FIXITY OF SPECIES AND SUBSPECIES. 173 



as sharply defined from its neighbors as are most of the subspecies of C. nauseostia 

 or of C. parryi. Chrysothamnus greenei and C. pulchellus both are somewhat variable, 

 as is indicated by their subspecies. However, their variations are chiefly those of habit, 

 foliage, and size of head and flowers, and therefore perhaps not of the importance that 

 their taxonomic status would seem to indicate. 



Chrysothamnus parryi, as here extended, comprises an extensive series of variations. 

 In addition to the ten accepted subspecies there are a number of other forms, some 

 representing combinations of minor characters while others are intermediate or outlying 

 forms obviously brought about by unusual environmental conditions. A few of these 

 are described under the heading of Minor variations. The conclusion is that C. parryi is 

 in a highly unstable state. 



Table 18. — Variation in plants of Chrysothamnus nauseosus growing under apparently uniform conditions. 



But, as already indicated, the species now undergoing the greatest amount of change, 

 and the one which exhibits the greatest variety of forms, is C. nauseosus. All of the 20 

 subspecies described in this paper are presumably more than ecologic forms and it is 

 almost certain that at least most of them include still other units the characters of 

 which are inherited. Even such characters as the amount and color of the tomentum 

 are heritable, as indicated by plants now growing in the Botanical Garden of the Uni- 

 versity of California. In these plantings, for example, the very white herbage of sub- 

 species hololeucus stands out in strong contrast to the gray of subspecies gnaphalodes 

 and to the yellowish-green of subspecies viridulus and leiospermus in neighboring rows. 

 On the other hand, the distinctive fragrance of certain forms seems to be lost when these 

 are grown in the coastal districts. Other evidence as to the importance of minor char- 

 acters is obtained by a comparison with geographic distribution. While a few subspecies 

 extend over wide areas, most of them have a definite geographical range from which the 

 most closely related subspecies are excluded. However, such pairs often meet at the 

 borders of their ranges, and here they seem to grade insensibly into each other. WTiether 

 this is actually the case or the intermediate forms are the result of hybridization is a 

 question that will require a genetic analysis for its answer. The most remarkable 

 mixing of two subspecies in this manner which has been thus far noted, takes place at 



