delivered the opinion of the Court:
It may be that the construction placed by the court below upon the section 1638 of the code as excluding suits for divorce from its purview was too narrow and illiberal. But without passing in any manner upon that question, we are of the opinion that, under the act of Congress of June 30, 1902, it is our duty to reverse the decree appealed from and to remand the cause for further proceedings according to law.
Under our system of jurisprudence, the subject of divorce, as is well understood, is entirely a matter of statutory cognizance. It is unknown to the common law; and, independently of statutory enactment, it is unknown in equity. The right of divorce, so far as it may be regarded as a right, is wholly the creation of statute; and the legislative authority, which created it, may likewise take it away without violating any vested .right or impairing the force of any natural principle of justice. Of course, the efficacy of decrees or judgments' of the courts granted under the provisions of statutory enactments in force at the time at which such decrees or judgments were rendered cannot be impaired or affected by subsequent legislation on the subject antagonistic to the previously existing law. And it may safely be said, although the point is not necessary to the decision of the present case, that it is not within the scope of the legislative authority in our country to undo that which the courts have done by decree final and absolute pronounced under previous law. And we may also say, with special reference to the case before us, that, if the decree in the case had been permitted to become absolutely final, however erroneous it *449might have been, there would have been no valid authority either in Congress or the courts to direct its reopening. Under such circumstances vested rights would have accrued; but except under such circumstances it is within the province of the legislative authority to ordain upon the subject as it pleases, to make, to alter, or to abolish law, and to change the course of procedure in regard thereto at any time. And if there is no reservation in regard to existing conditions or to existing suits, these must conform forthwith to. the new legislation. These are statements of the law which cannot well be questioned or controverted.
Now, such being the law and such the power of Congress over the subject of divorce in this District, it is clear to us, that, under the amendatory act of June 30, 1902¿ we can proceed with and dispose of the case before us only under the provisions of the old law and without reference to the new law as enacted in the code. Such is the specific requirement of the act of June 30, 1902. The word " mayj’ as used in that act, according to well established rules of construction of statute law, must be construed to mean “ shall ” so far as the action of the courts is concerned, although the permissive signification of the term is to be retained so far as respects the action of suitors. For it was not intended, of course, that they should be compelled to proceed, either under the old or the new law, whether they wished so to do or not; but it was undoubtedly the intention of Congress that, if they elected to proceed, the further action of the court in their cases should be under the old law, which was by necessary implication continued in force for that purpose. Now, the act of June 30, 1902, is imperative upon all courts alike; and this court is as much required to give effect to it as is the court below in any case yet pending before it to which the act is applicable. And we could not give effect to it by an affirmance of the decree in this, cause, assuming of course, that the decree was correct, upon which question, as we have said, we decline under the circumstances to pass. For that decree beyond question was based upon the new law and not upon the old. We can only give *450effect to the act of June 30, 1902, by reversing the decree and remanding the cause to the court below for further proceedings in accordance therewith. The petition in this case was pending on December 31, 1901; and the act of June 30, 1902, is therefore by its express terms applicable ten it.
It is true that a construction of a statute which will give to the statute a retroactive effect is not favored. But there are statutes which are intended to have a retroactive effect' and which serve a useful purpose in that regard; and when they violate no vested rights — and the act of Congress of June 30, 1902, as applied to the present case and to cases pending in the court below on December 31, 1901, and not finally disposed of at the date of its passage, certainly violates no vested right — such statutes must be upheld. The act of June 30 1902, is in terms to a certain extent retroactive;- for it seeks to affect legal proceedings instituted under a different condition of the law; but it is not for that reason objectionable, in view of the complete control of the subject of divorce which appertains to the legislative authority.
It is of no importance in our opinion that there was here a decree of the lower court finally and completely disposing of the case so far as that court could dispose of it. That decree was suspended in its operation by the appeal to this court; and the cause was still pending undisposed of, and subject to the operation of Congressional legislation on the matter of divorce. If, for any reason, the court below, after sustaining the demurrer in this case, had postponed the entry of a final decree dismissing the petition, and in the interval the act of Congress of June 30, 1902, had been enacted, we apprehend that it would have been the duty of that court, upon the application of the parties, forthwith to-rescind its action on the demurrer, and to proceed with the cause as directed by the act. This should have been the result, whether the act of June 30, 1902, is to be regarded as entirely new legislation, or whether it is to be considered as a legislative interpretation of the previously enacted section 1638 of the code.
*451Of course, what we say here is to be restricted rigidly and exclusively to the subject of divorce and to Congressional legislation in regard thereto. Our decision is not to be regarded as applicable in any other cases, civil or criminal, where the subject-matter is not so completely under the control of Congress as is that of divorce.
Adjudicated cases on the question under consideration, naturally enough, are not easily to be found; and we do not know that any are needed in support of our decision, when the statute seems to- be so plain. But there have been numerous analogous cases, wherein decrees and judgments, correct in themselves when rendered, have been reversed for some supervening cause accruing afterwards. See Secretary v. McGarrahan, 9 Wall. 298; United States v. Boutwell, 17 Wall. 604; Commissioners v. Sellew, 99 U. S. 624; United States v. Schurz, 102 U. S. 378; Thompson v. United States, 103 U. S. 480; United States v. Chandler, 122 U. S. 643; United States v. Lamont, 155 U. S. 303; United States v. Lochren, 164 U. S. 701; Warner Valley Stock Company v. Smith, 165 U. S. 28; United States v. Butterworth, 169 U. S. 600. It is true that all these cases, except one, were suits for mandamus to compel the incumbent of an office to perform some alleged ministerial duty. The exceptional one, the case of Warner Valley Stock Company v. Smith, was a proceeding in equity for an injunction. In all of them it was held that, upon the resignation or withdrawal from office of the person against whom the proceeding was directed, the cause was abated by reason of such resignation or withdrawal, notwithstanding that the resignation or withdrawal supervened after decree or judgment rendered and while appeal was pending to a higher tribunal. In most or all of them, of course, the judgment or decree could not have been enforced, even if it had been affirmed. But they all illustrate the rule that, for causes supervening after appeal taken, the appellate court, without reference to the question of the correctness of the judgment or decree appealed from, may reverse the judgment or decree, and remand the cause. In the case of Warner Valley Stock Company v. Smith, and *452in that of United States v. Butterworth, that was the course pursued by the Supreme Court of the United States. We perceive no difference in principle between abatement resulting from the resignation or withdrawal of an officer and the prohibition of an act of Congress in a matter over which Congress has practically unlimited control.
It is our conclusion that, for the reasons stated, the decree appealed from should be reversed, and the cause remanded to the Supreme Court of the District of Columbia, with directions to vacate such decree, and for such further proceedings therein according to law as may be right and just and consistent with the more recent legislation of Congress. And it is so ordered.
The Chief Justice concurs in the result, but for different reasons than those therein stated.