_Introductory—Distinction Between Expert Witnesses and Other Witnesses._—One of the most important positions that a medical man is called upon to assume by virtue of his professional character, is the position of expert witness. Most writers on medical jurisprudence confine themselves, in the discussion of this subject, to presenting the medico-legal rules which appertain to this position, and concede its high importance. Before defining what is meant by the term expert witness, or treating of the rules which determine the status of such witnesses in court, and their duties, it seems advisable to introduce the subject by a brief consideration of the distinction between ordinary witnesses and expert witnesses. When medicine and law are united in the purpose of investigating facts, and bringing about a legal determination as to what are or are not facts, they co-operate with each other in this way. The law furnishes the machinery for the inquiry and the rules which determine how it shall be prosecuted. The medical man, however, is called upon as an assistant to the law, because of his skill and experience in his profession, which enable him to ascertain and interpret the circumstances from which the facts sought to be established are to be inferred. Ordinary witnesses testify concerning matters of observation, the court and jury being endowed with the sole power to determine the credibility of their evidence and the true result of their observations. Yet the border line of distinction between witnesses who testify merely to actual matters of observation, and those who give their opinions upon and draw inferences from established facts and circumstances, is a wavering one. The general rule of evidence is well understood, that hearsay evidence is inadmissible, and yet, like any other rule, this has its exceptions which grow out of the necessities of given cases. This is so because there are and always will be, matters brought before courts for investigation and determination, long after the witnesses who personally saw the facts and circumstances of the particular case in which such matters are concerned have died, or have withdrawn beyond the reach of the process of the court. Possibly no record in writing of such facts and circumstances has been left, or if in writing it lacks the sanctity of being a judicial writing, and hence is no better evidence than any other form of hearsay evidence. For this reason in matters of family history, pedigree, custom, and the like, hearsay evidence is permitted, and is entitled to as much weight as any other evidence, if the witnesses giving it be trustworthy.