6 CONCEPT AND CAUSES OF SUCCESSION. 



the majority of primary successions, just as the chresard is the controlling 

 cause of vegetation structure, though it is dependent on the one hand upon 

 soil structure, and this upon physiography, and on the other upon the ram- 

 fall etc. 



Apart from the gain in clearness of analysis, greater emphasis upon the 

 proximate cause seems warranted by the fact that it is the chresard to which 

 the plant responds, and not the soil-texture or the physiography^- In like 

 manner, the invasion of a new area is a direct consequence of the action of the 

 causative process and not of the remote forces behind it. The failure to con- 

 sider the sequence of causes has produced confusion in the past (c/. Chapter III) 

 and will make more confusion in the future as the complex relations of vegeta- 

 tion and habitat come to be studied intensively. The difficulties involved 

 are well illustrated by the following conclusion of Ravmkiaer (1909) : 



"Every formation is before all dependent upon the temperature, aiid on the 

 humidity originating from the precipitation; the precipitation is distributed 

 in different ways in the soil according to its nature and surface, and hence 

 comes the division into formations. It therefore can not be said that one 

 formation is edaphic and another not; on the other hand, they may all be 

 termed edaphic, dependent as they are on the humidity of the soil; but as 

 the humidity is dependent upon the precipitation, it is most natxiral to say 

 they are all climatic." 



ESSENTIAL NATURE OF SUCCESSION. 



Developmental aspect. — The essential nature of succession is indicated by 

 its name. It is a series of invasions, a sequence of plant communities marked 

 by the change from lower to higher life-forms. The essence of succession lies 

 in the interaction of three factors, namely, habitat, life-forms, and species, in 

 the progressive development of a formation. In this development, habitat 

 and population act and react upon each other, alternating as cause and effect 

 until a state of equilibrium is reached. The factors of the habitat are the 

 causes of the responses or functions of the community, and these are the causes 

 of growth and development, and hen«e of structure, essentially as in the indi- 

 vidual. Succession must then be regarded as the development or Ufe-history 

 of the climax formation. It is the basic organic process of vegetation, which 

 results in the adult or final form of this complex organism. AU the stages 

 which precede the climax are stages of growth. They have the same essential 

 relation to the final stable structure of the organism that seedling and growing 

 plant have to the adult individual. Moreover, just as the adult plant repeats 

 its development, i. e., reproduces itself, whenever conditions permit, so also 

 does the climax formation. The parallel may be extended much further. 

 The flowering plant may repeat itself completely, may undergo primary 

 reproduction from an initial embryonic cell, or the reproduction may be 

 secondary or partial from a shoot. In Uke fashion, a climax formation may 

 repeat every one of its essential stages of growth in a primary area, or it may 

 reproduce itself only in its later stages, as in secondary areas. In short, the, 

 process of organic development is essentially alike for the individual and the 

 community. The correspondence is obvious when the necessary difference 

 in the complexity of the two organisms is recognized. 



