124 STRUCTURE AND UNITS OF VEGETATION. 



has not been wholly ignored in the formational concept. All of the writers 

 upon retrogression and regeneration of communities have had an inkling of 

 this fact, but have nowhere expressed it in the formational concept. Drude 

 suggests the idea more or less incidentally in his definitions (1890 : 29; 1896 : 

 286) when he speaks of a formation reaching an end in itself. Poimd and 

 Clements (1898 : 216; 1900 : 315) distinguished formations as either primitive 

 or recent with respect to origin, and stated that formations originate at the 

 present day by one of two principal methods, by nascence or by modification. 

 Schimper's (1898) much-discussed division of formations into climatic and 

 edaphic was really based upon development, but he failed to recognize the 

 fundamental and universal nature of edaphic formations as processes of 

 development. In his physiographic ecology, Cowles (1901) dealt primarily 

 with development, though this fact was obscured by the emphasis laid upon 

 physiography. However, he used the term "society" in place of "forma- 

 tion," and his developmental ideas were not embodied in the formational con- 

 cept. Clements (1902; 1904 : 6; 1905 : 199; 1907 : 219) advanced the concept 

 that the formation was essentially developmental in character, and stated 

 that it may be regarded as a complex organism whic];i shows both functions 

 and structure, and passes through a cycle of development similar to that of 

 the plant. Transeau (1905 : 886) also adopted a similar view in the statement 

 that " each formation is made up of many societies, bearing a definite succes- 

 sional relation to one another." He made no concrete applications of his 

 view, and hence it remains ambiguous. Moss (1907 : 12; 1910 : 36) proposed 

 a view similar to the two preceding, in which, however, the limitation of the 

 formation was grounded primarily upon the habitat. Tansley (1911 : 9) has 

 adopted Moss's concept, and defines the formation as follows: 



" In the normal primary development of a formation, the associations involved 

 show intimate relations and transitions one to another, and the whole set of 

 associations has a definite flora dependent on the type of soU. It is for these 

 reasons that we consider the entire set of plant communities on a given type 

 of soil, in the same geographical region, and imder given climatic conditions, 

 as belonging to one formation, in spite of the diversity of the plant forms in the 

 different associations. The plant formation thus appears as the whole of the 

 natural and semi-natural plant-covering occupying a certain type of soil, char- 

 acterized by definite plant communities and a definite flora." 



As has elsewhere been shown, the developmental value of this concept has 

 been greatly reduced by linking the habitat to a type of soil. 



THE FORMATION. 



Developmental concept of the formation. — In spite of the growing tendency 

 just indicated, no attempt has hitherto been made to put the formation either 

 chiefly or wholly upon a developmental basis. While this view has been 

 stated and restated in the preceding pages, it seems desirable to repeat it here 

 at some length. The unit of vegetation, the climax formation, is an organic 

 entity. As an organism, the formation arises, grows, matures, and dies. Its 

 response to the habitat is shown in processes or functions and in structures 

 which are the record as well as the result of these functions. Furthermore, 

 each climax formation is able to reproduce itself, repeating with essential 

 fidelity the stages of its development. The life-history of a formation is a 



