474 GLEASON: THE PLANT ASSOCIATION 
expression of similar environmental conditions and similar sur- 
rounding plant population. If they are for convenience described 
under the same name, this treatment is in no wise comparable to 
the inclusion of several plant individuals in one species. 
VII. Succession 
21. Any change in the association, from any cause whatever, 
either in the component species or in their relative numbers of 
individuals, marks a step in the development of vegetation. If 
this change eventually becomes so great as to involve the re- 
_ placement of the original association by a different one, the process 
is known as succession. 
Under this definition, the inclusion of any developmental phe- 
nomenon under the term succession depends entirely upon the 
concept of the association held by the investigator. According 
to general custom, the association is defined broadly enough to 
permit considerable variation in its structure. Any structural 
changes within this limit may be referred to minor or periodic 
environmental changes or to the immigration of relatively unim- 
portant species, and are not considered as succession. | 
Just as there is a transition zone in space between two con- 
tiguous associations, where the species of both mingle, so is there 
also a transition period between the disappearance of the original 
association and the complete establishment of the new one, during 
which relic species of the former and pioneer species of the latter 
exist side by side. Causes similar to those which decide the width 
of the transition zone also determine the duration of the transition 
period. This will be relatively long between associations of 
| similar or slight environmental control and in cases of gradual 
environmental change, and relatively short between associations 
of different or great environmental control and in cases of rapid 
environmental change. : 
22. Arrival within migration distance of a new population 
with similar environmental demands (or overlapping ranges 
demands) results in fresh immigration and a consequent change 
in the population of an association. This immigration is limited 
to those members of the new population which are adjusted to the 
environmental control of the original association. If these im- 
