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24 the nature and extent of its own powers in the choice of means for giving effect to its constitutional prerogatives, and it has long stood as an accepted canon of judicial action, that judges should be very slow to oppose their opinions to the legislative will in cases in which it is not made demonstrably clear that there has been a plain violation of some unquestionable constitutional principle, or some explicit constitutional provision. Of encroachments upon state as well as of encroachments upon federal powers, the federal authorities are, however, in most cases the only, and in all cases the final, judges. The States are absolutely debarred even from any effective defense of their plain prerogatives, because not they, but the national authorities, are commissioned to determine with decisive and unchallenged authoritativeness what state powers shall be recognized in each case of contest or of conflict. In short, one of the privileges which the States have resigned into the hands of the federal government is the all-inclusive privilege of determining what they themselves can do. Federal courts can annul state action, but state courts cannot arrest the growth of congressional power.