238 THE FORMATION 
termed mass invasion; it is characteristic of transition areas or regions, and 
along valleys or other natural routes for migration it gives rise to species 
guilds. The movement of species guilds constitutes one of the most com- 
plex and interesting problems in the whole field of invasion, the solution 
of which can be attempted only after the thorough analysis of the simpler 
invasions between formations. A better understanding of the meaning of 
invasion by species guilds is imperative for the natural limitation of regions, 
as at present such groups constitute alien associations in many regions other- 
wise homogeneous, 
286. Influence of levels. The invasion of a formation may occur at three 
different levels: (1) at the level of the facies, (2) below the facies, (3) 
above the facies, depending directly upon the relative height of invaders 
and occupants. The invasion level is an extremely simple matter to deter- 
mine, except in the case of woody plants, such as shrubs and trees, which 
attain their average height only after many years. Its importance is funda- 
mental. The level at which invasion occurs not only determines the 
immediate constitution of the formation, whether its impress shall still be 
given by the occupants or by the invaders or by both together, but it also 
. decides the whole future of the formation, i. e., whether the invaders or 
occupants shall persist unmodified or modified. The problem is an extremely 
complex one, but the careful analysis of invasion at each level throws a 
flood of light upon it.. The entrance of invaders of the same general height 
as the facies of a formation results regularly in mixed formations. This is 
well illustrated by the structure of the transition areas between two forma- 
tions of the same categary, i. e., forests, meadows, etc. It is seldom, 
however, that the facies and invaders are so equally matched in height and 
other qualities that they remain in equilibrium for a long period. One or 
the other has a slight advantage in height, or the one suffers shading or 
crowding better than the other, is longer-lived or faster-growing, with the 
result that invader yields to occupant, or occupant to invader. It is a well- 
known fact that many mixed formations represent intermediate stages of 
development. 
Invasion at a level different from that of the facies is inevitably followed 
by modification. If the invasion takes place below the facies, the invaders 
will be exterminated gradually, or slowly assimilated. In either case, there 
is little structural change in the formation, and its stability is affected 
slightly or not at all. If the invaders overtop the facies in any considerable 
number, the entire formation undergoes partial or complete modification, 
or in extreme cases it disappears, as is typically the case in succession. A 
peculiar variation of invasion at a level above the facies is seen where woody 
