After first determining whether the question under investigation is a proper subject of expert inquiry, the next question for the court to decide is: Has the witness offered the necessary qualifications of an expert? This is a matter resting largely within the discretion of the court. But if the witness be shown to have given the subject special study, or if he be a licensed member of a skilled profession or trade, having the years of experience usually sufficient for the ordinary members thereof to acquire a thorough knowledge of the subject, he will generally be permitted to testify and to give his opinions concerning hypothetical questions covering the facts proven in the case. He will not be permitted to express his opinion as to the general merits of the controversy, for to permit him to do so would be to allow the witness to usurp the province of the jury. It is for the court or jury to form opinions and judge of the merits of the case.

It is not necessary that the opinion of an expert be derived from his own observation and experience, but he may give his opinion based upon information derived from books.1 In Mitchell vs. The State, 58 Ala., 417, which was an indictment for murder by poisoning by arsenic, a physician was allowed to give his opinion as to the cause of death, although it did not appear that he had ever attended cases of that character, and in passing upon the admissibility of the evidence, the court held that a physician, who has had long experience in the practice of his profession, and knowledge of the symptoms of the malady of the deceased, was competent to testify as an expert.

State vs. Terrell, 12 Rich (S. C), 321.

And in State vs. Wood, 53 N. H., 484, which was an indictment for murder caused by an abortion alleged to have been produced by the defendant, it was held that a physician, testifying as an expert, may give his opinion founded upon his reading and study, alone.

It is not required that the expert testimony be the best obtainable, nor that the witness at the time he testifies be actively engaged in the profession or trade concerning which he is about to testify. But where he has abandoned the same for twenty years, courts are loath to receive the opinions of experts whose knowledge is so antiquated, more recent and better expert evidence being readily obtainable.

It has sometimes been held that an expert in one occupation may testify in another and kindred one when the nature of the first occupation is such as to give him superior opportunities for acquiring special knowledge and experience in the other. This rule, however, has been severely and justly criticised by courts and should be applied with great caution.

Before proceeding with the examination of an expert witness, it is necessary to interrogate him regarding his special qualifications and to satisfy the court that he is competent to give his opinion on the subject to which his testimony relates. The question as to his fitness should be determined by the court at once, before he is allowed to testify, and should not be postponed to the cross-examination to determine his disqualifications. If a witness testify that he is unable to give his opinion on a certain subject or hypothetical question, he is disqualified by his own testimony.

The court may, in its discretion, permit opposing counsel to cross-examine the expert as to his qualifications before allowing him to give his opinion, and it is the general practice for the opposing party to exercise his privilege of cross-examination on the matter of qualifications after the witness has been examined in full by the party offering him. That is the more convenient practice. And in Finch vs. Chicago, etc., R. Co., 46 Minn., 250, the court said: "We think it is the understanding of the judges and the bar that while the court may, in its discretion, permit a preliminary cross-examination, it is not bound to do so, but may allow the opinion to be given when the direct examination shows prima facie that the witness is qualified."