82 INVASION 



scatter the seeds of a many-seeded fruit, but these constitute 

 relatively rare exceptions. This distinction in the matter of 

 invasion is of value in studying the relative rapidity of the 

 latter, and the establishment of new centres, but it is of 

 greatest importance in explaining the historical arrange- 

 ment of species in a formation, and hence has a direct 

 bearing upon alternation. It is entirely independent of the 

 number of invaders, which, as we have seen, depends upon 

 seed-production, mobility, distance, occupation, etc., but is 

 based solely upon mode of arrangement, and will be found 

 to underlie the primary lynes of abundance, copious and 

 gregarious. In this connection, it should also be noted that 

 the contingencies of migration, especially the concomitant 

 action in the same direction of two or more distributive 

 agencies, often results in the penetration of a group of indi- 

 viduals belonging to two or more species. This may well be 

 termed mass invasion (invasio solida): it is character- 

 istic of transition areas or regions and, along valleys or 

 other natural routes for migration, it gives rise to species 

 guilds (Artengenossenschaften, Drude). The movement of 

 species guilds constitutes one of the most complex and inter- 

 esting 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 otherwise homogeneous. 



The invasion of a formation may occur at three different 

 levels: (1) at the level of the fades, (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 determine, except in the case of 

 woody plants, such as shrubs and trees, which attain their 

 average height only after many years. Its importance is 

 fundamental : 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 



