LAWS 133 



mesostatic, as it is altogether exceptional that layered for- 

 mations are either xerophytic or hydrophytic. 



LAWS OF SUCCESSION. 



The investigation of succession has so far been neither 

 sufficiently thorough nor systematic to permit the postu- 

 lation of definite laws. Enough has been done, however, to 

 warrant the formulation of a number of rules, which apply 

 to the successions studied, and afford a convenient method 

 for the critical investigation of all successions upon the 

 basis of initial causes, and reactions. Warming has already 

 brought together a few such rules, and an attempt is here 

 made to reduce the phenomena of succession, including its 

 causes and effects, to a tentative system. At present it is 

 difficult to make a thoroughly satisfactory classification of 

 such rules, and they are here arranged in general conform- 

 ity with the procedure in succession. 



I. Causation. The initial cause of a succession is the for- 

 mation or appearance of a new habitat, or the efficient 

 change of an existing one. 

 n. Reaction. Each stage reacts upon the habitat in such 

 a way as to produce physical conditions more or less 

 unfavorable to its permanence, but advantageous to 

 the invaders of the next stage, 

 in. Proximity and Mobility. 



(1) 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 dis- 

 tance. 



(3) The pioneer species are regularly derived from 

 different formations, as the latter nearly always con- 

 tain 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 



