52 



THE PLANT SPECIES 



are set apart from all their congeners by a prominent disconti- 

 nuity in the variation pattern so that botanists have long been 

 in agreement as to their validity, it is much easier to cite the few 

 large genera in which all the species are good than the numerous 

 genera in which at least some of the species are poorly behaved. 

 Plant genera, like plant families, which have attained any con- 



Table I. The Frequency of Good Species in Some Genera of Seed Plants 



siderable size and yet present a complete assemblage of well- 

 defined species are exceptional. 



It would be of interest to have some definite data on the rela- 

 tive frequency of good and poor species in various plant genera. 

 Table I shows the proportion of good species in eleven genera 

 representing as many plant families and all the major life forms. 

 All these genera have been more or less carefully studied by 

 modern methods. They were selected primarily for that reason 

 and secondarily in order to illustrate a wide range of evolutionary 



