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 
liumic 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 influenced 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 sticcession 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 comiplex 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 efféct 
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 catises and effects, to a tentative system. At present it is difficult to 
