I fear that the dramatic title of this discourse is rather misleading. I do not mean that the skin, as an organ, is of universal diagnostic value in relation to internal disorders. What I mean is that the skin may be of great diagnostic assistance in many groups of general conditions or diseases. In other words, it may reflect, as a mirror, certain fundamental truths relative to the pathology of diseases in general. Dermatology, since the time of Hebra, has become a study of morphology. The lesions seen on the skin have been studied both histologically and clinically, generally without regard to the underlying connection they may have with any other organ or system whatsoever. Most of the literature on skin diseases consists of a clinical description of the disease and its histologic character. This custom has been extant for the past sixty years, and it has been only in