172 GENUS CHRYSOTHAMNUS. 



FIXITY OF SPECIES AND SUBSPECIES AND OF THEIR CHARACTERS. 



The best evidence regarding the fixity of forms and of their characters is obtained 

 by experimental methods. In the case of perennials like Chrysothamnus, such experi- 

 ments must necessarily extend over a long period of years, in order to permit of the 

 growth of several generations from seed of known origin. Such work has been under- 

 taken in the present case, but results are now available from only the first generation. 

 Transplants of rooted specimens of a few of the forms have also been made and the 

 resulting changes studied. The data from these experiments were used in arriving at 

 the opinions here expressed, but much more extensive observations were made upon the 

 wild plants, especially as to the extent of variation under different ecologic conditions 

 and in various geographic areas. Finally, a close statistical and analytical study has 

 been made of series of specimens brought in from the field for this purpose. As a result 

 of the evidence assembled from these various sources, it is found that the genus includes 

 a few species in which the characters are remarkably constant, species which are hence 

 known to be non-plastic and which do not break up into minor segregates. But it is 

 also found that certain other species are in a highly plastic condition, such species 

 comprising a large number of closely related subspecies, many of which are in turn 

 breaking up through mutation and gradual variation into innumerable races and ecads. 

 The variability of the characters themselves has been already indicated to some extent 

 in the preceding chapter on discussion of criteria. 



The most nearly constant of the species are those which are assumed, for other 

 reasons, to be the most primitive ; namely, C. paniculatus and C. teretifolius. These two 

 are sharply set off from the other members of the genus and do not intergrade into 

 each other. As evidence of the constancy of their characters may be cited an exami- 

 nation of 11 specimens of teretifolius, all from different localities, in which the length 

 of the corolla did not vary by more than 0.5 mm., the length of the lobes by only 0.2 

 mm., and the ratio of lobe-length to corolla-length by 3 per cent. As contrasted with 

 this, the corolla of C. nauseosus varies by as much as 6 mm. in length, the lobes by 2 

 mm., and the ratio by over 27 per cent. This suggests that nauseosus is a composite 

 species, while teretifolius is not. But even within a well-defined subspecies of the 

 former, for example graveolens, the corolla varies to the extent of 2 mm., the lobes by 

 0.7 mm., and the ratio between the two by 7.5 per cent. Similar figures have been 

 obtained for length of involucre, relative lengths of style-appendage and stigma, and 

 other characters. The conclusion is therefore inevitable that teretifolius is in a set 

 condition as contrasted with the highly variable and composite nauseosus. 



With C. teretifolius (or the less well-known paniculatus) as the species with characters 

 the most definitely fixed, and with C. nauseosus as the one which is the most plastic, 

 the others exhibit varying degrees of mobility. Three of them are so local in their 

 distribution, at least as far as present collections indicate, that they can not be defi- 

 nitely assigned as to variability, but all of them will doubtless be found to be fairly 

 constant in their characters. These are gramineus, albidus, and pyramidatus. C. vaseyi 

 is also well fixed in its characters, there being no varieties, while even its ecologic forms 

 are few and not well marked. C. viscidijlorus presents a very different case. It un- 

 doubtedly represents the extreme of plasticity among the Typici. The ten subspecies 

 here described represent only a small proportion of the forms known in the field. It 

 seems that the species is capable of occupying areas of considerable diversity, and this 

 probably accounts for its wide distribution. The large number of habitats occupied is 

 equaled by the large number of forms. While most of these have arisen through eco- 

 logic factors and are fluctuating in their nature, others are quite certainly more deep- 

 seated in their origin. This is indicated by the absence of intergrading forms. The 

 more striking of these are given subspecific rank in the present paper. It is believed 

 that none of them, with the possible exception of linifolius, is as well established and 



