TAXONOMIC PRINCIPLES ] } 



Even such a promising criterion as chromosome number was 

 often found to be a poor guide for the identification or circumscrip- 

 tion of certain plant taxa. For example populations, and even in- 

 dividuals within populations, of Claytonia virginiana (Roth well, 1959; 

 W. Lewis, 1962) and Cardamine pratensis (Banach, 1950; and others) 

 tolerate a wide range of chromosome numbers. While polyploids of a 

 normally diploid entity are often ecologically, if not morphologically, 

 distinct, they are sometimes interspersed within populations which 

 appear to be fairly uniform from an ecological and morphological 

 point of view. Examples of diploid and tetraploid populations or in- 

 dividuals which can be distinguished in no other way than by their 

 chromosome number are becoming increasingly common in the taxo- 

 nomic literature, and this fact has understandably diminished the 

 hopes of many workers who would wish to use cytogenetical data as 

 the final criterion for categorical disposition. 



Fortunately, most workers, while recognizing the value of 

 cytogenetical data for systematic purposes, have been aware of the 

 taxonomic chaos that might ensue at the specific and infraspecific 

 levels if any attempt were made to define rigidly the formal categories 

 in terms of reproductive affinity or chromosome number. The formal 

 categories, which are established by international agreement under an 

 appropriate code, have been erected and modified subsequently by 

 several generations of taxonomists. The taxa are usually circum- 

 scribed by discontinuities, and more often than not they are natural 

 biological entities classified according to their relative morphological 

 similarities or differences (which presumably is a reflection of their 

 genetical similarities or differences). 



The "experimental categories" (see below) are in reality no 

 better defined than the formal categories and, as indicated above, 

 they suffer an inherent classificatory deficiency in that they may or 

 may not reflect relative genetic differences between and among 

 taxa. Lewis (1957) has clearly set forward the value of experimental 

 systematics from the standpoint of taxonomy by pointing out that 

 while such approaches do not permit an objective definition of the 

 species, they do provide an orientation for the concept. Hecht and 

 Tandon (1953) have appropriately stated that: 



The delimitation of two species upon the basis of their failure to form 

 a hybrid is untenable wherever single or few gene differences or simple 

 structural heterozygosity leads to the formation of nonviable combina- 

 tions. Incipient species may owe their origins to differences such as 

 these, but the accumulation of further differences must follow before 

 what was once a single species may be considered as two. 



