There remains the question as to enjoining criminal prosecutions. Should the jurisdiction of a court of equity be ousted simply because the state has authorized its officers to enforce unconstitutional regulations affecting property rights by a criminal instead of a civil action? The Supreme Court has held that, notwithstanding the general principle that a court of equity has no jurisdiction of a bill to stay criminal proceedings, it may nevertheless enjoin a state officer from instituting such proceedings where property rights are about to be invaded and destroyed through the instrumentality of an unconstitutional statute providing for its enforcement by criminal proceedings. The nature of an essentially civil question or controversy, such as one between shippers or passengers on the one side and a railroad company on the other as to the reasonableness of rates, cannot be changed by legislative fiat. The exercise of such a jurisdiction to restrain criminal proceedings has been found necessary in many recent cases where a defense on a criminal trial before a jury would afford no fair or adequate protection to those whose property rights were affected. The litigation, for example, under a bill in equity to restrain the enforcement of an unconstitutional criminal statute regulating rates presents a controversy of a civil nature with the officer and not with the state, and the only question is, whether a court of equity should intervene, or should leave those against whom criminal proceedings are threatened to their defense by demurrer to the indictment or trial on the merits. The latter will always be done when a defense at law will afford reasonably fair and adequate protection. But when a defense at law will not afford due protection and irreparable injury to property is threatened, there exists no reason why a court of equity should not intervene in such a case and grant protection and relief. It may seem to many doubtful whether the two leading cases which are now attracting so much attention, namely, In re Ayers[43] and Fitts _vs._ McGhee,[44] necessarily presented any question under the eleventh amendment, and whether they should not have been disposed of solely upon the ground that a court of equity ought not to have enjoined the threatened suits or prosecutions. Probably neither of the suits in equity discussed in these two cases would have been maintainable under the general principles of equity jurisprudence even if the state had been suable in a court of the United States, for no irreparable injury was threatened and the opportunity of defense at law seemed to afford reasonable protection.