The experience of all ages has conclusively proved that a large proportion of such morbid phæenomena as occur in consequence of local climatic conditions are capable equally of being produced sooner or later in countries and neighbourhoods the climate of which is entirely different by help of the _genius epidemicus_; and that the readiness with which they are so produced varies in direct ratio with the degree in which the climate is associated with and seconds the favourable factors. It is indeed extremely difficult, in view of the low level of development to which the science of Epidemics, in general no less than in particular, has as yet attained, to show this as applicable in any given case, more especially if it is a question of the epidemic condition of some disease of which the pathological relations themselves are far from being as yet adequately known. Still this must not prevent us from making at any rate an attempt at investigation of the question, how much or how little effort has been manifested by such influence in the course of years. But the influence of the genius epidemicus on diseases in general is a twofold one. _Either_ it supplies the capital, most essential external circumstances conditioning the production of a disease, in fact is related to it as cause to effect. In virtue of it the disease is an _epidemic_ disease, coming into existence for the first time concurrently with the development of the genius epidemicus, disappearing again with the cessation of its prevalence, and once again springing up if and when the genius epidemicus makes a second re-appearance. _Or else_ the most essential external conditioning circumstances are specifically independent of the genius epidemicus; while the latter takes merely a remote share in the way of favouring or counteracting the production of the disease, manifesting its influence rather in modifying the form and direction of such morbid reactions as have arisen in the organism without its intervention at all,—in other words _the disease is subject to epidemic influence_.