580 A TEXTBOOK OF BOTANY [Ch. XIV 



tation and geography that this phase of ecology is often 

 called Ecological Plant Geography. 



When a plant formation is examined more closely, it is 

 found composed, of course, from vegetation forms. Rarely, 

 however, if ever, does a formation consist of one form, but 

 usually there are several, and oftentimes many, complexly 

 but definitely interrelated. What now are the factors which 

 determine the presence of the respective vegetation forms in 

 particular formations? In the first place, while mere acci- 

 dent plays some part in special instances, in the long run the 

 natural methods of dissemination of plants are so effective 

 that all the kinds in a given region are able to migrate, and 

 are constantly sifting by their seeds, to all parts of it, though 

 they only develop in those places to which they are fitted. 

 Thus the forms in a formation represent those which are 

 retained from the entire drifting flora. Second, any given 

 habitat is naturally preempted and held by the largest 

 vegetation form of the country which happens to be best 

 fitted to it. Such a form is called dominant, and chiefly 

 determines the character of the formation. Theoretically we 

 would expect a single dominant form in each habitat, as 

 often occurs, e.g. in pine forests, maple groves, etc. Com- 

 monly, however, two or even more forms share the domi- 

 nance, as when Maples and Birches grow intermingled in 

 our New England forests, or, more strikingly, when decidu- 

 ous and evergreen trees grow intermingled, as so often in 

 our woods. In such case it is still uncertain whether this 

 shared dominance is a result of practically identical relations 

 to habitat, the two forms thus standing in a state of balanced 

 contention, or whether in some one of several conceivable 

 ways they are complementary to one another, one using what 

 the other does not need. Shared dominance is more common 

 towards the tropics, and single dominance towards the poles. 

 Vegetation forms which share dominance are often called 

 commensals (table-mates) , and are like or unlike, as the case 

 may be. Third, to the dominant forms some others may be 



