COMPETITION 171 



determine tho space occupied by a plant, irrespective of 

 the room adjacent plants may allow it. The explanation of, 

 competition upon physical grounds likewise invalidates the 

 view that plants possess spheres of influence, other than 

 the areas within which they exert a demonstrable reaction 

 upon the physical factors preaent. 



Competition plays a very important role in alternation. 

 It produces minor examples of alternation in the physical 

 units of an asymmetrical series. Its greatest influence, 

 J&owever, is exerted in modifying the effects of asymmetry. 

 The reaction of occupants emphasizes, or reduces the effect 

 of asymmetry, and has a corresponding action upon alter- 

 nation. This result of competition is typical of succession, 

 in which the sequence of stages arises from the interaction 

 of occupant and invader. 



Alternation involves two ideas, viz., the alternation of 

 different species or formations with each other, and the 

 alternation of the same species or formation in similar but 

 separate situations. This is the evident result of asym- 

 metry, in response to which contiguous areas are dissimilar 

 and remote ones often similar. Individuals of the same 

 species, or examples of the same formation may be said to 

 alternate between two or more similar situations, while 

 different species, or formations are said to alternate with 

 each other, occurring of course in situations different in 

 character. From the nature of alternation, the two phe- 

 nomena are invariably found together. 



It is possible to distinguish three kinds of alternation : (1) 

 of a formation, association, layer, facies or species in similar 

 situations; (2) of similar or corresponding formations, 

 species, etc., in similar situations; (3) of facies and other 

 species with respect to number. The last two are merely 

 variations of the first, arising out of slight differences in 

 the physical factors of the alternating areas, the adjacent 

 flora, or the course of competition. For this reason, they 

 are not sharply set off, and it is undesirable to distinguish 

 them by actual terms. The alternation of different examples 

 of the same formation is a significant feature of greatly 



