Application of genetic theory to clinical situations must always remain, in a sense, unfinished business. In the nature of the circumstances it is impossible, and presumably will always remain impossible, to prove that a given trait is transmitted from man to man by the intervention of genes. The closest one can come to proof is to demonstrate that the facts are consistent with genetic theory and that no alternative hypothesis explains them so well; and as new data are gathered the demonstration of concordance becomes a series of approximations which may approach infinitely close to proof without ever quite attaining it.