To the same effect see Muldowney v. Illinois Central R. R. Co., 36 Iowa, 472; Wharton on Evidence, Sec. 436; Greenleaf on Evidence, Sec. 441.
Qualifications of this General Rule.—The extent to which an expert witness can go in giving his opinion is limited to matters of science and skill, and does not extend to the expression of views on matters of legal or moral observation, or the manner in which others would probably be influenced if the parties had acted in one way rather than in another. Campbell v. Richards, 5 B. & Ad., 345.
So it has been held that the question whether a physician has honorably and faithfully discharged his duty in a given case, either to his medical profession or to his patient, is not a question of science but of pure ethics, upon which the jury is as competent to decide as any one else, and in such a case an opinion would not be allowed to be given either by another medical practitioner or by a professor in the science of morals. Rogers on Expert Testimony, Sec. 11, citing Ramadge v. Ryan, 9 Bing., 333.
There are also some matters of fact which apparently transcend the dividing line between common experience and judgment and scientific experience and judgment, as to which expert testimony is not receivable, but the jury and court must weigh the facts and draw the inferences for themselves. An interesting example of this is found in the case of Manke v. The People, 78 N. Y., 611 (17 Hun, 410), cited in Stephens’ “Digest of the Law of Evidence,” p. 107, note H, decided in the New York Court of Appeals a few years ago. In that case one Adolf was killed by a gunshot, and pieces of paper were found near the scene of the homicide bearing certain marks. An expert was called upon to say whether they were powder-marks, and whether the condition of the paper was such that in his opinion it was wadding which had been fired from a gun. This evidence was held to be inadmissible by the General Term of the Supreme Court, and this decision was affirmed by the Court of Appeals. These courts held that the question as to whether this was a wad fired from a gun was a matter which the jury was as competent to judge of as the witness. In delivering the opinion at General Term, Presiding Justice Talcott said that this case was very close to the border line, but in his judgment it was beyond the province of experts and within the province of jurors.
Nevertheless, in that case the evidence of chemists who had examined the wadding, and had discovered the marks on it which were said to be powder-marks, and upon analysis had determined that they were powder-marks, or that they were marks of powder which had exploded, would have been clearly admissible.
The subjects concerning which medical men may be called upon to testify as experts are as numerous as the diseases, injuries, mental and physical conditions of the human race which fall within the range of the practice of medicine and surgery. It is therefore practically impossible to give them in detail.[185]
Practical Suggestions and Admonitions Embodied in Rules.—It is deemed advisable that the following practical suggestions and admonitions to physicians, concerning their duties as expert witnesses, shall here be given.
First: A physician should refuse to testify as an expert unless he is conscious that he is really qualified as an expert.
Second: After accepting the responsibility, his first duty should be to make a diligent examination and preparation for his testimony, unless it is upon a subject with which he is familiar and which he is satisfied that he has already exhausted, by reading the best authorities that he can find, and by careful reflection upon particular questions as to which his opinion will be asked.
Third: Where he is to make an examination of facts, such as the post-mortem examination of a body, a chemical analysis or an examination of an alleged insane person, he should insist upon having plenty of time and full opportunity for doing his work thoroughly. He should take particular pains to make his examination open and fair, and, if possible, should invite opposing experts to co-operate with him in it.