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314 HARVARD LAW REVIEW. decisions which the lapse of time and the change in judicial officers will often produce. The writ of error to a State court would be no protection to a contract if we were bound to follow the judgment which the State court had given, and which the writ of error brings up for revision here." (6) But there is an- other ground for coming into the courts of the United States. A case may come there, as this one has, not because of any ques- tion arising under the Constitution or laws of the United States, but simply because the plaintiff and defendant are citizens of dif- ferent States or countries. In such a case the court is adminis- tering the law of the State. In this sort of case the general rule is, that, since the court is applying the law of the State, it will follow, in determining what that law is and in construing it, the decisions of its highest court. If the question has not ever come up in the State court, or if there be no settled rule there, the United States court must, of course, decide for itself. But, even after such an independent decision has been made, if the highest court of the State should arrive at a different conclusion, the United States court wiU, in general, change from its own previous decision, and will adopt that of the State courts.* Nothing could more plainly mark the secondary character of the jurisdiction of United States courts in this region of it. But there are various qualifications of these doctrines. The most conspicuous of them is the principle of Swift v. Tyson, i6 Pet. I (1842), in which the novel and much-contested doctrine was laid down, that upon questions of what are called general commercial law, the courts of the United States did not under- take to follow the State courts. This declaration was not required for the decision of that case, but it has been followed, and is an established rule of the United States jurisprudence. Its sound- ness in point of principle is, perhaps, open to question; at any rate, it is undergoing much criticism at the present day. The same principle is laid down as regards the construction of ordi- nary language (Lane v, Vick, 3 How. 464, 476) ; but in that case there was a strong dissenting opinion of McKinley, J., concurred in by Taney, C. J. Again, when the United States court has already decided a question, and a later decision of the State differs from this, the United States court may at least wait awhile before chang- ^ Green v. Neal's Lessee, 6 Pet. 291 ; Carroll County Supervisors v. United States, 18 Wall. 71.