SUCCESSION 239 
plants invade grassland, when the trees or shrubs become more or less uni- 
formly scattered in an open woodland or open thicket. Here the grassland 
takes on an altogether different appearance superficially, though it is usually 
unchanged, except beneath and about the invaders, where either adaptation 
or extermination results. Finally, it should be borne in mind that the inva- 
sion of a particular formation, especially in the case of layered thickets and 
forests, often takes place at two levels, at the height of the facies and below 
the facies. 
INVESTIGATION OF INVASION 
287. The methods to be used in the study of invasion are those already 
described elsewhere. The migration circle is of the first importance because 
it makes it possible to secure an accurate record of actual movement. Quad- 
rat and transect’ are valuable, but from their nature they are more service- 
able for ecesis than for migration. All of these should be of the permanent 
type, in order that the fate of invaders may be followed for several years at 
least. Permanent areas furnish evidence of the changes. wrought in the 
actual vegetation, while denuded ones can serve only to show the potential 
migration and ecesis of the constituent species. Transition zones and areas 
are special seats of invasion; they are best studied by means of the belt tran- 
sect and the ecotone chart. The movement of a line of invaders or of scat- 
tered outposts is traced by the use of labeled stakes at the points concerned. 
It is clear that this. method will yield conclusive data in regard to the great 
invasions between regions, such as the movement of species guilds, the ad- 
vance of the forest frontier, etc. When invasion is scattered, factor instru- 
ments can not be used to advantage, but where the invading line is well 
marked, or where extra-formational areas occur, a knowledge of the phys- 
ical factors is a great aid. 
An invasion that has been completed can not be studied in the manner in- 
dicated. A method of comparison must be used, in order to determine the 
original home of the invaders. For this an exact knowledge of the contig- 
uous formations and of the abundance of the species common to all is a pre- 
requisite. With this as a basis, it is usually a simple task to refer all the 
species of the formation concerned to their proper place in the groups, indig- 
enous, derived, and adventitious. 
SUCCESSION 
288. Concept. Succession is the phenomenon in which a series of inva- 
sions occurs in the same spot. It is important, however, to distinguish clearly 
between succession and invasion, for, while the one is the direct result of the 
