168 ALTERNATION 



that are dissimilar. Tiiis similarity must be one'of vege- 

 tation or habitat form, not one of systematic position. The 

 latter is in fact of no significance, except where there is a 

 certain correspondence between the two. Leaf, stem and 

 root characters determine the outcome, and those species 

 most alike in these features will be in close competition, 

 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 in- 

 vaders in the different stages of succession. Those invad- 

 ing 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. 

 Stabilisation results when the entrance of invaders of such 

 form as to exert an efficient reaction is no longer possible, 

 jn 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 composition, can not be displaced by another. 



It is obvious that the vegetation forms and habitat forms 

 of associated species are of fundamental importance in de- 

 termining the course and result of competition. Identity of 

 vegetation form regularly produces close competition, and 



