EVIDENCE. the occurrence, (c) Expert testimony. A wit- ness ,n:i be allowed to testify as to his opinion because, by reason of experience or special study an d investigation, he is better qualified to i i an opinion than the jury. Thus, physician i ngi neers, handwriting experts, etc., are allowed t" give opinion evidence in order to aid the jury in reaching :i correct conclusion. They are Dot 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 com- petent 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 un- derstand the nature and obligation of an oath, and an insane person may testify upon a matter concerning which his understanding is not af- feeted by his insanity. At common law one con- victed of a felony within the jurisdiction was in- competent 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. Xor could either the husband or wife testify for or against the other at common law. This dis- ability has been removed to some extent by stat- ute iii most jurisdictions, but not generally so as to permit testimony' as to confidential com- munications between husband and wife. It was the policy of the common law to pro- tect the witness from being compelled to incrimi- nate 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 rela- tionships, 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 con- fidential 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 ! 1 ' EVIDENCE. probable thai the President of i lie I Initi and the Governors of Mate- are privileged from appearing as witnesses under any circumstai VII. Examination of Witnessed. Witnesses inn;, bi ela sifted as favorable or opposing. A favorable witness i- one railed by a pari;. to - tify in support of his contention in tin contro versy, and an opposing witness is our called by the other party t<> the controversy to testifj in his behalf. The favorable witness on on. ide it therefore the opposing witness of the other. Aa a general rub- one is not allowed to ask Lis own (or favorable) witness leading qui lions, that is, questions which by their form in. inaii' i he answer desired. The extent to which leading questions may be asked, however, n in the discretion of the trial judge, and should the witness prove hostile leading questions may he asked. One may not impeach the credibility of his own witness ; that is. he is not allowed to in- troduce testimony to show generally that the wit- ness is not worthy of belief, lb- 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 i-. may examine him as an opposing witness. On cross-examinations it is permissible to ask lead- ing questions. The cross-examiner may also at- tack the credibility of the witness, and for tnat purpose may ask questions not otherwise rele- vant. The witness, however, may refuse to an- swer questions of this class which tend to incrimi- nate or degrade him. The credibility of an op- posing witness may also be attacked by intro- ducing testimony to show that he is generally unworthy of belief. The Burden of Proof. From the nature of plead- ing and the trial of an action at law it follows that upon one party or the other to the controversy rests the burden of introducing some evidence in order to establish his contention. 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. Tims at vari- ous stages of the trial the burden of introducing evidence may shift from one side to the other. It is evident, therefore, that the common expres- sion 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 ease by the preponderance of evidence. In criminal trials the burden of proof rests upon the prosecution, which is required to prove it- 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 doc- trines of judicial notice, and of presumption. It
