46 THE PLANT SPECIES 



cept applicable to the majority of higher organisms, does repre- 

 sent a very important loss. If in any given group of organisms 

 we do not find an evolutionary development into reproductively 

 isolated populations, we can, after all, fall back upon a morpho- 

 logical criterion and call the various forms by some other name 

 than species, such as taxon or binom. Biologists who reserve the 

 category of species for morphologically similar groupings of in- 

 dividuals, on the other hand, have not left any alternative desig- 

 nation for the reproductively isolated system of breeding pop- 

 ulations. 



The reproductively isolated population, whether it is called a 

 species or some other name, undoubtedly represents a major 

 biological unit, which must be investigated and discussed. This 

 task will be facilitated if the unit in question can simply be re- 

 ferred to as "the species" or, if any qualification is needed, as 

 "the biological species." For purely operational purposes, if for 

 no other reason, the adoption of the biological definition of the 

 species is justified in discussions of the origin, nature, and be- 

 havior of reproductively isolated population systems. 



The substitution of a biological for a morphological definition 

 of the species does not provide a panacea for the solution of every 

 complex situation in nature. The naive assumption that it should 

 do so, more often tacitly implied than explicitly stated, is respon- 

 sible for the disillusionment in the biological species concept felt 

 by some biologists whenever they run into difficulties of inter- 

 pretation. Such difficulties should be expected, considering the 

 wide diversity of life cycles, breeding systems, and population 

 structures existing among sexual organisms. All that can be 

 claimed for the biological species concept is that it provides a 

 framework that works better than any other yet proposed for the 

 analysis of species problems. 



The remainder of this paper will be devoted to a consideration 

 of certain problems involved in the recognition and demarcation 

 of biological species in the higher plants. Tin's inquiry will give 

 attention to the difficulties encountered in setting the limits of 

 species, the causes of those difficulties, and the biological signifi- 

 cance of the causes. 



