— This is an action to recover a balance of $350 for goods, wares and merchandise sold and delivered to defendant. The answer pleaded in bar a special contract. It was therein alleged and admitted *375by the replication, that, in the month of September, 1890, the plaintiff contracted with defendant to furnish and place in the school building of the latter two number 71 hot-air furnaces which should be of sufficient capacity to heat said building to seventy degrees Fahrenheit when the temperature outside was zero. The answer further alleged, and the replication denied, that the plaintiff guaranteed that said furnaces should be complete with all necessary connections to heat said building, and that they should be free from emitting gas and smoke into said building, for which the defendant was to pay $850. The answer set forth in minute detail wherein the furnaces failed to come up to the requirements of the contract it alleged. The replication on the other hand charged that the reason why the furnaces did not heat the building was on account of the neglect and fault of the defendant, which was quite elaborately therein specified. The defendant had judgment and the plaintiff appealed.
At the very threshold of the trial the court committed an error, afterwards repeated and emphasized in the giving of an instruction for the plaintiff, which we think is fatal to the judgment. The court over the objections of the plaintiff permitted the defendant, upon whom under the issue made by the pleadings the burden of proof was primarily cast, to introduce in evidence a written proposition made by plaintiff to the defendant and by the latter accepted, to the effect: First. That it would place in defendant’s school building two number 61 furnaces complete, set in brick with all proper connections necessary to heat said building for the sum of $440. Second. That it would guarantee that said furnaces should be free from emitting gas or smoke into the building; and third, that said furnaces would heat said building to seventy degrees Fahrenheit when the temperature would be zero; The introduction *376of this contract in evidence did not tend to prove that alleged in the answer. Evidence showing a contract to deliver two number 61 furnaces for $440 with the specific guarantees therein enumerated to be free from emitting gas and smoke, and of a certain heating capacity, would not in the least tend to prove the contract alleged in the answer for the delivery of two number 71 furnaces for $850 with guarantees therein specified. There is a wide variation between the two.
It is insisted that the parol contract for the delivery of the two number 71 furnaces for $850 was made after that in writing which was admitted in evidence, and that the former is but a modification of the latter. No doubt this is a correct view of the matter, but the diffi- • culty is that the answer pleads no such written contract and parol modification thereof. It is the law of this state that parties by a parol agreement upon sufficient consideration may modify or change the terms of a written contract. Bunce v. Beck, 43 Mo. 266; Henning v. Ins. Co., 47 Mo. 425. But when they do so they must declare on the agreement as it stands modified. This is usually done by setting out the original agreement and the modification of it. Every substantive fact which the plaintiff must prove to maintain his action should under the practice act be alleged so that an issue can be made'thereon. Harrison v. Railroad, 50 Mo. App. 332; Lanitz v. King, 93 Mo. 513. Here the allegata and probata do not correspond. It is necessary to allege the original contract’and the modification thereof, for otherwise it would be impossible in any case to determine just what the contractual obligations of the parties really are. The original contract not having been alleged it could not be proved. If the original written contract and the subsequent modifications of it had been alleged as it should have been, then the con*377tract introduced in evidence would not have been subject to objection.
The defendant’s first instruction told the jury that it is admitted by both plaintiff and defendant in this case that the original contract as to the furnishing and putting in the furnaces in controversy was in writing, and the jury are further instructed by such written contract the plaintiff agreed and guaranteed that it would put' in defendant’s school building two number 61 hot-air furnaces, and that such furnaces should heat the building to a temperature of seventy degrees Fahrenheit when the thermometer should be at zero as shown by the temperature outside; also that said furnaces should not emit smoke or gas into the building; and if you further believe and find from the evidence, that after making the aforesaid written contract the plaintiff’s agent, James Halpin, inspected and examined the school building and thereupon concluded that furnaces of larger capacity were required for heating the building, and proposed a modification of said original contract, and that said proposed modification was only as to the size and heating capacity of the furnaces, which were to be number 71 instead of number 61, and as to the price of such larger furnaces, which were to be $850 instead of $440; and if you further believe that such proposed modifications were accepted by defendant, and that the terms of the original written contract as to furnaces as to heating the building and as to not emitting gas and smoke still remained a part of the modified contract, then you are instructed that if the furnaces put in by plaintiff did not comply with the agreement and guaranty as" to heating the building and as to emitting gas and smoke, your findings will be for defendant.
The assumption by this instruction that it is admitted by the pleadings by both plaintiff and defendant *378that the original contract for putting in the furnaces was in writing, we do not find sustained by an inspection of the pleadings. This instruction submits to the jury the proper theory of the case, if such had been the structure of the pleadings. It presents a very plain exposition of the provisions of the written contract and then leaves it to the triers of the fact to determine whether plaintiff and defendant entered into a subsequent parol modification thereof as to the size, heating capacity and price, etc., of said furnaces. It is quite clear that the essential facts embraced in its hypothesis ai-e not within the limits of the pleadings, and for this fault-it must be condemned. Aultman-Taylor Co. v. Smith, 52 Mo. App. 351; Wright v. Fonda, 44 Mo. App. 634; George v. Railroad, 40 Mo. App. 433; Moffatt v. Conklin, 35 Mo. 455; Bank v. Murdock, 62 Mo. 73; Crews v. Lackland, 67 Mo. 621; Leonx v. Harrison, 88 Mo. 495; Merrett v. Poulter, 96 Mo. 240; Noll v. Railroad, 97 Mo. 74; Bender v. Dungan, 99 Mo. 130. And the defendant’s other instructions, except the sixth and seventh, are subject to a like infirmity.
No error is perceived in the action of the eour int-refusing plaintiff permission to pi’ove an agreement entered into between plaintiff and defendant whereby the latter promised to repair the said building and the-flues therein. The replication alleged no such agreement. If this was one of the modifications of the original written contract and the plaintiff desired to invoke its provisions he should have pleaded it, and not having done so he is pi’ecluded from' making the proof he offered. And for similar reasons the rulings of the court in rejecting other offers of proof by the plaintiff were properly rejected. In this connection it is not out-of place to state that the plaintiff’s^ seventh instruction in respect to the agreement to repair was outside of the limits of the pleadings and should not have been given *379for that reason. But of this plaintiff can have no advantage, and we call attention to it in view of a probable further trial of the cause.
We can discover no error in the action of the court in refusing certain instructions asked by plaintiff, except as to the third, which informed the jury that, although they may believe and find from the evidence ■ that the plaintiff failed to heat said building to 70 degrees, the temperature outside being zero, without the escape of smoke and gas, yet if the jury further-find from the evidence that plaintiff erected and placed in said school building two number 71 hot-air furnaces in good and workmanlike manner, and that they were tacitly accepted by the defendant, and were used and possessed by defendant, and were of value to said defendant in warming and heating said building, then the jury will find for the plaintiff the actual value of' said furnaces not exceeding the contract price.
The plaintiff’s third instruction we think should have been given. It is true that the word “tacitly” should have been omitted; still we cannot think in view of the evidence that the meaning of this term could have been misunderstood. Juries are presumed to understand the meaning and import of common English words of which this is one. The evidence tended to show that the defendant retained the furnaces in their building and paid part of the purchase price, and while there was no formal acceptance of the furnaces under the contract it was not improper to have, as the instruction did, left it to the jury to determine whether or not under the circumstances in evidence there was not a tacit or implied acceptance of it.
Defendant’s case seems to place the sale and putting-in of the furnaces as an ordinary sale of a chattel, as distinguished from what is known as a builder’s contract. Defendant’s instructions were upon the theory *380that plaintiff should remove the furnace on being notified of their failure to fill the contract. If the contract wás a builder’s contract, then the instructions should have been further qualified by allowing defendant the damage sustained, if any. The Globe L. & H. Co. v. Doud, 47 Mo. App. 439; Mohney v. Reed, 40 Mo. App. 112. The well established rule in this state now is, that if one party without fault of the other fails to perform his part of the contract in such a manner as to sue on it, still, if the other party has derived benefit from the part performed, it would be unjust to allow him to retain that without páying anything. The law, therefore, generally implies a promise on his part to pay such remuneration as the benefit conferred is reasonably worth and to recover that quantum in an action of indebitatus assumpsit, and this doctrine applies to cases arising on contracts for sales and on contracts for work and labor and materials. There is no longer any distinction as to the application of this doctrine to any of these cases. It is equally applicable to all alike. Nothing more need be said in respect to the propriety of the action of the trial court in refusing this instruction than to refer to the case of Smith v. Keith & Perry, decided by us and reported in 36 Mo. App. 567, where the principles of the rule embodied in this instruction were considered and discussed in the light off the authorities there collated.
We have not overlooked the question as to whether or not the plaintiff has not by its own instructions adopted the very errors of which it complains in respect to the admission of evidence and the giving of instructions for defendant. While some of the plaintiff’s instructions approach in that direction within “a hair breadth” of the forbidden line, they do not transcend it. We do not think they should be considered as inviting or adopting the errors to which we have adverted *381in noticing the evidence and instructions of defendant. The instructions given for both sides are in other respects, we think, unexceptionable.
Entertaining these convictions it follows that the judgment must be reversed and the cause remanded.
All concur.