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 occupatiort, 

 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 distri1>utive agencies, often results in the penetration of a 

 group of individuals belonging to two or more species. This may well be 



