44 THE PLANT SPECIES 



another . . ." (Mason, 1950). The presence of reproductive iso- 

 lating mechanisms is a feature of species, but not of subspecies; 

 the smallest population possessing such reproductive barriers is 

 the species, and not the polytypic section or genus. 



The difficulties of the typological species concept have led 

 some students to adopt, not the biological, but still another, nomi- 

 nalistic concept of the species. The proponents of this concept 

 stipulate that the species is (are) an "empty category," "mental 

 units rather than biological units," "highly abstract fictions," an 

 "abstract category in the taxonomic structure" which "has no 

 foundation in reality and obviously cannot be objectively de- 

 fined" (Mason, 1950; Davidson, 1954; Burma, 1954). 



The nominalistic concept is logically sound although scientifi- 

 cally barren. If all that is implied by the word "species" is the 

 category in the taxonomic hierarchy corresponding to the specific 

 name in the binomial system of nomenclature, there can be no 

 disagreement as to its artificiality. In this sense of the word, the 

 unicorn, if properly described according to the Rules of Nomen- 

 clature, would be as real a species as the horse. 



According to Mason again (1950), "The wisdom of past expe- 

 rience has dictated that the taxonomist purposely refrain from 

 defining these categories [species, genus, family, order, etc.] in 

 any way that will impose restrictions on the freedom with which 

 he may express the interrelationships that he construes to exist." 

 With regard to the genus, family, and order the absence of an 

 objective definition is not so much a matter of wisdom as of 

 necessity. With regard to the species the attempt to preserve a 

 wide elasticity of usage, besides being unnecessary, is also un- 

 wise, since it leads to a chaotic condition in taxonomic work 

 which could be avoided. Every branch of natural science is 

 sooner or later compelled to define its basic units in objective 

 terms. It would be remarkable if plant taxonomy were to prove 

 unique in this respect. In the meantime, as so aptly pointed out 

 by Camp (1951), the adherence to a subjective criterion of its 

 basic unit is responsible for the fact that plant taxonomy, in many 

 of its branches, is not yet a science but an art. 



The opinion that the species is not a definite entity has been 



