10 BIOCHEMICAL SYSTEMATICS 



which involves a subjective judgment. The biological status of these 

 taxa would not be changed if they were called families or, for that mat- 

 ter, orders. However, one should understand that any changes in the 

 nomenclature of the categories of a portion of a taxonomic system or 

 arrangement should be followed consistently throughout that portion 

 of the system under consideration. 



It is evident that the taxon which lends itself most readily to 

 experimental techniques, that is, the species, is also the taxon which 

 is most likely to intergrade morphologically and genetically with 

 some closely related taxon. Thus the species is the most difficult 

 taxon for which to discern discontinuities and to estabUsh parameters 

 for recognition purposes. As one proceeds from the species to the 

 genus, family, order, and so on, though the discontinuities between 

 these various taxa becomes increasingly large, and consequently easier 

 to circumscribe and identify, nonetheless the subjectiveness of these 

 categories increases. 



Or, stated another way, it is easier for the taxonomist to 

 circumscribe and hence recognize the major taxonomic categories in 

 spite of the fact that the lesser specific and infraspecific categories are 

 better defined biologically and lend themselves to experimental 

 genetical and populational studies. 



EXPERIMENTAL CATEGORIES 



The development of cytogenetics and its application to taxon- 

 omy made possible a quasi-experimental approach to plant classifica- 

 tion. It was natural that early workers in this area of systematics felt 

 that a panacea was in the making and that with detailed (cytogene- 

 tical) study much of the difficulty in defining or circumscribing for- 

 mal categories would soon become a matter of the mere accumulation 

 and application of such data. Unfortunately, this has not proven to 

 be the case. It soon became apparent that sometimes obviously 

 closely related taxa would not hybridize while morphologically more 

 distinct taxa hybridized with ease, often both in the experimental 

 garden and in nature. Many studies which were conceived to establish 

 genetic affinities between taxa of given groups more often succeeded 

 in showing degrees of reproductive success or failure rather than 

 demonstrating comparative genomic differences. 



Such reproductive data are often difficult to obtain, and even 

 where assembled the data may contribute little to the solution of the 

 species problem since, at least in the higher plant groups, taxa show 

 all degrees of reproductive affinity, depending on the time and cir- 

 cumstances under which hybridization occurs (either artificial or 

 natural). 



