Though the opinion that heredity is an important influence in epilepsy is widespread, only one epileptic person in five is able to name any relative who has been similarly affected. Like an underground stream, the predisposition to seizures can flow unrevealed and unsuspected through numerous generations and then suddenly, assisted perhaps by some injury to the brain, appear in a certain individual as epilepsy. This predisposition, carried as it is in abnormal genes, has never been demonstrated in the clinical laboratory. There are reasons for believing that a study of cortical electrical activity might clarify the problem of inheritance in epilepsy. This cortical rhythm, as recorded by the electro-encephalograph, is a fundamental constitutional characteristic. Evidence of this is the individuality of each person's electro-encephalogram and, what is even more convincing, the similarity of the electro-encephalograms of similar twins.1This similarity is observed both in twins with normal and in