CYTOLOGY AND TAXONOMY 249 



of nature's continual experimentation among all the plant species that populate 

 a given area. (Clausen, Keck and Hiesey.) 



The above investigations on western plants include detailed studies of 

 interspecific relationships within certain genera, the taxonomic, ecological, 

 cytological, and genetical evidence all being brought to bear upon the 

 problem. One of these genera is Layia, of which the analysis to date is 

 summarized graphically in Fig. 176. Inspection of this diagram may 

 serve better than a long description to suggest the complexity of problems 

 of this kind and the amount of time and labor required for their solution. 

 One must choose between solving them quickly and solving them well. 



Conclusions.^ — Cytology has contributed to taxonomy in rwo impoi- 

 tant ways. First, it has added a new category of characters to those 

 commonly employed in classification. Obviously, chromosomes are 

 not very useful in the field; nevertheless, the.y are available and should 

 always be a part of anj^ thorough taxonomic analysis. They do not 

 always prove valuable, but when they do they compensate well for the 

 effort expended upon them. In numerous cases they have enabled 

 workers to decide whether a plant type newly observed in the field is a 

 Mendelian variant, a heteroploid derivative of a familiar species, or a 

 stable and fertile interspecific hybrid. In other words, visible characters 

 of the chromosomes frequently' indicate the invisil)le genie constitution 

 primarily responsible for the external characters. 



Cytology's second contribution to taxonomy lies in the clues it gives to 

 the origin of the species and other taxonomic units. Evolution involves 

 the origin of heritable variations, a selective process operating among the 

 \'ariants, and some isolating factor that permits a variant to become 

 modified independently of neighboring tj^pes. Darwin began with the 

 x'ariants; now cytology, with genetics, is revealing the inner causes of 

 the variations. It is also revealing some of the internal reasons for the 

 selection of certain variants to the exclusion of others, for cytological 

 phenomena, especiallj^ at meiosis, often show why certain chromosome 

 coml^inations are viable and stable while others are not. An internal 

 cause of isolation is also evident in cytological behavior which either 

 prevents successful crossing with related types or leads to the sterility 

 of hybrids in case the.y are formed. Finally, cytological studies some- 

 times indicate clearly in what order a series of differing types should 

 be read. 



C.ytology itself has gained greatl.y from its association with taxonomy. 

 Cytologists have the satisfaction of seeing their subject given wider 

 usefulness in its application to a biological problem of the first rank. 

 The.v are gaining a greater familiarity with the work of other biologists 

 less confined to the laboratory than they, and through this the.y are 

 de\('lo}^ing a deeper appreciation of the significance of their own subject. 



