IX. CLASSIFICATION OF SERES. 



Historical. — While the division of successions into progressive and regressive 

 by Nilsson (1899) may be regarded as an early attempt at classification, the 

 first system of classification for successions was proposed by Clements (1904: 

 107, 138; 1905 : 241). Cowles (1901 : 86) had already advanced his physio- 

 graphic grouping of the plant societies in the region of Chicago. While this 

 necessarily threw successions into topographic groups, his whole intent was to 

 classify plant societies or associations upon a genetic and dynamic basis (I. c, 

 178), and hence he did not consider the classification of successions. Later 

 (1911 : 161), he discusses the causes of vegetative cycles, and proposes a classi- 

 fication upon this basis. These two systems are the only ones yet suggested, 

 and as they have much in common it is desirable to consider them in detail 

 before taking up the system proposed here. 



Clements's System. — ^This was based primarily upon development, with 

 especial reference to reaction, and secondarily upon initial causes, in which 

 topographic causes were recognized as paramount. The division into normal 

 and anomalous successions, and the subdivision of normal successions into 

 primary and secondary were both based upon development. The subdivisions 

 of primary successions were all grounded upon topographic processes, and 

 those of secondary successions upon topographic and biotic agencies, while 

 anomalous successions were primarily due to climatic changes. The essential 

 features of the classification are indicated by the following outline: 



I. Normal Buccessiong. 



1. Primary euccessions. 



(1) By elevation. . 



(2) By volcanic action. 



(3) In residuary soils. 



(4) In colluvial soils. 



(5) In alluvial soils. 



(6) In aeolian soils. 



(7) In glacial soils. 



2. Secondary successions. 



(1) In eroded soils. 



(2) In flooded soils. 



(3) By subsidence. 



I. Normal successions— Continued. 



2. Secondary successions — Continued. 



(4) In landslips. 



(5) In drained and dried-out soils. 



(6) By animal agencies. 



(7) By human agency. 



a. Bums. 



b. Lumbering. 



c. Cultivation. 



d. Drainage. 



e. Irrigation. 



II. Anomalous successions. 



With reference to the initial physical or biological cause, a normal succession 

 was defined as one which begins with a bare area and ends in a climax, while 

 anomalous succession was defined as that in which an ultimate stage of a 

 normal succession is replaced by another stage, or in which the direction of 

 movement is radically changed. The former was stated to be of imiversal 

 occurrence and recurrence; the latter operates upon relatively few ultimate 

 formations. Anomalous successions were regarded as the usual result of a 

 slow backward-and-forward swing of climatic conditions. Primary succes- 

 sions were defined as those that arise on newly formed soils, or upon surfaces 

 exposed for the first time. Such areas have in consequence never borne vege- 

 tation before. They present extreme conditions for ecesis, and possess few 

 or no dormant disseminules. Accordingly, primary successions take place 

 slowly and exhibit many stages. Secondary successions arise on denuded 



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