V. GRANT 57 



reproduction, but not self-pollination, are important contributing 

 factors. A part of the species problems is due to the continuity of 

 the speciation process. As far as the higher plants are concerned, 

 however, the latter factor seems to be a minor source of difficulty 

 in comparison with the other three causes mentioned. 



Biological Species Concept Reconsidered 

 in the Light of Its Difficulties 



Four factors which contribute to the difficulty in application 

 of the biological species concept have been considered. These 

 factors are: (1) sibling species; (2) the gradual formation of 

 species from races; (3) hybridization; and (4) asexual reproduc- 

 tion. Each factor has a somewhat different bearing on the ques- 

 tion of the objective reality of the species as a biological unit. 



Sibling Species. The difficulties of the species concept arising 

 from the existence of sibling species are practical, not theoretical, 

 difficulties. The distinction between sibling species and normal 

 species is relative. Most sibling species when studied long enough 

 and hard enough are found to be separable morphologically. Ex- 

 ternal diagnostic characters were eventually discovered even for 

 the example par excellence of a pair of cryptic species, Drosophila 

 pseudoobscura and D. persimilis. 



The naming of sibling species in plants as a result of biosystem- 

 atic studies has provoked a certain discontent among many 

 herbarium curators and floristic taxonomists. The biosystematist 

 does indeed have a responsibility to determine and annotate as 

 many large herbarium collections as is feasible. He also has a 

 responsibility to science not to suppress his findings merely in 

 order to facilitate the task of herbarium filing. Two courses are 

 open to the curator or general taxonomist who is confronted with 

 undetermined material of sibling species. Let him determine it 

 as to species group rather than as to species. Or let him scrutinize 

 the material with more than average thoroughness. 



If the taxonomic procedures in different phyla are compared, 

 as Dobzhansky and Epling (1944) pointed out, the amount of 

 effort considered necessary for making a determination is not a 

 standard quantity. Dipterists are in the habit of killing and pin- 



