V. GRANT 53 



patterns. The standard of reference is in each case the total num- 

 ber of species for a given region according to some recent revi- 

 sional study or Munz' A California Flora (in press). 



As Table I shows, the number and percentage frequency of 

 discrete species varies from genus to genus within wide limits. 

 Of the genera represented only Asclepias apparently is without 

 problematical species for the area considered, and only Ceano- 

 thus is without any good species in the area considered; but there 

 may be critical species of Asclepias in Mexico and Central Amer- 

 ica, and, conversely, Ceanothus seems to have about two good 

 species in eastern North America. All the genera thus contain 

 some good species as well as some poorly defined ones. 



The frequency of clearly demarcated species varies from group 

 to group within a single phylad. Thus within the family Pole- 

 moniaceae the two small genera, Allophyllum and Langloisia, 

 both consisting of herbaceous annual plants, may be contrasted. 

 There are four species of Langloisia, all of which are clearly 

 marked; not one of the five species of Allophyllum, on the other 

 hand, has well-defined boundaries. The 40 per cent of good 

 species estimated for the genus Gilia in this same family is an 

 average figure which covers extreme fluctuations from one infra- 

 generic group to the next. Of the species composing the sec- 

 tions Saltugilia and Giliastrum, for example, 60 per cent are good, 

 whereas the sections Gilia and Arachnion contain only 20 and 14 

 per cent good species. 



Of interest to the zoologist will be the conclusion that critical 

 species and species groups are the rule rather than the exception 

 in higher plants. Of interest to those botanists who regard the 

 species as a completely subjective concept is the demonstration 

 that at least some discrete units, generally recognized as species, 

 are to be found in almost every genus. 



Causes of the Species Problem 



The difficulties encountered in the practical application of a 

 biological definition of the species in many groups of higher 

 plants may be referred to several causes. 



One factor, which has long been acknowledged to contribute 



