ALTERNATION 287 
In accordance with the above principles, the competition is closer between 
species of like form than between those that are dissimilar. This similarity 
must be one of vegetation or habitat form, not one of systematic position. 
The latter is in fact of no significance, except where there is a certain cor- 
respondence between the two. Leaf, stem, and root characters determine the 
outcome, and those species most alike in these features will be in close com- 
petition, regardless of their taxonomic similarity or dissimilarity. This is as 
conclusive of the competition between the species of the same genus as it is 
between those belonging to genera of widely separated families. From this. 
may be deduced a second principle of competition, viz., the ‘closeness of the 
competition between the individuals of different species varies directly with 
their similarity in vegetation or habitat form. This principle is of primary 
importance in the competition which arises between occupants and invaders 
in the different stages of succession. Those invading species that show the 
greatest resemblance to occupants in leaf, stem, and root form experience the 
greatest difficulty in establishing themselves. The species, on the contrary, 
which are so unlike the occupants that they come in at a clear advantage or 
disadvantage, establish themselves readily, in the one case as a result of the 
reaction, in the other by taking a subordinate position. This principle lies 
at the base of the changes in succession which give a peculiar stamp to each 
stage. A reaction sufficient to bring about the disappearance of one stage 
can be produced only by the entrance of invaders so different in form as to 
materially or entirely change the impress of the formation. Stabilization 
results when the entrance of invaders of such form as to exert an efficient 
reaction is no longer possible. In forests, while many vegetation forms can 
still enter, none of these produce a reaction sufficient to place the trees at a 
. disadvantage, and the ultimate forest stage, though it may change in compo- 
sition, can not be displaced by another. 
It is obvious that the vegetation forms and habitat forms of associated 
species are of fundamental importance in determining the course and result 
of competition. Identitv of vegetation form regularly produces close com- 
petition, and the consequent numerical reduction or disappearance of one or 
more species. Dissimilarity, on the other hand, tends to eliminate competi- 
tion, and to preserve the advantage of the superior form. Species of trees 
compete sharply with each other when found together; the same is true of 
shrubs, or rosettes, etc. The relation of the shrubs to the trees, or of the 
rosettes to the shrubs of a formation is one of subordination rather than of 
competition. The matter of height and width often enters here also to such 
a degree that the tallest herbs compete with the bushes and shrubs, and 
rosettes with mats or grasses. The amount and disposition of the leaf sur- 
face are decisive factors in the competition between species of the same 
