68 ECESIC CAUSES. 



study of vegetation, distant migration has appeared more striking and inter- 

 esting than local. It is in no degree as important in the study of succession, as 

 local migration is primarily responsible for the population of new areas. Here, 

 again, exact observations and experiments are few, but most* of the evidence 

 available shows that effective invasion in quantity is always local. This is 

 doubtless true of great migrations such as those of the glacial and post- 

 glacial times, when populations moved hundreds of nules. These were appar- 

 ently only the gross result of repeated local movements, acting in the same 

 general direction through long periods. 



Up to the present time the study of succession has been almost wholly con- 

 fined to examining and correlating communities during one or a few seasons. 

 The development has not been followed in the various portions of its course, 

 but has been reconstructed from the end results, i. e., the communities. While 

 the whole course of a primary sere can be obtained in no other way, every one 

 of its stages permits quantitative study of its own development. Secondary 

 seres may often be studied as processes in their entirety, owing to their much 

 shorter course. In such work the position of the bare area with reference to 

 the migration agents active is of the first consequence. An area surrounded 

 by a community of the successional series will be quickly colonized by immi- 

 gration from aU sides. One lying in the ecotone between two associations 

 will have its development influenced by the prevailing direction of movement. 

 This is well illustrated by the behavior of new areas just below timber-line on 

 mountains. The area belongs to the forest climax, but it is invaded and held 

 by alpiae species for a very long time, it not permanently. This is due to 

 the ease with which seeds and fruits from the alpine area above are brought to 

 the area by gravity, and to the extreme difficulty the forest migrules find in 

 moving up the slope. Man and animals are the only agents which can over- 

 come this effect. The only exception is furnished by small comose seeds, such 

 as those of the fireweed and aspen, which may be carried hundreds of feet up 

 mountain sides by the wind (plates 15 c, 16 a). 



ECESIS. 



Nature and r61e. — Ecesis is the adjustment of the plant to a new home 

 (Clements, 1904 : 50; 1905 : 220; 1907 : 261). It consists of three essential 

 processes, germination, growth, and reproduction. It is the normal conse- 

 quence of migration, and it results sooner or later in competition. Ecesis 

 comprises all the processes exhibited by an invading germule from the time it 

 enters a new area imtil it is thoroughly established there. Hence it really 

 includes competition, except in the case of pioneers in bare areas. The ecesis 

 of a social plant is the same as that of an isolated invader in essentials, but it 

 takes place under conditions modified by the neighboring plants. Hence it 

 promises clearer analysis if ecesis is considered first and competition subse- 

 quently. 



Ecesis is the decisive factor in invasion. Migration is wholly ineffective 

 without it, and at present, indeed, is usually measured by it. The relation 

 between the two is most intimate. Ecesis in bare areas especially depends in 

 a large measure upon the time, direction, rapidity, distance, and amoimt of 

 migration. There is usually an essential alternation between the two, since 

 migration is followed by ecesis, and the latter then establishes a new group 



