In the world of science it is generally found that virile opinions are those which are fortified with a rather effeminate equipment of evidence. One of the subjects of which this is true is mental inheritance. In early days it was simple and attractive to place the blame for the occurrence of mental disorders on heredity. It is now becoming apparent that preventive policies based on genetic principles divorced from exact scientific knowledge of human inheritance are no more likely to succeed, than curative efforts unfortified by a scientific understanding of the environment. Mental inheritance is not a problem of genetics alone; it demands all that the geneticist can contribute to the recognition of how the characteristics of an organism are influenced by those of its ancestors; it calls for all that the nerve physiologist can impart about the way in which the characteristics of an organism are influenced by the changing pattern of stimuli to which it is subject.