264 THE FORMATION 



accumulation of vegetable matter is enormous. The plant remains decom- 

 pose slowly and incompletely under the water, giving rise to the various 

 humic acids. These possess remarkable antiseptic qualities, and have an 

 injurious effect upon protoplasm. They affect the absorption of water by 

 the root hairs, though this is also influence(| by poor aeration. The same 

 acids are found in practically all inland marshes and swamps, but the 

 quantity of decomposing vegetation in many is not great enough to produce 

 an efficient reaction. Formations of this type usually start as freshwater 

 swamps. The succession is apparently hydrostatic, but no thorough study 

 of its stages has as yet been made. 



323. Succession by modifying atmospheric factors. All layered forma- 

 tions, forests, thickets, many meadows and wastes, etc., show reactions of 

 this nature, and are in fact largely or exclusively determined by them. The 

 reaction is a complex one, though it is clear that light is the most efficient 

 of the modified factors, and that humidity, temperature, and wind, while 

 strongly affected, play subordinate parts. In normal successions, the effect 

 of shade, i. e., diffuse light, enters with the appearance of bushes or shrubs, 

 and becomes more and more pronounced in the ultimate forest stages. The 

 reaction is exerted chiefly by the facies, but the effect of this is to cause 

 increasing diffuseness in each successively lower layer, in direct ratio with 

 the increased branching and leaf expansion of the plants in the layer just 

 above. In the ultimate stage of many forests, especially where the facies are 

 reduced to one, the reaction of the primary layer is so intense as to pre- 

 clude all undergrowth. Anomalous successions often owe their origin to 

 the fact that certain trees react in such a way as to cause conditions in 

 which they produce seedlings with increasing difficulty, and thus offer a 

 field favorable to the ecesis of those species capable of enduring the dense 

 shade. Successions of this kind are almost invariably mesostatic, as it is 

 altogether exceptional that layered formations are either xerophytic or 

 hydrophytic. 



LAWS OF SUCCESSION 



324. The investigation of succession has so far been neither sufficiently 

 thorough nor systematic to permit the postulation 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, includ- 

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



