In his paper on “Wages, Lunacy, and Crime in South Wales,” Dr. R. S. Stewart asks how such a state of things as he describes is “to be reconciled with the lately expressed opinion that ‘insanity is, and ever will be, the product of two factors, stress and heredity?”’ These words he puts in quotation marks, but I am unable to trace them to any other source. The doctrine that insanity is a function of two variables, heredity and stress, was first enunciated by me some thirteen years ago, and can scarcely therefore be regarded as a novelty now; but, as it is clear, from Dr. Stewart's question and other remarks that he has a very erroneous notion of the doctrine, and as, moreover, it has been misapprehended and therefore misrepresented by others, perhaps you would allow me space to re-state it; although I think that any one who does me the honour to read it in its original form, as set forth in Sanity and Insanity, would scarcely fall into the errors that I have alluded to.