INVASION 237 
nuded, while vegetation with respect to the degree of occupation is open 
(sporadophytia), or closed (pycnophytia). Each type of area presents dif- 
ferent conditions to invaders, largely with respect to the factors determining 
ecesis. Naked habitats, rocks, talus, gravel slides, and dunes, while they 
offer ample opportunity for invasion on account of the lack of occupation, 
are really invaded with the greatest difficulty, not only because they contain 
originally few or no disseminules, but also because of their xerophytic 
character and the difficulty of obtaining a foothold, on account of the ex- 
treme density or instability of the soil. Denuded habitats, blowouts, sand 
draws, ponds, flood plains, wastes, fields, and burns, usually afford maximum 
opportunity for invasion. They invariably contain a large number of dis- 
seminules ready to spring up as soon as the original vegetation is destroyed. 
The surface, moreover, is usually such as to catch disseminules and to offer 
them optimum conditions of moisture and nutrition. Open formations are 
readily invaded, though the increased occupation renders entrance more 
difficult than it is in denuded areas. Closed formations, on the other hand, 
are characterized by a minimum of invasion, partly because invaders from 
different formations find unfavorable conditions in them, but chiefly be- 
cause the occupation of the inhabitants is so complete that invaders are 
unable to establish themselves. 
Invasion takes place by the penetration of single individuals or groups 
of individuals. This will depend in the first place upon the character of 
the disseminule. It is evident that, no matter how numerous the achenes may 
be, the invasion of those anemochorous species with comate or winged seeds 
or one-seeded fruits will be of the first type, while all species in which the 
disseminule is a several or many-seeded fruit or plant, as in hooked fruits, 
tumble-weeds, etc., will tend to produce a group of invaders. Occasionally 
of course, the accidents of migration will bring together a few one-seeded 
disseminules into a group, or will 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 centers, but it is of greatest importance in explaining 
the historical arrangement 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 types of abundance, copious, and 
gregarious. In this connection, it should also be noted that the contingen- 
cies 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 individuals belonging to two or more species. This may well be 
