54 THE PLANT SPECIES 



to the imperfect delineation of many species, is the process of 

 gradual speciation. We find species in all stages of divergence 

 from the ancestral forms. Some examples of borderline cases 

 between races and species, as manifested in superspecies com- 

 posed of very different and discontinuous allopatric forms, in 

 overlapping rings of races, and in species groups exhibiting mar- 

 ginal sympatry, have been discussed by Mayr (1942). 



Whether to regard a pair of populations in transition from 

 races to species as one or the other entity may at times require 

 an arbitrary decision. Such arbitrary decisions have to be made 

 here and there in most, if not all, of the genera listed in Table I. 

 This difficulty of interpretation is, however, in these genera and 

 elsewhere in higher plants, minor in comparison with the diffi- 

 culties arising from other causes yet to be considered. 



A second contributing factor to the species problem is the oc- 

 currence of sibling species. Species which are virtually indistin- 

 guishable morphologically are not uncommon in the higher plants. 

 Every specialist can cite examples from his own group. 



Cryptic species in plants are frequently associated with poly- 

 ploidy. Two diploid species, AA and BB, combine to form a new 

 allotetraploid species, AABB. The new population does not 

 merely fill the morphological gap between the original diploids, 

 but proceeds to segregate variations in the direction of each 

 parental species. The result is that until cytotaxonomic and cyto- 

 genetic work is carried out the true biological species will not 

 be recognized. Instead there will stand some artificial and ad- 

 mittedly unsatisfactory classification. In the hands of a conserva- 

 tive taxonomist the entire assemblage may be treated as one spe- 

 cies, whereas a morphologically minded taxonomist is likely to 

 recognize two species consisting respectively of AA plus certain 

 variants of AABB, and BB plus other variants of AABB. As poly- 

 ploidy continues to the higher levels of hexaploidy, octoploidy, 

 and so on, the number of species involved in a morphologically 

 indistinguishable complex of course increases. 



Some of the most familiar plants comprise clusters of sibling 

 species. The sagebrush or Artemisia tridentata group, for exam- 

 ple, is a complex consisting of an unknown number of biological 



