It is generally said that no person has a vested right in rules of evidence.1 Hence the law concerning the competency of witnesses in force at the time of the trial, and not that in force when the contract is entered into, controls.2 Thus A conveyed to B and B married C. A died. The conveyance from A to B was attacked. C was held to be a competent witness because she was not a party to the contract or cause of action on trial.3 The judgment below was reversed. Before the second trial a statute was passed providing that no party to a suit or proceeding whose right of action or defense is derived from one who is or, if living, would be incompetent to testify, shall be admitted to testify. This statute was held to apply to a new trial of this cause.4 On the other hand, " subsequent legislation which affects unreasonable changes in the rules of evidence." of a subsisting contract "impairs the obligations of the contract within the restriction of the Federal Constitution."5 An unreasonable change in the rules of evidence is therefore void as applicable to pre-existing contracts.6 The application of so general a rule as this to specific states of fact results in some contradictory decisions. If a subsequent statute makes it impossible to prove a material or relevant fact, which under the law of evidence in force when the contract right was acquired could have been proved, it impairs the obligation of such prior contract.7 Thus a statute which provides that evidence of certain land claims shall not be admitted unless archived or recorded is unconstitutional as against prior claims the evidence of which consists of original public documents in the custody of the Mexican government, since the originals cannot be obtained for record and the statute makes no provision for recording copies.8 If the change in the law of evidence does not make proof of the fact in question absolutely impossible, but does make it so burdensome as to be practically impossible, the statute is unconstitutional. Thus certain bonds with detachable interest coupons had been issued by the state of Virginia. Many of these coupons had been detached and sold. A subsequent statute of Virginia provided that such coupons could be collected only on producing the original bond from which they had been cut. This requirement was held by the Supreme Court of the United States to be " an unreasonable condition, in many cases impossible to be performed," and hence invalid.9 This statute further forbade proof of the genuineness of the coupons by expert evidence. The state court had held such statute to be constitutional.10

12 Blouin v. Ledet, 109 La. 709; 33 So. 741.

13 Central Trust Co. v. Ry., 65 Fed. 257.

14 Soule v. Seattle, 6 Wash. 315; 33 Pac384, 1080. (However, a change in the method of assessment from assessment according to valuation to assessment according to frontage does not impair the obligation of a contract for such improvement. )

1 Callanan v. Hurley, 93 U. S. 387; Bannon v. Burnes, 39 Fed. 892; Martin v. Cole, 38 la. 141; Immegart v. Gorgas, 41 la. 439; Morrill v. Douglass, 17 Kan. 291; In re Douglas. 41 La. Ann. 765; 6

So. 675; Virden v. Bowers, 55 Miss. 1; Bell v. Coats, 54 Miss. 538; Griffin v. Dogan, 48 Miss. 11; Raley v. Guinn, 76 Mo. 263; Abbott v. Lindenbower, 42 Mo. 162; O'Bryan v. Allen, 108 Mo. 227; 32 Am. St. Eep. 595; 18 S. W. 892; Ensign v. Barse, 107 N. Y. 329; 14 N. E. 400; 15 N. E. 401; Brown v. Slauson, 23 Wis. 245; Smith v. Cleveland, 17 Wis. 556.

2 O'Bryan v. Allen. 108 Mo. 227; 32 Am. St. Rep. 595; 18 S. W. 892.

3 O'Bryan v. Allen, 95 Mo. 68; 8 S. W. 225.

4 O'Bryan v. Allen, 108 Mo. 227; 32 Am. St. Rep. 595; 18 S. W- 892.

5 Davis v. Supreme Lodge, 165 N. Y. 159; 58 N. E. 891.

6McGahey v. Virginia, 135 U. S. 662; Davis v. Supreme Lodge, 165 N. Y. 159; 58 N. E. 891.

7 A statute " which forbids the introduction of evidence of a prior contract, admissible and made necessary to the validity and existence of the contract by the law in force at the time it was made, unless it provides some other method of making sufficient proof of the necessary facts accessible to the person called upon to make the proof, it seems to us impairs the obligation of a contract as fully as though such subsequent law in terms declared that the contract should no longer be operative or be enforced through the courts, for it destroys the only means through which the contract may be established or enforced." Texas-Mexican Ry v. Locke, 74 Tex. 370, 399; 12 S. W. 80.

8 Texas-Mexican Ry. v. Locke, 74 Tex. 370; 12 S. W. 80.

In some cases changes in the law of evidence, making admissible evidence which before was inadmissible, is held to impair the obligation of contracts as to the party prejudiced thereby. So a statute providing that oral evidence is admissible to identify land insufficiently described in a written contract within the statute of frauds cannot apply to a prior contract.11

This principle has even been applied to a change in rules affecting competency of witnesses. Thus where a physician was incompetent to testify as to knowledge acquired by him in pro-fessional confidence, and hence could not testify as to the cause of the death of certain relatives of the insured so as to defeat the policy by showing the falsity of certain statements in the application, a subsequent statute making him competent has been held not to apply to pre-existing contracts.12 A conclusive presumption is really a rule of substantive law; and a statute creating a conclusive presumption really creates a new rule of substantive law. Thus a statute making a tax deed conclusive evidence of a fee has been held invalid.13 The converse of this proposition is not always true. By the weight of authority a presumption which was conclusive when contract rights were acquired may be modified by statute so as to be prima facie only. Thus after a tax sale, a statute destroying the conclusive effect of a tax deed and making it prima facie only, does not impair the obligation of the contract with the purchaser.14 So a statute which makes a tax deed conclusive as to all but jurisdictional facts is held valid,15 as where it is made conclusive as to the fact that the tax is duly levied and that the necessary prerequisites have been complied with by the public officials,16 or as to every fact necessary to its validity except that the tax has not been paid and that the realty is not exempt from taxation.17

9 McGahey v. Virginia, 135 U. S. 662.

10 Cornwall v. Commonwealth, 82 Va. 644; 3 Am. St. Rep. 121.

11 Lowe v. Harris, 112 N. C. 472; 22 L. R. A. 379; 17 S. E. 539. (The contract in question was as follows: " Wilkesboro, N. C, Apr. 19, 1880. James Harris has paid the twenty dollars on his land. Owes me six more on it." The statute was passed in 1891.)

12 Davis v. Supreme Lodge, 165 N. Y. 159; 58 N. E. 891. The real point of this decision was that the subsequent statute was not intended to alter the former rule of evidence. This result was in part reached, however, because any other construction would have rendered the new statute unconstitutional.

13 Dawson v. Peter, 119 Mich. 274; 77 N. W. 997.