This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
In at least another respect there is some authority for saying that the parol evidence rule is merely a rule of evidence. If A and B have agreed upon certain terms orally and subsequently they reduce the contract to writing and intentionally omit certain oral terms which impose a liability upon one party alone, it has been held that a written modification of the written contract made after such written contract is executed and delivered, and without any new consideration, may incorporate such oral negotiations into
12Sweetland v. Shattuck, 66 Cal. 31, 4 Pac. 885; Barton v. Koon, 20 S. D. 7, 104 N. W. 521; Ray v. Rood, 62 Vt 293, 19 Atl. 226; Eaves v. Vial, 08 Va. 134. 34 S. E. 978.
This principle has been applied to oral evidence of a trust. Poole v. McGahan, 124 Ind. 583, 24 N. E. 723: Merritt-Allen Co. v. Torrence (Ia.), 102 N. W. 154. (In this case the greater part of the corpus of the trust was personalty and oral evidence was admissible as to such property.)
13 Le Mesnager v. Hamilton, 101 Cal. 532, 40 Am. St. Rep. 81, 35 Pac. 1054; Beckwith v. Talbot, 2 Colo. 639.
14 See Sec. 2154.
15 Ryan v. Logan County Bank, 132 Ky. 625, 116 S. W. 1179, 119 S. W. 768.
such written contract.1 Where the admission of such evidence might have been justified on the ground that the oral agreement was the inducement for the written agreement,2 or on the ground that oral evidence might be admitted to show the connection between two different writings,3 the court preferred to place its holding upon the ground that the parol evidence rule is essentially a rule of evidence.4
If the parol evidence rule were a rule of substantive law, and if the oral terms were in a part of the contract, the addition to the contract by subsequent agreement of terms which impose an obligation upon one party alone would be without legal effect, because no consideration would exist for such modification.5 In this case, too, the inconsistency between the result thus reached and the theory that the parol evidence rule is a rule of substantive law, does not seem to be noticed.