170 ALTERNATION 



depend largely upon the similarity existing between the 

 two. The arrangement in such cases is sparse. A species 

 with great seed-production and great mobility usually shows 

 both kinds of arrangement, the position of the individuals, 

 and the competition between them varying accordingly. 

 This is due to the intermittent action of distributing agents, 

 making it possible for the seeds to fall directly to the 

 ground during the times that winds, etc., are absent. The 

 three types of arrangement indicated above are termed 

 gregarious, copious, and gregario-copious. They furnish 

 the basis for the investigation of abundance,' which deals 

 essentially with the number and arrangement of the indi- 

 viduals of competing species. The effect of distance, i. e., 

 the interval between individuals, upon competition is funda- 

 mental. The competition increases as the interval dimin- 

 ishes, and the reverse. 



The view here advanced, i. e., that competition is nurely 

 physical in nature, renders untenable the current concep- 

 tions of vegetation pressure, occupation, etc. Masses of 

 vegetation are thought to force the weaker species toward 

 the edge, thus initiating an outward or forward pressure. 

 As has been shown above, no such phenomtsnon occurs in 

 vegetation. This movement is nothing but simple migra- 

 tion, followed by ecesis, and has no connection with 

 "weaker" species, or the development of a vital pressure. 

 The direction taken by the migrating disseminules is 

 essentially indeterminate. Migration seems to be outward, 

 or away from the mass, merely because the ecesis is greater 

 at the edge, where the increased dissimilarity between 

 plant forms diminishes the competition. The actual move- 

 ment is outward, but it takes place through the normal 

 operation of competition. In this connection, it should be 

 pointed out that the common view that plants require room 

 is inexact, if not erroneous. This is difficult of proof, as it 

 is impossible to distinguish room as such from the factors 

 normally present, light, heat, water and nutrient salts, but 

 it seems obvious that the available amounts of these will 

 'Pound and Clements. Phytogeography of Nebraska 2 ed. 69 1900 



