SUCCESSION 265 



make a thoroughly satisfactory classification of such rules, and they are 

 here arranged in general conformity with the procedure in succession. 



I. Causation, The initial cause of a succession is the formation or ap- 

 pearance of a new habitat, or the efficient change of an existing one. 

 II. Reaction, Each stage reacts upon the habitat in such a way as to pro- 

 duce physical conditions more or less unfavorable to its permanence, 

 but advantageous to the invaders of the next stage. 

 III. Proximity and mobility, 



(i) The pioneers of a succession are those species nearest at hand 

 that are the most mobile, 



(2) The number of migrants from any formation into a habitat varies 



inversely as the square of the distance, 



(3) The pioneer species are regularly derived from different forma- 



tions, as the latter nearly always contain permobile species 

 capable of effective ecesis, 



(4) The plants of the initial stages are normally algae and fungi, 



with minute spores, composites, and grasses, which possess 

 permobile fruits, or ruderal plants, on account of their great 

 seed production. 

 IV. Ecesis. 



( 1 ) All the migrants into a new, denuded, or greatly modified habitat 



are sorted by ecesis into three groups: (i) those that are 

 unable to germinate or grow, and soon die; (2) those that 

 grow normally under the conditions present; (3) those that 

 pass through one or more of the earlier stages in a dormant 

 state to appear at a later stage of the succession. 



(2) Wherever ruderal vegetation is present, it contributes a large 



number of the pioneer species of each succession, on account 

 of the thorough ecesis. In other regions this part is played 

 by subruderal native species. 



(3) Annuals and biennials are characteristic of the early stages of 



secondary successions, on account of their great seed produc- 

 tion and ready ecesis. 



(4) In layered formations, heliophytes appear before sciophytes; 



they ultimately yield to the latter, except where they are able 

 to maintain a position in the primary layer. 



(5) Excessive seed production and slight mobility lead to the im- 



perfect ecesis of individuals in dense stands, and in conse- 

 quence usually produce great instability. 



