306 Annals of the Carnegie Museum. 



heterogeneous, while in those areas which possess physical or biological 

 definiteness, habitats, and vegetation centers, it is relatively homo- 

 geneous. This fundamental peculiarity has given us the concept of 

 the formation, an area of vegetation, or a particular association, which 

 is homogeneous within itself, and at the same time essentially differ- 

 ent from contiguous areas, though falling into a phylogenetic series 

 with some and a biological series with others. From its nature, the 

 plant-formation is to be considered the logical unit of vegetation, 

 though it is not, of course, the simplest example of association." 



In newly formed areas of soil, no formation is likely to be permanent 

 for any considerable length of time, for there will be changes in the 

 environment, which will render the area less suitable to the plants 

 occupying it than to certain other plants ; or, if not really rendered 

 unsuitable for the former, the conditions may become such that other 

 plants may occupy the area and crowd them out by competition. Cer- 

 tain recent investigations '■'' have shown that certain highly toxic secre- 

 tions may be given off by many species of plants, these toxic secre- 

 tions being poisonous in each case to different species to a different 

 extent. 



By succession the ecologist refers to the successive appearance and 

 replacement of different formations in a given area. In such a process 

 invasion is followed by a reaction upon the habitat. This reaction 

 may result in the replacement of one formation by another represent- 

 ing a later stage in the succession. A normal succession begins with a 

 habitat bare of plants {^nudation') and ends with the more or less per- 

 manent occupation of the habitat {^stabilization') by a formation desig- 

 nated as \.\i& climax-formation oi 'CiXQ. %\XQ,Q.t%'iAOXi. Normal successions 

 begin with new soils in primary successions, or with denuded soils in 

 secondary successions. In a primary succession with a new soil the 

 conditions are evidently not suitable for a luxuriant vegetation ; 

 usually such successions consist of many stages and reach stabilization 

 very slowly as compared with a secondary succession.^* 



In the discussion of the structure of the vegetation of a given area, 



^'Schreiner, Oswald, and Reed, Howard S. "Some Factors Influencing Soil 

 Fertility." U. S. Dept. Agr., Bureau Soils, Bull. 40: 1-40. 1907. And Living- 

 ston, B. E. " P'urther Studies on the Properties of Unproductive Soils." U. S. Dept. 

 Agr., Bureau of Soils, Bull. 36: 1-71. 1907. 



'* For a further elucidation of the ecological terms used in this article the reader 

 is referred to Clements, F. E. '* Research Methods in Ecology," Lincoln, Nebraska, 

 1905. 



