138 TORREYA 



The idea is current that a district in which a plant shows much variation or 

 has many closely related species must be its original home. According to the 

 above point of view this would only mean that the plant has been present within 

 the district for a comparatively long time and has developed in different direc- 

 tions under the pressure of varying conditions there . . . The similarity or dis- 

 similarity of two types alone will hardly be able to settle discussions concerning 

 relationship between them." This latter conclusion is arrived at by Hulten be- 

 cause of the complication resulting from "parallel selection" of biotypes by 

 separated but climatically similar regions. 



We have seen that the location of greatest differentiation of a type may be 

 at the center of origin of the group and, also, that the criterion can not be un- 

 critically applied for a number of reasons. 



Criterion 2. Location of Dominance or Greatest Abundance of 



Individuals 



In connection with this criterion it first is necessary to note that dominance 

 is a matter of the control of a community through reaction and coaction, and 

 abundance is only a matter of numbers of individuals. It is true that certain 

 forms may exert dominance through mere numbers, and that is possibly more 

 frequent among plants than animals, but often it is true that less abundant forms 

 are dominants by virtue of their life-form or strong actions. 



Species that are dominants in a certain community (and there are usually 

 not very many such species relative to the floristic composition of the commu- 

 nity as a whole) usually range more widely than the area of the community. 

 For example, beech, sugar maple, hemlock, and yellow birch all range more 

 widely than the northern hardwood climax association in which they are co- 

 dominants. It seems to me that dominance for a species can have no meaning 

 except in terms of community dynamics. If, however, we consider a genus, there 

 may be some instances in which the regions where certain species are commu- 

 nity dominants or codominants are also the regions where there is a large con- 

 centration of species of the genus. This appears to be true for Quercus and 

 Hicoria in the Ozark and Cumberland regions. Even here, however, a different 

 interpretation is likely. These are ancient land areas in which evolution has 

 long been going on and the numbers of species and their dominance may be un- 

 related phenomena, and also unrelated to center of origin. 



The center of greatest abundance of individuals, center of frequency, has 

 a special meaning only in connection with the distribution of the members of a 

 population, a subspecies, a species, etc. The assumption that the center of abun- 

 dance is also the center of origin for the type has to be based, it seems to me, 

 on an hypothesis that the species arose in the habitat where it is best capable 

 of abundant reproduction and establishment. This is a gratuitous assumption. 



