Since its first formulation, the concept of manic-depressive illness has been subject to successive modification and, on the whole, to progressive enlargement. It was Kraepelin, following on the attempts of Baillarger, Falret and Magnan, who grouped together all the various nosographic forms distinguished by isolated depressive or manic crises, periodic or alternating (and even the form designated as involutional melancholia), including them all under the sole class name of manic-depressive psychosis. The morbid entity thus defined was regarded as distinguished by the periodicity of the crises, each with a tendency towards social remission. Aetiologically the causation was seen as preponderantly hereditary.