202 THE FORMATION 



to association. We may lay down the general principle that immobility tends 

 to maintain the association of the individuals of the same generation, i. e., 

 the association of like forms, while mobility tends to separate the similar 

 individuals of one generation and to bring unlike forms together. With 

 the mobile algae, separation of the members of each generation is the rule, 

 unless the individuals come to be associated in a thallus, or are grouped in 

 contact with the substratum. Flowering plants that are relatively immobile, 

 especially in the seed state, drop their seeds beneath and about the parent 

 plants, and in consequence dense association of the new plants is the rule. 

 In very many cases, however, this primitive tendency is largely or completely 

 negatived by the presence of special dissemination contrivances, which are 

 nearly, if not quite, as effective for many terrestrial plants as the free float- 

 ing habit is for algae. From this point, the whole question of mobility be- 

 longs to migration, just as the adjustment between the parent plants and 

 their offspring, or between plants established and the mobile plants to be 

 established, belongs to competition. 



If association were determined by reproduction and immobility alone, it 

 would exhibit areas dissimilar in the mass of individuals, as well as areas 

 dissimilar in the kinds of individuals. Some areas would be occupied by 

 plants of a single species, others by plants of several or many species. This 

 tendency of association to show differences is, however, greatly emphasized 

 by the fact that vegetation is fundamentally attached to and dependent upon 

 a surface that exhibits the most extreme physical differences. For this rea- 

 son, new differences in association appear, due not only to the morphological 

 differentiation of vegetation forms, but also to the changes in the degree and 

 manner of association produced directly by the different habitats. Associa- 

 tion might then be defined as a grouping together of plant individuals, of 

 parents and progeny, which is initiated by reproduction and immobility, and 

 determined by environment. It is a resultant of differences and similarities. 

 In consequence, association in its largest expression, vegetation, is essen- 

 tially heterogeneous, while in those areas which possess physical or biolog- 

 ical definiteness, habitats and vegetation centers, it is relatively homogene- 

 ous. This fundamental peculiarity has given us the concept of the 

 formation, an area of vegetation, or a particular association, which is hcwno- 

 geneous within itself, and at the same time essentially different from con- 

 tiguous areas, though falling into a phylogenetic series with some and a 

 biological series with others. From its nature, the plant formation is to be 

 considered the logical unit of vegetation, though it is not, of course, tlie 

 simplest example of association. 



