HISTORICAL DEVELOPMENT 3 
by virtue of which all other lines of botanical inquiry languished. This 
tendency had spent itself to a certain degree by the opening of the roth 
century, and both plant distribution and plant physiology began to take 
form. The stimulus given the former by Humboldt (1807) turned the 
attention of botanists more critically to the study of vegetation as a field 
in itself, and the growing feeling for structure in the latter led to Grise- 
bach’s concept of the formation, which he defined as follows: “I would 
term a group of plants which bears a definite physiognomic character, such 
as a meadow, a forest, etc., a phytogeographic formation. The latter may 
be characterized by a single social species, by a complex of dominant species 
belonging to one family, or, finally, it may show an aggregate of species, 
which, though of various taxonomic character, have a common peculiarity ; 
thus, the alpine meadows consist almost exclusively of perennial herbs.” 
The acceptance of the formation as the unit of vegetation took place slowly, 
but as a result of the work of Kerner (1863), Grisebach (1872), Engler 
(1879), Hult (1881, 1885), Goeze (1882), Beck (1884), Drude (1889), 
and Warming (1889), this point of view came tc be more and more preva- 
lent. It was not, however, until the appearance of three works of great 
importance, Warming (1895), Drude (1896), and Schimper (1898), that 
the concept of the formation became generally predominant. With the 
growing recognition of the formation during the last decade has appeared 
the inevitable tendency to stereotype the subject of ecology in this stage. 
The present need, in consequence, is to show very clearly that the idea of 
the formation is a fundamental, and not an ultimate one, and that the proper 
superstructure of ecology is yet to be reared upon this as the foundation. 
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 miuch 
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 de- 
velopment 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 
