INITIAL CAUSES. 35 



dition in the new area has two consequences of the first importance. It 

 determines the conditions of ecesis and hence the life-forms and species which 

 can act as pioneers. It Ukewise determines the direction of development 

 from drier to wetter or wetter to drier, and consequently the reactions possible. 

 The degree of departure from the cUmatic mean controls the life-history and 

 determines the number of stages possible between the pioneer and the climax 

 vegetation. 



The most critical factor in origin, then, is the amoimt of water-content in 

 comparison with the mean for the climax area. This is directly affected by 

 the texture of the soil, and this by the initial process or agent. The two 

 extremes possible are water at one end and rock at the other. The former has 

 an excess of water-content and a lack of solid material for fixing the habitat; the 

 latter has a surplus of stability and a deficit of water. Between the two occur 

 all possible combinations of water and solid materials in the form of the various 

 soils. While there is no ecological warrant for excluding rock and water from 

 soils, it will perhaps be clearer if the term is restricted to the usual meaning 

 of a mixture of comminuted rock and water. Apart from the amount of water 

 present in a new area, the stability of the substratum itself must be taken into 

 account. This is of the first consequence in extremely mobile soils, such as 

 those of dunes and blow-outs, where it determines the form and sequence of 

 the pioneers and calls forth a peculiar reaction. The usual course of succes- 

 sional development is a response to the increase or decrease of the holard, 

 i. e., to the ratio between water and rock, as already suggested. This ratio 

 expresses itself in three chief forms, water, rock, and soil. These produce 

 primary distinctions in the development of vegetation, and are used as the 

 physical basis of the system proposed in Chapter IX. 



Kinds of initial causes. — All initiating processes and agents agree in then- 

 fundamental relation to succession, viz, the production of a bare area charac- 

 terized by a more extreme condition, usually as to the holard. Moreover, 

 processes very different in themselves produce areas essentially similar or 

 identical as to the sere developed. A pond or lakelet may be formed by 

 physiographic processes, such as flooding, filling, or erosion, by a swing of 

 climate, by a rise in the water-table, by the action of ice, of gravity as in talus, 

 by beavers, or by man in a variety of ways. Many of these do, and all of 

 them may, occur in the same climax area, and would then result in identical 

 or similar seres. A sandy bank may be formed by currents, waves, ice, wind, 

 gravity, or biotic agencies, but the agent has relatively little effect upon the 

 succession. It is the wet, loose condition of the bare sand and the surrounding 

 vegetation which determine the development. The secondary importance of 

 the process is further indicated by the behavior of dune-sand when carried by 

 the wind into streams or lakes or heaped into dimes. The water-content of 

 the two areas is so controlling that the resulting seres converge only at or near 

 the climax. In case base-leveling is regarded as a process, it is obvious that 

 here is a process that produces the most diverse bare areas and seres. 



The classification of initial causes from the standpoint of the development 

 of vegetation necessarily groups together the most diverse agents and processes. 

 This is shown to be the case in the classification of seres outlined later. For 

 the sake of a complete account of initial causes it is most convenient to treat 

 them here from the standpoint of the nature of the agent or process, however 



