306 THE FORMATION 
single formation. Since they are often absent, or mixed with other facies of 
greater importance, they can not be considered the sole tests of the forma- 
tion. This view is reinforced by the fact that prairie, sand-hill, plains, and 
foot-hill all have their characteristic principal and secondary species, in ad- 
dition to facies that are more or less typical. In certain formations, doubt- 
less, Bouteloua and Andrepogen are relicts, in others invaders, while in the 
formations actually constituted by them they are dominant. The final solu- 
tion of such problems is quite impossible, however, until the comparative 
study of large areas can be based upon the accurate detailed investigation of 
the component formations. 
EXPERIMENTAL VEGETATION 
356. Scope and methods. The experimental study of the formation as a 
complex organism rests upon methods essentially similar to those discussed 
under experimental evolution. The scope of the two fields is practically the 
same, moreover, in that both deal with the experimental development of an 
organism and the structures that result. The actual problems are naturally 
very different, since the formation is a complex of individual plants, but the 
fundamental -basis of habitat, function, and structure is common to both. 
However, the functions now to be considered are aggregation, invasion, com- 
petition, etc., and the structures, zones, consocies, societies, communities, and 
families. The latter may properly be regarded as adaptations called forth 
by the adjustment, i. e., aggregation, migration, ecesis, etc., of the formation 
to the physical factors of the habitat. As consequences of measured factors, 
formational adjustment and adaptation must themselves be carefully meas- 
ured and recorded. For these purposes, the methods of quadrat and tran- 
sect, of chart, photograph, and formation herbarium are used. Invaluable 
as they are for any scientific inquiry into vegetation, such methods form the 
very foundation of experimental study in which accuracy is the first 
desideratum. 
It has already been shown that nature’s own experiments in the production 
of new forms furnish the best material for experimental evolution. This 
statement is equally true of experimental vegetation. The formation of new 
habitats by weathering and transport, and the denuding of old ones, yield 
experimental plots of the greatest value. This is likewise the case in the 
great majority of formations, where invasion or competition is active. These 
are the phenomena that must be considered in any careful study of vegeta- 
tion, but in taking them up from the experimental standpoint, greater atten- 
tion must be paid to detail, and the changes must be followed closely for a 
longer time. The method that makes use of existing changes in vegetation 
