The author here seeks to analyse the distribution in families of temperament, as expressed in mood, and to test the hypothesis that it is dependent on heredity. Mood is divided into two main clashes (as seen in manic depressive states): the hyperkinetic or exalted, and the hypokinetic or depressed. The hyperkinetic temperament is divided into two grades: a less developed called nervous (and sometimes sanguine), and a more developed called choleric. The hypokinetic temperament is likewise divided into two grades: a less developed called phlegmatic, and a more developed called melancholic. In some families there is a prevailing tendency to the first class of conditions, and in other families for the second class, while yet other families show a mixed state. How can we bring under one general scheme the inheritance of these various types of mood? After several trials the following hypothesis was selected to test. There is in the germplasm a fectot E, which induces the more or less periodic occurrence of an excited condition (or an exceptionally strong reactibility to exciting conditions), and its absence e, which results in an absence of extreme excitability. There are also the factor C, which makes for normal cheerfulness of mood, and its absence c, which permits a more or less periodic depression. Moreover, these factors behave as though in different chromosomes, so that they are inherited independently of each other and may occur in any combination.