42 THE PLANT SPECIES 



knowledge of individual species gained since the nineteenth 

 century an expanded concept was required. Criteria had to be 

 sought that would lead to the demarcation of species not only 

 within a local community but also on the wider stage of the 

 world fauna and flora. The result of this search, conducted by 

 numerous students during the past seventy years, is the bio- 

 logical species concept, which defines species on the basis of 

 reproductive isolation or the presence of barriers to gene ex- 

 change. 



The period between primitive man and the modern evolution- 

 ists, that is, the period during which geographical variation and 

 the intergradation of morphological characters were known, but 

 the idea of reproductive isolating mechanisms had not yet been 

 formulated to explain the limits of this variation, was marked by 

 an interregnum in the species concept. The Linnaean or typologi- 

 cal species concept, which emphasized morphological difference 

 rather than discontinuity as the criterion of species and recog- 

 nized species universally throughout the plant and animal king- 

 doms, prevailed for two centuries during this interregnum, and 

 it has inevitably left its stamp on modern systematics. 



In its original conception the Linnaean species was a fixed and 

 immutable entity. In the nineteenth century, with the increase 

 in knowledge concerning natural variation and the rise of the 

 theory of evolution, the fixed category became regarded more 

 often as an arbitrarily determined stage above the level of the 

 variety. The morphologically defined species, which seemed to 

 be an objective reality in the period of Linnaeus and Cuvier, 

 became frankly recognized as a subjective concept in the nine- 

 teenth and early twentieth centuries. 



Critique of the Species Concept 



The problem of defining the species in plants has been com- 

 plicated by the fact that different botanists have applied the 

 name of species to entities ranging in magnitude from homo- 

 zygous biotypes (Lotsy) to species groups (Engler). A common 

 practice among botanists is to recognize as a species any popu- 

 lation exhibiting distinctive morphological characters combined 



