210 THE FORMATION 
THE DEVELOPMENT OF THE FORMATION 
259. A strict account of development should trace the results of the vari- 
ous activities of vegetation in their proper sequence. This is aggregation, 
migration, ecesis, reaction, and competition. These functions are so inti- 
mately and often so inextricably associated that it is hardly feasible to dis- 
cuss development by treating each one separately. In consequence, the two 
fundamental phenomena, invasion and succession, which they produce, are 
taken as the basis of the discussion. These, moreover, are different only in 
degree; succession is merely complete, periodic invasion. Nevertheless, the 
subject gains in clearness by a separate treatment of each. 
INVASION 
260. By invasion is understood the movement of plants from an area of 
a certain character into one of a different character, and their colonization 
in the latter. This movement may concern an individual, a species, or a 
group of species. From the nature of invasion, which contains the double 
idea of going into and taking possession of, it usually operates between con- 
tiguous formations, but it also takes place between formational zones and 
patches. More rarely and less noticeably, there may be invasion into a re- 
mote vegetation, as a result of long carriage by wind, water, birds, railroads, 
or vessels. Movement or migration, however, represents but one of the two 
ideas involved in invasion. Migration merely, carries the spore, seed, or 
propagule into the area to be invaded. In ecesis, the spores or seeds germi- 
nate and grow, after more or less adjustment, and in case the latter becomes 
sufficiently complete, the new plants reproduce and finally become estab- 
lished. With all terrestrial plants, invasion is possible only when migration 
is followed by ecesis, because of the inherent differences of formations or of 
areas of the same formation. In the case.of surface floating forms, such as 
Lemnaceae, and of the plancton, ecesis is of much less importance, on ac- 
count of the uniformity of the medium and the lack of attachment, and 
migration is often practically synonymous with invasion, 
MIGRATION 
261. Migration has been sometimes used loosely as a synonym for inva- 
sion, but it is here employed in its proper sense of removal or departure, i. e., 
movement, and is contrasted with ecesis, the making of a home, the two 
ideas being combined in invasion, which is a moving into and a taking pos- 
session of. An analysis of migration reveals the presence of four factors, 
mobility, agency, proximity, and topography. Not all of these are present 
