opinion: Mr. Justice Brennan
  delivered the opinion of the Court.
  The Isbrandtsen Co., Inc., filed a petition in the United States Court of Appeals for the District of Columbia Circuit to review, under 5 U. S. C. § 1034, an order of the Federal Maritime Board approving a rate system proposed by the Japan-Atlantic and Gulf Freight Conference (the Conference). Under the proposed system a shipper would pay less than regular freight rates for the same service if he signs an exclusive-patronage contract with the Conference. Contract rates would be set at levels 9% percent below noncontract rates. The Court of. Appeals set aside the Board’s order on the ground that this system of dual rates was illegal per se under § 14 of the Shipping Act, 1916, 39 Stat. 733, as amended, 46 U. S. C. § 812 Third. We granted certiorari. 353 U. S. 908.
  The Conference is a voluntary association of 17 common carriers by water serving the inbound trade from Japan, Korea, and Okinawa to ports on the United States Atlantic and Gulf Coasts. Five of the carriers are American lines, eight are Japanese, and four are of other nationalities. The Conference presently operates under a Board-approved Conference Agreement made in 1934. Prior to World War II, the Conference had no direct liner competition and little tramp competition.
  After the war, Isbrandtsen entered the trade as the sole non-Conference line maintaining a regular berth service in the Japan-Atlantic trade. From 1947 to early 1949, Isbrandtsen operated from Japan to Atlantic Coast ports via the Suez Canal. Since 1949 Isbrandtsen has operated an approximately fortnightly service from Japan to United States Atlantic Coast ports via the Panama Canal as part of its Eastbound, Round-the-World Service.
  Although Conference membership is open to any common carrier regularly operating in the trade, Isbrandtsen has refused to join. Isbrandtsen’s practice, between 1947 and March 12, 1953, was to maintain rates at approximately 10 percent below the corresponding Conference rates. The general understanding of shippers and carriers in the trade was that Isbrandtsen underquoted Conference rates by 10 percent. This practice of undercutting Conference rates during the years 1950, 1951, and 1952, captured for Isbrandtsen 30 percent of the total cargo in the trade although Isbrandtsen provided only 11 percent of the sailings.
  Since outbound tonnage from the United States exceeds the inbound tonnage, the Japan-Atlantic and Gulf trade is presently overtonnaged, and both Isbrandtsen and Conference vessels have had substantial unused cargo space after loading cargoes in Japan. Total sailings in the trade rose from 109 in 1949 to more than 300.in 1953. (Cf. note 6.) The re-entry of the Japanese lines in the trade after World War II, four in 1951 and four in 1952, greatly contributed to the excess of tonnage. For the years 1951, 1952, and the first 6 months of 1953, the Japanese lines carried approximately 15 percent, 49 percent, and 66 percent, respectively, of the trade’s total liner cargo. For the years 1950, 1951, 1952, and the first 6 months of 1953, American flag lines, including Isbrandtsen but excluding two others, carried 53 percent, 46 percent, 34 percent, and 21 percent respectively.
  When, in late 1952, Isbrandtsen announced a plan to increase sailings from two to three or four sailings a month, the Conference foresaw a further increase in Isbrandtsen’s participation which, because of the nationalistic preference of Japanese shippers, would probably be at the expense of the non-Japanese Conference lines. To meet this outside competition the Conference first attempted, in November of 1952, a 10-percent reduction in rates, but Isbrandtsen answered with a reduction of its rates 10 percent under the Conference rates.
  On December 24, 1952, the Conference proposed the dual-rate system and filed its plan with the Board as required by the Board’s General Order 76, 46 CFR § 236.3, which permitted proposed rate changes to become effective after 30 days unless postponed by the Board on its own motion or on the protest of interested persons. Protests were filed by Isbrandtsen and the Department of Justice. The Secretary of Agriculture intervened as an interested commercial shipper opposed to the proposal. On January 21, 1953, the Board ordered a hearing on the protests but refused, pending the Board’s determination, to suspend operations of the dual-rate system. Isbrandtsen, therefore, filed a petition in the United States Court of Appeals for the District of Columbia Circuit for a stay of the Board’s order insofar as it authorized the Conference to institute the dual-rate system. The court announced on February 3, 1953, that the Board’s order would be stayed and the stay was entered on March 23, 1953.
  The Conference response to the stay was to open rates to allow each line to fix its own rates. At a meeting on March 12, 1953, the Conference voted to open Conference rates on 10 of the major commodities moving in the trade. The action was primarily directed at Isbrandt-sen’s competition; the Board found that “it was hoped that the rate war would lead to Isbrandtsen’s joining the Conference or to the institution of the dual rate system or other system.” On succeeding dates in the spring of that year, the Conference opened rates on most of the major items in the trade. In the resulting rate war, the level of rates dropped to about 80 percent and later to about 30 percent to 40 percent of the pre-March 12 rates. In some instances, rates fell below handling costs. Isbrandtsen attempted to keep on a competitive basis in the rate war but, when pegging of minimum rates in May did not improve its position, in July it set its rates at 50 percent of the pre-March 12 Conference rates. Since that date, Isbrandtsen has carried little cargo in the trade. Meanwhile the Board proceeded with the hearing and issued its report on December 14, 1955, followed on December 21, 1955, and January 11, 1956, by orders approving the proposed dual-rate system. The question for our decision is whether the Court of Appeals correctly set aside the Board’s orders.
  It has long been almost universal practice for American and foreign steamship lines engaging in ocean commerce to operate under conference arrangements and agreements. At least by 1913 it was recognized that such agreements might run counter to the policy of the antitrust laws; several cases were pending against foreign and domestic water carriers for alleged violations of the Sherman Act. The House Committee on Merchant Marine and Fisheries of the 62d Congress, of which committee Representative J. W. Alexander was Chairman, undertook an exhaustive inquiry into the practices of shipping conferences. The work of this Committee is set forth in two volumes of hearings, a volume of diplomatic and consular reports, and a fourth volume containing the Committee’s report, known as the Alexander Report. Contemporaneously a British inquiry was conducted by the Royal Commission on Shipping Rings. The Royal Commission’s report was available to the House Committee and was considered by it in formulating recommended legislation. See Hearings, at 369.
  Both inquiries brought to light a number of predatory practices by shipping conferences designed to give the conferences monopolies upon particular trades by forestalling outside competition and driving out all outsiders attempting to compete. The crudest form of predatory practice was the fighting ship. The conference would select a suitable steamer from among its lines to sail on the same days and between the same ports as the nonmember vessel, reducing the regular rates low enough to capture the trade from the outsider. The expenses and losses from the lower rates were shared by the members of the conference. The competitor by this means was caused to exhaust its resources and withdraw from competition.
  More sophisticated practices depended upon a tie between the conference and the shipper. The most widely used tie, because the most effective, was the system of deferred rebates. Under this system a shipper signed a contract with the conference exclusively to patronize its steamers, and if he did so during the contract term, and for a designated period thereafter, a rebate of a certain percentage of his freight payments was made to him at the end of the latter period. In this way, the shipper was under constant obligation to give his patronage exclusively to the conference lines or suffer the loss of the rebate, which often amounted to a considerable sum.
  But the Alexander Committee also found evidence of other predatory practices. Shippers who patronized outside competitors were denied accommodations for future shipments even at full rates of freight, or were discriminated against in the matter of lighterage and other services. Outside competition was also met by dual-rate contracts, by contracts with large shippers at lower rates for volume shipments, and by contracts with American railroads giving conference vessels preference in the handling of cargoes at the docks, and delivering through shipments of freight to conference vessels. Report, at 287-293.
  The Alexander Committee recommended against a flat prohibition of shipping combinations because it found that the restoration of unrestricted competition among carriers would operate against the public interest by depriving American shippers of desirable advantages of conference arrangements honestly and fairly conducted. The Committee mentioned advantages such as “greater regularity and frequency of service, stability and uniformity of rates, economy in the cost of service, better distribution of sailings, maintenance of American and European rates to foreign markets on a parity, and equal treatment of shippers through the elimination of secret arrangements and underhanded methods of discrimination.” Id., at 416. The Committee believed that these advantages could be preserved “only by permitting the several lines in any given trade to cooperate through some form of rate and pooling arrangement under Government supervision and control,” ibid., and further “that the disadvantages and abuses connected with steamship agreements and conferences as now conducted are inherent, and can only be eliminated by effective government control; and it is such control that the Committee recommends as the means of preserving to American exporters and importers the advantages enumerated, and of preventing the abuses complained of.” Id., at 418.
  In passing the Shipping Act of 1916, 39 Stat. 728, 733, as amended, 46 U. S. C. § 812 Third, Congress followed the basic recommendations of the Alexander Committee. The Act does not forbid shipping conferences in foreign commerce but requires all conference agreements covering the subjects mentioned in § 15 to be submitted for Board approval. No power to fix rates is granted to the Board. Subject to familiar limitations, the power vested in the Board is to approve agreements not found to be unjustly or unfairly discriminatory in violation of §§16 and 17 or otherwise in violation of the Act. Approved agreements are exempted from the antitrust laws.
  But it must be emphasized that the freedom allowed conference members to agree upon terms of competition subject to Board approval is limited to the freedom to agree upon terms regulating competition among themselves. The Congress in § 14 has flatly prohibited practices of conferences which have the purpose and effect of stifling the competition of independent carriers. Thus the deferred-rebate system (§14 First) and the fighting ship (§14 Second) are specifically outlawed. Similarly, § 14 Third prohibits another practice, common in 1913: to “[rjetaliate against any shipper by refusing . . . space accommodations when such are available . . that prohibition, moreover, is enlarged to condemn retaliation not only when taken “because such shipper has patronized any other carrier” but also when taken because the shipper “has filed a complaint charging unfair treatment, or for any other reason.” (Emphasis added.)
  But in addition to these specifically proscribed abuses, Congress, as previously noted, was aware that other devices — some known but not so widely used, and others that might be contrived — might be employed to achieve the same results. Therefore, coordinate with these three clauses aimed at specific practices, a fourth category, couched in general language, was added: “resort to other discriminating or unfair methods . . . In the context of § 14 this clause must be construed as constituting a catchall clause by which Congress meant to prohibit other devices not specifically enumerated but similar in purpose and effect to those barred by § 14 First, Second, and the “retaliate” clause of § 14 Third.
  The reason the “resort to” clause was added to the statute as an independent prohibition of practices designed to stifle outside competition is revealed in the Alexander Report. From information contained in the Report of the British Royal Commission and a communication from a major New York carrier organization, the Alexander Committee was aware that the outlawing of the deferred-rebate system would lead conferences to adopt a contract system to accomplish the same result. The British Royal Commission believed that ties to shippers were justified and that the abuses of the deferred-rebate system should be tolerated in the interest of achieving a strong conference system. Hearings, 369-381. However, the Alexander Committee, and the Congress in adopting the Committee’s proposals, reached a different conclusion. Congress was unwilling to tolerate methods involving ties between conferences and shippers designed to stifle independent carrier competition. Thus Congress struck the balance by allowing conference arrangements passing muster under §§ 15, 16, and 17 limiting competition among the conference members while flatly outlawing conference practices designed to destroy the competition of independent carriers. Ties to shippers not designed to have the effect of stifling outside competition are not made unlawful. Whether a particular tie is designed to have the effect of stifling outside competition is a question for the Board in the first instance to determine.
  Since the Board found that the dual-rate contract of the Conference was “a necessary competitive measure to offset the effect of non-conference competition” required “to meet the competition of Isbrandtsen in order to obtain for its members a greater participation in the cargo moving in this trade,” it follows that the contract was a “resort to other discriminating or unfair methods” to stifle outside competition in violation of § 14 Third.
  The Board argues, however, that Congress, although aware of the use of such contracts, did not specifically outlaw them and therefore implicitly approved them. But the contracts called to the attention of Congress bear little resemblance to the contracts here in question. Those joint contracts were described by the Alexander Committee as follows:
  “Such contracts are made for the account of all the lines in the agreement, each carrying its proportion of the contract freight as tendered from time to time. The contracting lines agree to furnish steamers at regular intervals and the shipper agrees to confine all shipments to conference steamers, and to announce the quantity of cargo to be shipped in ample time to allow for the proper supply of tonnage. The rates on such contracts are less than those specified in the regular tariff, but the lines generally pursue a policy of giving the small shipper the same contract rates as the large shippers, i. e. are willing at all times to contract with all shippers on the same terms.” Report, at 290.
  These contracts were very similar to ordinary requirements contracts. They obligated all members of the Conference to furnish steamers at regular intervals and at rates effective for a reasonably long period, sometimes a year. The shipper was thus assured of the stability of service and rates which were of paramount importance to him. Moreover, a breach of the contract subjected the shipper to ordinary damages.
  By contrast, the dual-rate contracts here require the carriers to carry the shipper’s cargo only “so far as their regular services are available”; rates are “subject to reasonable increase” within two calendar months plus the unexpired portion of the month after notice of increase is given; “[e]ach Member of the Conference is responsible for its own part only in this Agreement”; the agreement is terminable by either party on three months’ notice; and for a breach, “the Shipper shall pay as liquidated damages to the Carriers fifty percentum (50%) of the amount of freight which the Shipper would have paid had such shipment been made in a vessel of the Carriers at the Contract rate currently in effect.” Until payment of the liquidated damages the shipper is denied the reduced rate, and if he violates the agreement more than once in 12 months, he suffers cancellation of the agreement and the denial of another until all liquidated damages have been paid in full. Thus under this agreement not only is there no guarantee of services and rates for a reasonably long period, but the liquidated-damages provision bears a strong resemblance to the feature which Congress particularly objected to in the outlawed deferred-rebate system. Certainly the coercive force of having to pay so large a sum of liquidated damages ties the shipper to the Conference almost as firmly as the prospect of losing the rebate. It would be anomalous for Congress to strike down deferred rebates and at the same time fail to strike down dual-rate contracts having the same objectionable purpose and effect. Events have proved the accuracy of the prediction that the outlawing of the deferred-rebate system would lead conferences to adopt a contract system, as here, specially designed to accomplish the same result.
  It is urged that our construction “produces a flat and unqualified prohibition of any discrimination by a carrier for any reason” and converts the rest of the statute into surplusage. But that argument overlooks the revealed congressional purpose in § 14 Third. That purpose, as we have said, was to outlaw practices in addition to those specifically prohibited elsewhere in the section when such practices are used to stifle the competition of independent carriers. The characterizations “unjustly discriminatory” and “unjustly prejudicial” found in other sections (§§ 15, 16 and 17) imply a congressional intent to allow some latitude in practices dealt with by those sections, but the practices outlawed by the “resort to” clause of § 14 Third take their gloss from the abuses specifically proscribed by the section; that is, they are confined to practices designed to stifle outside competition.
  Petitioners argue that our construction of § 14 Third is foreclosed by this Court’s decisions in United States Navigation Co. v. Cunard S. S. Co., 284 U. S. 474, and Far East Conference v. United States, 342 U. S. 570. A reading of those opinions immediately refutes any suggestion either that this issue was expressly decided in those cases or that our holding here is not fully consistent with the disposition of those cases. In Cunará the petitioner had filed a complaint in the District Court alleging that respondents had conspired to maintain “a general tariff rate and a lower contract rate, the latter to be made available only to shippers who agree to confine their shipments to the lines of respondents.” 284 U. S., at 479. The differentials were alleged to be unrelated to volume or regularity of shipments, but to be wholly arbitrary and unreasonable and designed “for the purpose of coercing shippers to deal exclusively with respondents and refrain from shipping by the vessels of petitioner, and thus exclude it entirely from the carrying trade between the United States and Great Britain.” Id., at 480. An injunction was sought under the Sherman and Clayton Acts. The Court held that the questions raised by this complaint were within the primary jurisdiction of the Shipping Board and therefore the courts could not entertain the suit until the Board had considered the matter. In Far East Conference the Court similarly held that the Board’s primary jurisdiction' precluded the United States from bringing antitrust proceedings against a shipping-conference maintaining dual rates.
  The Board and the Conference argue that, if the Court in these earlier cases had thought that § 14 Third in anyway makes dual rates per se illegal and thus not within the power of the Board to authorize, it would not have found it necessary to require that the Board first pass upon the claims. But in the Cunará case the Court said:
  “Whether a given agreement among such carriers should be held to contravene the act may depend upon a consideration of economic relations, of facts peculiar to the business or its history, of competitive conditions in respect of the shipping of foreign countries, and of other relevant circumstances, generally unfamiliar to a judicial tribunal, but well understood by an administrative body especially trained and experienced in the intricate and technical facts and usages of the shipping trade; and with which that body, consequently, is better able to deal.” 284 U. S., at 485.
  Similarly, in the Far East Conference case:
  “The Court [in Cunará] thus applied a principle, now firmly established, that in cases raising issues of fact not within the conventional experience of judges or cases requiring the exercise of administrative discretion, agencies created by Congress for regulating the subject matter should not be passed over. This is so even though the facts after they have been appraiseá by specializeá competence serve as a premise for legal consequences to be juáicially áefmeá. Uniformity and consistency in the regulation of business entrusted to a particular agency are secured, and the limited functions of review by the judiciary are more rationally exercised, by preliminary resort for ascertaining and interpreting the circumstances underlying legal issues to agencies that are better equipped than courts by specialization, by insight gained through experience, and by more flexible procedure.” 342 U. S., at 574-575. (Emphasis added.)
  It is, therefore, very clear that these cases, while holding that the Board had primary jurisdiction to hear the case in the first instance, did not signify that the statute left the Board free to approve or disapprove the agreements under attack. Rather, those cases recognized that in certain kinds of litigation practical considerations dictate a division of functions between court and agency under which the latter makes a preliminary, comprehensive investigation of all the facts, analyzes them, and applies to them the statutory scheme as it is construed. Compare Denver Union Stock Yard Co. v. Producers Livestock Marketing Assn., ante, p. 282. It is recognized that the courts, while retaining the final authority to expound the statute, should avail themselves of the aid implicit in the agency’s superiority in gathering the relevant facts and in marshaling them into a meaningful pattern. Cases are not decided, nor the law appropriately understood, apart from an informed and particularized insight into the factual circumstances of the controversy under litigation.
  Thus the Court’s action in Cunará and Far East Conference is to be taken as a deferral of what might come to be the ultimate question — the construction of § 14 Third — rather than an implicit holding that the Board could properly approve the practices there involved. The holding that the Board had primary jurisdiction, in short, was a device to prepare the way, if the litigation should take its ultimate course, for a more informed and precise determination by the Court of the scope and meaning of the statute as applied to those particular circumstances. To have held otherwise would, necessarily, involve the Court in comparatively abstract exposition.
  This consideration, moreover, is particularly compelling in light of our present holding. Since, as we hold, § 14 Third strikes down dual-rate systems only where they are employed as predatory devices, then precise findings by the Board as to a particular system’s intent and effect would become essential to a judicial determination of the system’s validity under the statute. In neither Cunará nor Far East Conference did the Court have the assistance of such findings on which to base a determination of validity. We conclude, therefore, that the present holding is not foreclosed by these two cases.
  Finally, petitioners argue that this Court should not construe the Shipping Act in such a way as to overturn the Board’s consistent interpretation. “[T]he rulings, interpretations and opinions of the [particular agency] . . . , while not controlling upon the courts by reason of their authority, do constitute a body of experience and informed judgment to which courts and litigants may properly resort for guidance. The weight of such a judgment in a particular case will depend upon the thoroughness evident in its consideration, the validity of its reasoning, its consistency with earlier and later pronouncements, and all those factors which give it power
  to persuade, if lacking power to control.” Skidmore v. Swift & Co., 323 U. S. 134, 140. But we are here confronted with a statute whose administration has been shifted several times from one agency to another, and it is by no means clear that the Board and its predecessors have taken uniform and consistent positions in regard to the validity of dual-rate systems under § 14 Third. See Isbrandtsen Co. v. United States, 96 F. Supp. 883, 889-891. In view of the fact that in the present case the dual-rate system was instituted for the purpose of curtailing Isbrandtsen’s competition, thus becoming a device made illegal by Congress in § 14 Third, we need not give controlling weight to the various treatments of dual rates by the Board under different circumstances.
  Affirmed.
  4 F. M. B. 706. The Federal Maritime Board and its predecessors are hereinafter referred to as “the Board.” Its predecessors were the United States Shipping Board (1916 to 1933); the United States Shipping Board Bureau in the Department of Commerce (1933 to 1936); and the United States Maritime Commission (1936 to 1950).
  The Federal Maritime Board was named a respondent in Is-brandtsen’s petition. The United States was also named as statutory respondent pursuant to 5 U. S. C. § 1034 but, appearing by the Department of Justice, joined Isbrandtsen in attacking the Board order. The Secretary of Agriculture intervened and joined in the Justice Department's brief. The Conference intervened by leave of the court. The same parties are before this Court.
  99 U. S. App. D. C. 312, 239 F. 2d 933.
  Section 14 provides:
  “No common carrier by water shall, directly or indirectly, in respect to the transportation by water of passengers or property between a port of a State, Territory, District, or possession of the United States and any other such port or a port of a foreign country — ■
  “First. Pay or allow, or enter into any combination, agreement, or understanding, express or implied, to pay or allow a deferred rebate to any shipper. The term ‘deferred rebate’ in this chapter means a return of any portion of the freight money by a carrier to any shipper as a consideration for the giving of all or any portion of his shipments to the same or any other carrier, or for any other purpose, the payment of which is deferred beyond the completion of the service for which it is paid, and is made only if, during both the period for which computed and the period of deferment, the shipper has complied with the terms of the rebate agreement or arrangement.
  “Second. Use a fighting ship either separately or in conjunction with any other carrier, through agreement or otherwise. The term ‘fighting ship’ in this chapter means a vessel used in a particular trade by a carrier or group of carriers for the purpose of excluding, preventing, or reducing competition by driving another carrier out of said trade.
  “Third. Retaliate against any shipper by refusing, or threatening to refuse, space accommodations when such are available, or resort to other discriminating or unfair methods, because such shipper has patronized any other carrier or has filed a complaint charging unfair treatment, or for any other reason.
  “Fourth. Make any unfair or unjustly discriminatory contract with any shipper based on the volume of freight offered, or unfairly treat or unjustly discriminate against any shipper in the matter of (a) cargo space accommodations or other facilities, due regard being had for the proper loading of the vessel and the available tonnage; (b) the loading and landing of freight in proper condition; or (c) the adjustment and settlement of claims.
  “Any carrier who violates any provision of this section shall be guilty of a misdemeanor punishable by a fine of not more than $25,000 for each offense.”
  Isbrandtsen’s vessels are not equipped with refrigerated space or silkrooms, as are many of the Conference vessels, and do not compete for cargoes requiring these facilities.
  The comparative sailings and carryings are indicated in the following table:
  On January 21, 1954, the Court of Appeals handed down its final decision holding that § 15 of the Shipping Act required the Board to hold a hearing on the proposed dual-rate system before approval. 93 U. S. App. D. C. 293, 211 F. 2d 51.
  The Board did modify the exclusive-patronage contracts to delete from their coverage refrigerated cargoes for which Isbrandtsen did not compete.
  Proceedings of the House Committee on the Merchant Marine and Fisheries in the Investigation of Shipping Combinations under House Resolution 587, Hearings, 62d Cong. (Hereinafter “Hearings.”)
  H. R. Doc. No. 805, 63d Cong., 2d Sess. (Hereinafter “Report.”)
  H. R. Rep. No. 659, 64th Cong., 1st Sess. 27; see S. Rep. No. 689, 64th Cong., 1st Sess. 7. The Alexander Report was submitted in 1914 to the 63d Congress and a bill to carry out its recommendations was introduced but not passed. H. R. 17328, 63d Cong., 2d Sess. In the following Congress substantially the same bill was reintroduced, H. R. 15455, 64th Cong., 1st Sess., and became the Shipping Act of 1916.
  Section 15 provides:
  “Every common carrier by water, or other person subject to this chapter, shall file immediately with the Federal Maritime Board a true copy, or, if oral, a true and complete memorandum, of every agreement, with another such carrier or other person subject to this chapter, or modification or cancellation thereof, to which it may be a party or conform in whole or in part, fixing or regulating transportation rates or fares; giving or receiving special rates, accommodations, or other special privileges or advantages; controlling, regulating, preventing, or destroying competition; pooling or apportioning earnings, losses, or traffic; allotting ports or restricting or otherwise regulating the number and character of sailings between ports; limiting or regulating in any way the volume or character of freight or passenger traffic to be carried; or in any manner providing for an exclusive, preferential, or cooperative working arrangement. The term 'agreement’ in this section includes understandings, conferences, and other arrangements.
  “The Board may by order disapprove, cancel, or modify any agreement, or any modification or cancellation thereof, whether or not previously approved by it, that it finds to be unjustly discriminatory or unfair as between carriers, shippers, exporters, importers, or ports, or between exporters from the United States and their foreign competitors or to operate to the detriment of the commerce of the United States, or to be in violation of this chapter, and shall approve all other agreements, modifications, or cancellations.
  “Every agreement, modification, or cancellation lawful under this section shall be excepted from the provisions of [the Antitrust Acts] . . . .” 39 Stat. 733, as amended, 46 U. S. C. § 814.
  Both the section which became § 14 Third and the section which became § 15, as originally proposed, used the language “discriminating or unfair.” H. R. 17328, 63d Cong., 2d Sess. The bill which became the Shipping Act, H. R. 15455, 64th Cong., 1st Sess., substituted “unjustly discriminatory or unfair” in § 15 but left untouched “discriminating or unfair” in § 14 Third.
  The Board estimated that Isbrandtsen would lose approximately two-thirds of its 1952 volume. “. . . [I]t [is] probable that Is-brandtsen will retain 10 percent or more of the cargo moving in the trade as against the 26 percent carried by it in 1952 . . . .” 4 F. M. B. 706, 737, 1956 Am. Mar. Cas. 414, 451.
  The Court of Appeals made a partial application of the rule of ejusdem generis and related the “resort to” clause to retaliation, holding the dual-rate contract or suit was retaliatory and within the ban of the section. The Board urges that the Court of Appeals did not carry the rule of ejusdem generis far enough, that by carrying the rule “a hand’s breadth farther” and also relating- — and limiting— the “resort to” clause to the refusal of space accommodations and similar services to shippers, the dual-rate contract falls without the prohibition because the contract is concerned only with charges for services and not with denial of services. We do not believe that these constructions can be reconciled with the language of the statute or the scope of the congressional plan.
  Certainly it must be assumed that the Court would refrain from settling sub silentio an issue of such obvious importance and difficulty plainly requiring a clearly expressed disposition.
  Petitioners’ reliance on Swayne & Hoyt, Ltd., v. United States, 300 U. S. 297, is similarly misplaced. In that case the Court upheld the administrative determination that a dual-rate system gave an “undue or unreasonable preference or advantage” under § 16 of the Shipping Act. Because the Court sustained the finding as supported by substantial evidence it did not need to reach the more contentious problem of whether that particular contract was illegal under § 14 Third.
  Compare, e. g., Eden Mining Co. v. Bluefields Fruit & S. S. Co., 1 U. S. S. B. 41, and Contract Routing Restrictions, 2 U. S. M. C. 220, 226-227, with W. T. Rawleigh Co. v. Stoomvart, 1 U. S. S. B. 285, 290.