IV THE PAROL-EVIDENCE RULE:

This is properly a rule of substantive law which is, in effect, that the terms of a contract or other legal instrument should be deemed to be embodied wholly in the written instrument executed by the parties thereto, or, stated in terms of evidence, the rule is that parol or oral evidence shall not be introduced for the purpose of varying the terms of a written instrument. The rule, though necessarily subject to many exceptions, is founded upon the just notion that when parties have deliberately embodied their agreement or transact in writing, they should not thereafter be allowed to dispute its terms. The following are the most important cases in which to vary the express terms of a written instrument testimony may be given:

a) Where the parties did not intend to reduce all the terms of the agreement to writing.

(b) When the writing or agreement is varied by a subsequent parol agreement.

(c) When the evidence is introduced to show that the written instrument has never takes effect because of the non-occurrence of some agreed condition precedent.

(d) When a term of the instrument is ambiguous and parol evidence is necessary to explain the meaning, and upon analogous grounds where the term of a written instrument has a technical or local meaning requiring oral explanation.

(e) When the proof of a custom which is in law a part of a contract or other document varies the effect or meaning of the written language.

(f) When in equity an action is brought to reform or rescind a written instrument, or construe a conveyance as a mortgage.

V. OPINION EVIDENCE.

In general witnesses are allowed to testify only as to facts, and not as to their inferences or opinions based upon facts within their knowledge. To permit the witness to indulge in opinion testimony would be a usurpation of the function of the jury, whose duty it is to draw inferences of fact and to form an opinion, where an opinion is necessary to the verdict. Thus the witness, when the facts of a conversation are in issue, must testify as to the terms of the conversation and not his conclusions as to its meaning. There are, however, three important exceptions to the rule that opinion evidence is inadmissible. They are:

(a) Matters of common experience;

matters of common knowledge to a certain extent the result of inference. Thus, to testify that a certain day was cold, or that a knife was sharp, involves the operation of the witness's mind in drawing a conclusion; but since these are matters of common experience about which the conclusions of the witness are as trustworthy as those of a jury, such testimony is legally admissible as evidence.

(b) Matters not of common experience:

but about which the opinion of the witness is under the circumstances more trustworthy than any which could be formed by the jury. Thus, a witness may be so situated with reference to an event or combination of circumstances as to be able to draw a more accurate conclusion from them than the jury, which should rely wholly upon verbal testimony about the occurrence. Thus, the witness may be allowed to give his opinion of the distance between an approaching street-car and a pedestrian before warning of the approach was given; or, under certain circumstances, he may be allowed to give his opinion of the rate of speed at which the car was moving. His presence at the time of the event enables him to form a more accurate opinion than the jury, which can only rely upon a necessarily imperfect description of the occurrence.

(c) Expert Testimony:

A witness may be allowed to testify as to his opinion because, by reason of experience or special study and investigation, he is better qualified to form an opinion than the jury. Thus, physicians, engineers, handwriting experts, etc., are allowed to give opinion evidence in order to aid the jury in reaching a correct conclusion. They are not allowed, however, to express any opinion as to the truth or untruth of other evidence submitted to the jury, that being a matter of which the jury is qualified to judge. The testimony of experts, so far as it is opinion evidence is based upon the evidence already before the jury, assuming it or parts of it to be true. For that reason questions asked of expert witnesses are usually required to be hypothetical in form.

VI. WITNESSES' COMPETENCY:

A witness is not competent to testify until he has taken oath to testify truly. At common law an atheist or other unbeliever in the Christian religion was not a competent witness, because it was believed that he would not feel constrained by his oath to testify truly. At the present time a witness is generally allowed to testify on his oath or affirmation, no particular religious belief being requisite. A child is a competent witness if old enough to understand the nature and obligation of an oath, and an insane person may testify upon a matter concerning which his understanding is not affected by his insanity. At common law one convicted of a felony within the jurisdiction was incompetent as a witness unless pardoned. In most jurisdictions such a conviction now affects the credibility only, and not the competency of the witness. At common law a party to an action was not a competent witness in his own behalf, nor was one a competent witness if directly interested in the controversy. This disability has been generally removed by statute. Nor could either the husband or wife testify for or against the other at common law. This disability has been removed to some extent by statute in most jurisdictions, but not generally so as to permit testimony as to confidential communications between husband and wife.

It was the policy of the common law to protect the witness from being compelled to incriminate himself. He is therefore privileged from giving any testimony which tends to incriminate him or to subject him to a penalty or forfeiture. If the witness does not claim his privilege, his testimony is competent and subject to the usual rules of the law of evidence. Having once fairly waived his privilege, he must testify fully. Thus a defendant in a criminal trial is privileged from being compelled to testify; but having offered to testify in his own behalf, he must answer proper questions directed to him on cross-examination. At common law, also, an attorney and client were privileged from testifying as to any confidential communication between them. By statute this privilege has in most jurisdictions been extended to persons standing in other confidential relationships, e.g. physician and patient, clergyman or priest and layman, and in some jurisdictions, notably New York, attorneys, physicians, and clergymen are not competent to testify as to confidential communications received by them in their professional capacity. Upon the similar ground of public policy, a party is privileged from testifying as to his efforts or willingness to compromise the matter in controversy, and it is probable that the President of the United States and the Governors of States are privileged from appearing as witnesses under any circumstances.

VII. EXAMINATION OF WITNESSES:

Witnesses may be classified as favorable or opposing. A favorable witness is one called by a party to testify in support of his contention in the controversy to testify in his behalf. The favorable witness on one side is therefore the opposing witness of the other. As a general rule one is not allowed to ask his own (or favorable) witness leading questions, that is, questions which by their form indicate the answer desired. The extent to which leading questions may be asked, however, rests in the discretion of the trial judge, and should the witness prove hostile leading questions may be asked. One may not impeach the credibility of his own witness; that is, he is not allowed to introduce testimony to show generally that the witness is not worthy of belief. He may, however, contradict the testimony of the witness by other witnesses for the purpose of showing the truth as to a fact about which the first witness has testified. At the close of the direct examination, or the examination of a favorable witness, counsel for the other side may cross-examine, that is, may examine him as an opposing witness. On cross-examinations it is permissible to ask leading questions. The cross-examiner may also attack the credibility of the witness, and for that purpose may ask questions not otherwise relevant. The witness, however, may refuse to answer questions of this class which tend to incriminate or degrade him. The credibility of an opposing witness may also be attacked by introducing testimony to show that he is generally unworthy of belief.

THE BURDEN OF PROOF.

From the nature of pleading and the trial of an action at law it follows that upon one party or the other tot he controversy rests the burden of introducing some evidence in order to establish his intention. The burden of proof is said to rest upon the party against whom a judgment must be given if no evidence be introduced in his favor. The same doctrine is stated in slightly different terms by saying that the burden of proving a fact rests upon him who asserts the existence of the fact in his pleading, and not on him who denies it. The party on whom the burden rests may, by the introduction of some evidence, make out a prima facie case, and then arises the legal necessity for the other party to introduce evidence enough to destroy the prima facie case of his opponent. Thus at various stages of the trial the burden of introducing evidence may shift from one side to the other. It is evident, therefore, that the common expression that the burden of proof shifts during the progress of a trial is not exact, unless the word proof be taken in the sense of attempt to establish the truth of a fact, and not in its usual legal sense as such evidence as satisfies the mind. In civil trials the party on whom rests the burden of proof must sustain his case by the preponderance of evidence. In criminal trials the burden of proof rests upon the prosecution, which is required to prove its case beyond a reasonable doubt. In sustaining the burden of proof the party upon whom the burden rests is aided in making proof by the doctrines of judicial notice, and of presumption. It is unnecessary to prove facts of which the court will take judicial notice. In general these are facts of such common and universal knowledge that it would be idle to prove them by affirmative testimony. Thus (to cite a few of the innumerable cases) it is unnecessary to prove the calendar, the multiplication table, that water will freeze, or that ice will melt. The party sustaining the burden of proof is also aided in making proof by proving one fact or set of facts from which certain consequences are presumed to flow. All so-called circumstantial evidence is evidence intended for the purpose of creating a presumption of some other fact sought to be proven.

In general courts of equity follow the rules of evidence as adopted by the common-law courts. The important exceptions have been noticed above.

In the United States the Federal courts in civil cases follow rules of evidence applied by the local State courts, unless a different rule is required by Federal Statute. In criminal trials they follow the common law as interpreted by the Federal courts, and as modified by Federal statutes. In the several states the common-law rules of evidence are generally followed with comparatively few statutory modifications, the more important of which have been noted. Consult: Greenleaf, Treatise on the Law of Evidence (16th ed., Boston, 1899) ; Thayer, Preliminary Treatise on Evidence at Common Law (Boston, 1898); id., Cases on Evidence (Boston, 1900); Stephen, Digest of the Law of Evidence (May, editor, Boston, 1877); Abbott, Select Cases on Evidence (New York, 1895); id., Trial Evidence (New York, 1900); Powell, Principles and Practice of the Law of Evidence (7th ed., London, 1899); Best, Principles of Evidence (9th ed., London, 1902).