=5. Plant succession.= The fact that formations arise and disappear was perceived by Biberg as early as 1749, but it received slight attention until Steenstrup’s study of the succession in the forests of Zealand (1844 prox.). In the development of formations, as well as in their recognition, nearly all workers have confined themselves to the investigation of particular changes. Berg (1844), Vaupell (1851), Hoffmann (1856), Middendorff (1864), Hult (1881), Senft (1888), Warming (1890), and others have added much to our detailed knowledge of formational development. Notwithstanding the lapse of more than a half century, the study of plant successions is by no means a general practice among ecologists. This is a ready explanation of the fact that the vast field has so far yielded but few generalizations. Warming (1895) was the first to compile the few general principles of development clearly indicated up to this time. The first critical attempt to systematize the investigation of succession was made by Clements (1904), though this can be considered as little more than a beginning on account of the small number of successions so far studied. Future progress in this field will be conditioned not only by the more frequent study of developmental problems by working ecologists, but also, and most especially, by the application of known principles of succession, and by the working out of new ones. =6. Ecological phytogeography.= Until recent years, the almost universal tendency was to give attention to formations from the standpoint of vegetation alone. While the habitat was touched here and there by isolated workers, and plant functions were being studied intensively by physiologists, both were practically ignored by ecologists as a class. The appearance of Warming’s _Lehrbuch der oecologischen Pflanzengeographie_ (1896) and of Schimper’s _Pflanzengeographie auf physiologischer Grundlage_ (1898) remedied this condition in a measure by a general discussion of the habitat, and by emphasizing the importance of the ecological or physiological point of view. Despite their frank recognition of the unique value of the habitat, the major part of both books was necessarily given to what may be termed the general description of formations. For this reason, and for others arising out of an almost complete dearth of methods of investigation, ecology is still almost entirely a floristic study in practice, although there is a universal recognition of the much greater value of the viewpoint which rests upon the relation between the formation and its habitat.