For many years, heredity has been recognized as an important factor in all branches of medicine, but its mode of operation was never clearly defined, and indeed could not be understood until the clue given by Mendel was followed up. With this guide, it has been possible to begin to unravel the mystery, but progress is slow for several reasons: 1. It is impossible to control the material. It might be illuminating if one could mate two persons suffering from the same disease, and study the progeny, but instead one has to be content with taking the matings as one finds them. 2. The generations are so long and the records so imperfect that such statistics as are available are fragmentary and subject to error. 3. The stock is so thoroughly mongrel that it is almost impossible to say what traits may remain latent for many years, unable to make