¶1The printed abstract comprises over five hundred pages. Upon the return of the verdict the defendant filed a motion for a new trial, embracing fourteen grounds. This being overruled he filed his motion in arrest of judgment, including thirteen grounds. The references in appellant’s argument are to the written transcript, and not to the printed abstract. We have thus been need: lessly embarrassed in our consideration of the case. The objections urged are so numerous that, in order to bring the opinion within reasonable space, it is necessary to group many of them together, and to consider them all but briefly.
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¶5*134These witnesses were introduced as experts, and hence all the above questions were proper. The opinions of medical men, who are shown to be experts, as to the instruments producing, and the nature and consequence of wounds, or the causes of diseases, are competent evidence in a prosecution for homicide. State v. Morphy, 88 Iowa, 270. The fact that, upon cross-examination, it appeared that the examination of deceased was not as thorough as it might have been, does not affect the competency of their testimony. Dr. Cherry having stated that there might have been an aneurism sufficient to produce death; that they did not examine the lower portions of the body at all, and that it would have been impossible to have discovered an aneurism without an examination of the blood vessels, was asked by the State the following question : “ State what was the reason you made no more extensive examination than you did ? ” Ans. “ The reason we did not pursue our investigations further was, that we thought we had discovered, sufficient cause to produce death.” There was no error in admitting this testimony. Having discovered a cause sufficient to produce death, there existed no legal necessity that they shoxild dissect the entire body and examine all the blood vessels, for the purpose of discovering whether an aneurism existed, causing a rupture of any of them. And, having failed to make such dissection, we know of no reason why they should not be allowed to state the cause which induced them to forego such examination. True, if they had made such examination and been able to state that no other cause existed to which the death could be attributed, their evidence would have been more satisfactory and convincing. And a desire to give intelligent testimony in a trial involving no less than the life of the accused, should lead every physician to prosecute his investigations as far as the circumstances render possible. Yet, when this is not done, *135it is not improper to state the reason for the failure. The sufficiency of the reason is to be weighed by the jury.
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¶7It is claimed by appellant that, in criminal cases, the prosecution must adduce such evidence as will exclude every other hypothesis but that of the guilt of the accused. Citing Greenleaf on Evidence, § 13 a. If this, without qualification, were to be admitted to be the rule, which we doubt, yet the hypothesis must arise out of the evidence adduced, and not out of facts which, by possibility, may exist, and of which there is no proof.
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¶9He was then asked “ if the books lay down the extent of the time within which this external force may be applied, and produce these spots.” This question was excluded by the court.
¶10The witness was subjected to a rigid, se'aréhing and minute cross-examination, occupying thirty pages of'the *136printed record. While the greatest latitude should be allowed in cross-examination, so long as it tends to the eliciting of truth, and the furnishing of the jury with data upon which to base an intelligent finding, yet it must rest somewhat in the sound discretion of the trial court. And when there seems to have been no abuse of discretion, and no substantial prejudice has resulted, this court will not interfere. It is claimed that this witness had based his opinions upon the statements of the books, and that this question was proper in order to a subsequent introduction of the books, to show that the witness did not know what the books taught. But the books could have been introduced for that purpose, just as well without asking this question. While, therefore, the court might, with propriety, have admitted the question, there was no error in rejecting it.
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¶12The declarations are connected with no act of which they constitute part of the res gesteb. The declarants are living and competent witnesses.
¶13The question in issue is not one of pedigree, and if governed by the same rules governing questions of pedi*137gree, the declarations of persons living are inadmissible. 1 Greenl. on Ev., § 103.
¶14YII. Certain witnesses, having testified as to the mental condition of defendant, were permitted, against his objection, to answer the following question: “ I will ask you whether, in your opinion, Henry, the defendant, has not sense enough to know right from wrong ? ” The witnesses were not experts, but had detailed fully the facts upon which their opinions were based, and had stated, in answer to a question by the defense as to the mental condition of defendant, “ that he never was just right.” Having given an opinion as to defendant’s lack of mental power, it was proper, it seems to us, to ascertain from them, upon cross-examination, the degree of imbecility. It is objected, however, that the ability to distinguish right from wrong does not furnish the proper test of criminal responsibility. This may or may not be true. If the insanity consists in a want of intellectual power, this rule furnishes the test. If the insanity consists in an uncontrollable impulse, overcoming the will, and impelling one to do an act known to be wrong, it may not furnish the proper test. The testimony, however, is proper, inasmuch as it furnishes a rule which may apply to the case.
¶15In re-examination, one of these witnesses was asked to state whether he had ever seen the defendant angry, and what was his appearance then ? This was rejected. With respect to this it is sufficient to say, that the same witness upon direct examination stated that he had seen defendant angry, and detailed his appearance. There was no error in refusing to hear the testimony repeated.
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¶18"We notice specially instruction twenty-four, which is as follows: “ If you find from the evidence that the defendant killed or caused the death of the said John Porter, and if you further find that he was acting in self-defense in -so doing, it will be your- duty to acquit him; or if you find that John Porter attacked the defendant, and that the defendant only used such force as was necessary to repel the assault, he would not be guilty as charged, even if death did result therefrom. If,however,.you find that the! defendant inflicted the blow upon the deceased that caused *140death, then the burden of proof is upon the defendant to show that he did it in self-defense.”
¶19This instruction is clearly erroneous. Proof is the result of evidence. No fact can legally be said to be proved, unless it is established by at least a preponderance of evidence. The party upon whom rests the burden of proof, must establish the fact respecting which the burden is cast upon him, by at least a preponderance of evidence.
¶20Whether or not a homicide is committed in self-defense depends upon the circumstances under which the act is done. And these circumstances, whether introduced by the State, in making out the case in chief, or by the defendant in support of his defense, constitute part of the res gestee. If they do not, they are not admissible in evidence. And, however the circumstances are introduced, the rule of law is, that the jury, after weighing them all, must be satisfied of the defendant’s guilt beyond a reasonable doubt, or they must acquit. Under the rule laid down, the defendant must establish, by a preponderance of evidence, that he acted in self-defense, or the jury would be required to find him guilty; whereas he is entitled to an acquittal if he shows, by the facts attending the commission of the offense, proved either by himself or the State, that there is reasonable doubt that his act was willful. The rule is different when the matter of defense is wholly disconnected from the body of the offense. Tweedy v. The State, 5 Iowa, 434, and eases cited; The State v. Morphy, 33 id. 270; The State v. Felter, 32 id. 49.
¶21There was no eye witness of the homicide in question. The defendant was seen coming from deceased’s house, brandishing a chair, and was heard calling upon deceased to come on. The deceased was heard telling him to go out of the house and stay out. Defendant said: «‘The old man is trying to drive me off of the place.” Immediately thereafter the deceased was found sitting by the fireplace apparently very angry. Other circumstances *141were introduced showing the violent temper of deceased, and previous difficulties between him and the defendant. It was due the defendant that the cause should be submitted to the jury with proper instructions as to every reasonable phase of the case. For the error in the instruction above named the cause is reversed, and remanded for a new trial.
¶22Reversed.