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State or municipal quarantine regulations. The medical officers of the Marine Hospital Service and customs officers are required to aid in the enforcement of these rules. Provision is made to employ State quarantine officers in the national system, and to establish quarantines at ports where State or municipal ones do not exist, if necessary. Pains are taken that quarantine regula- tions under State laws shall not be interfered with. The details of the act are somewhat elaborate, and may be laid to provide a national system of quarantine supplementary to and concurrent with the system of the States. In the matter of the importation of infected cattle and hides, Congress has enacted health laws, more stringent than quarantine regulations, resembling the law of Missouri held unconstitutional in the case of Railroad Company v, Husen.^
Tha principal conclusion of this article may be stated as follows : A State may not constitutionally, in the exercise of its police power, either enter the domain of legislation, which, under the Constitution, exclusively belongs to Congress, or violate the prohibitions imposed by the Constitution upon the States. For example, it seems a State cannot constitu- tionally enact a law which operates, to the extent of regu- lation, directly and immediately upon foreign or interstate commerce itself. To this rule, quarantine laws, perhaps upon historical grounds, form an exception. In determining whether a statute is a regulation of commerce, the operation of the law, not the intention of the Legislature in passing it, is the test to be applied. State laws which operate upon foreign or interstate commerce only indirectly, secondarily, and remotely, are not unconstitutional as regulations of commerce. The power of Congress to regulate commerce with foreign nations and among the several States does not exclude commercial regulations of a subject by the States, except when the power of Congress is in exercise upon that subject. Congress may exercise its power of regulation either by express legislation, which, to the extent that the will of Congress is expressed, abrogates all conflicting State laws ; or by refraining from legislation, which, except in case of subjects to which a national uniform system of regulation is not appropriate, is to be taken generally as indicating the will of Con-
^ Supra^ p. 304- The acts of Congress are foand in 14 U. S. Stat, at Large, i, 3, or U. S, Rev. Stat. sa. 2493-2495 (1865 and x866).