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 homo- 
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, the 
simplest example of association, 
