50 THE PLANT SPECIES 



G. sinuata and G. crassifolia. Examples such as these should warn 

 us against automatically combining related allopatric forms in 

 the same species. The category of superspecies, proposed by Mayr 

 ( 1942 ) , is available for groups of geographically isolated but 

 nevertheless valid species. 



It is no more justifiable in certain cases to treat sympatric pop- 

 ulations as distinct species than to reduce all allopatric popula- 

 tions automatically to the status of subspecies. We now know 

 that the terminal races of a polytypic species may overlap in 

 range in one part of the distribution area and yet be connected 

 by more or less continuous intergradation in other parts of the 

 area. When the population system is considered as a whole, it is 

 commonly found impossible to split it naturally into more than 

 one species. 



How frequently the condition of sympatry arises within the 

 limits of a single polytypic species is a moot point. Goldschmidt 

 ( 1952 ) is "disappointed" that "only a few examples, quoted over 

 and over again, exist to demonstrate what should be a most fre- 

 quent occurrence." Why we should expect overlapping rings of 

 races to be a commonplace phenomenon is not clear; but if fresh 

 examples are desired, it may be recorded that three have turned 

 up in the last few years in the family Polemoniaceae. Thus Gilia 

 capitata, the largely allopatric G. tenuiflora-latiflora group, and 

 Ipomopsis aggregata all exhibit terminal overlap of the extreme 

 subspecies. The evolutionary significance of this situation cannot 

 be considered here. As far as the species problem is concerned, 

 it indicates that marginal sympatry is not, in itself alone, a re- 

 liable species criterion. 



Magnitude of the Species Problem in Plants 



A biological species exhibits three characteristics (Mayr, 1948, 

 1949). It possesses its own distinctive morphological characters, 

 which arc separated from those of other species by a prominent 

 liiutus in the variation pattern. It possesses its own particular 

 ecological requirements, which reduce the competition with 

 other sympatric forms. It lias a combination of reproductive iso- 



