When one takes his place as an expert before a court, a legal paradox is instituted on his behalf, by which he is allowed to testify—not as to what he knows, but to what he believes or forms an opinion upon, based necessarily on probabilities of analogy as well as experience. Nothing is required (in the absence of any statutory provision to the contrary) to entitle any one to give evidence as a medical witness, than that he has been educated in the science of medicine; and this he may be by study without practice, or by practice without study; it is not necessary that he should be a physician, or have studied for one, nor be a graduate, nor one licensed to practise, nor need he be or have been a practitioner [275]. One may be competent to testify as an expert, although his special knowledge of the particular subject of enquiry has been derived from the reading and study of standard authorities, and not from experience or actual observation. But one cannot qualify himself as an expert in a particular case merely by devoting himself to the study of authorities for the purposes of that case, when such reading and study is not in the line of his special calling or profession and is entered upon to enable him to testify in the case. In Vermont, however, it has been held that mere education as a physician, without some practice as such, is insufficient to qualify one as an expert; and in Arkansas, it is said, that competency must be shewn from study and experience. In New York, it has been held that one otherwise qualified, who is |112| a physician and surgeon, may give evidence, although not in full practice at the time; this fact merely goes to affect his credit [276].