The subject of the present paper has occupied my attention for some years, but I may state that what follows is the outcome of notes prepared for lectures to my students and is only a preliminary attempt to focus our present knowledge of the geographical distribution of some tropical diseases, and to indicate as far as possible the knowledge which we at present possess of those physical phenomena which influence the production of these diseases and the area of their distribution. Why, for instance, some diseases are confined to limited areas of distribution, whereas others are endemic in extensive districts, and others again periodically extend their ravages throughout clearly defined, though widespreading, regions.