THE MANNER OF INVASION 83 



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 prob- 

 lem 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 formations of the same category, 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 sufEers 

 shading or crowding better than the other, is longer-lived 

 or faster-gruwing, 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 in- 

 evitably 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 plants invade grassland, when the trees 

 or shrubs become more or less uniformly 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 in- 

 vaders, where either adaptation or extermination results. 

 Finally, it should be borne in mind that the invasion 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. 



