67. So, too, where a labor organization sought by a boycott to prevent the manufacture of articles intended for interstate commerce, and to prevent the re-selling of these articles in other States, the combination and plan were held to be restraint of commerce and in violation of the Anti-Trust act.[179] The cases strongly suggest that federal laws to regulate commerce may be essentially police regulations as, notably, laws requiring safety appliances on railroad trains and steamboats; laws regulating hours of labor and child labor; laws requiring arbitration of controversies between employers and employees operating in interstate commerce; the pure food law; the exclusion of lottery tickets from the mails, and the like. The Constitution contains no clause explicitly delegating the police power to the United States, and the exercise of police power by Congress has thus far been quite without exception under the commerce clause. Yet by parity of reasoning, the police power may be included under the power to declare war. 68. There is such a thing as the peace of the United States.[180] The enormous power of Congress under the commerce clause has undoubtedly promoted that peace: “domestic tranquillity” is one of the specified purposes in ordaining and establishing the Constitution. As absence of power to regulate commerce marked the weakness of the Articles of Confederation, so the special inclusion of that power among those delegated to Congress marks the strength of the Constitution. 69. Within their respective jurisdictions the United States and the several States have power to regulate commerce. The power over commerce, in either jurisdiction, is exercisable within the principle of self-preservation. Whatsoever exercise of this power is essential to the existence of either government belongs to that government and cannot be repugnant to the other, that is, under the dual system of American constitutional government. Simple as this principle may seem, its practical application in defining the two jurisdictions, or the authority of either government, involves all the issues in American constitutional law, and the decisions of the American judiciary in cases arising under the commerce clause of the Constitution. A notable instance of the authority given by the commerce clause is the power of Congress, over the transportation of the mails, to prevent “any unlawful and forcible interference” with them. “The strong arm of the government may be put forth to brush away all obstructions to the freedom of interstate commerce or the transportation of the mails”; “the United States have a property in the mails.” The contents of the mail-bags—that is, matter, lawfully mailable—are commerce in the sense in which that word is used in the Constitution.