132 Retrograde Varieties 



In systematic works the positive ones are as a 

 rule called " radiate," and the negative ones 

 " discoid." Discoid forms of the ordinary 

 camomile, of the daisy, of some asters (Aster 

 Tripolium), and of some centauries have been 

 described. Eadiate forms have been observed 

 in the tansy (Tanacetum vulgar e)^ the common 

 horse-weed or Canada fleabane (Erigeron cana- 

 densis) and the common groundsel (Senecio 

 vulgaris). Taken broadly the negative varie- 

 ties seem to be somewhat more numerous than 

 the positive ones but it is very difficult to come 

 to a definite conclusion on this point. 



Quite the contrary is the case with regard to 

 the color-varieties of red and blue flowers. 

 Here the loss of color is so common that every 

 one could give long lists of examples of it. Lin- 

 naeus himself supposed that no blue or red-col- 

 ored wild species would be without a white va- 

 riety. It is well known that he founded his 

 often criticized prescript never to trust to color 

 in recognizing or describing a species, on this 

 belief. 



On the other hand there are some red varie- 

 ties of white-flowered species. But they are 

 very rare, and little is known about their charac- 

 ters or constancy. Blue varieties of white spe- 

 cies are not found. The yarrow (Achillea Mil- 

 le folium) has a red-flowered form, which occurs 



