public acts;[4]
