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. Radiate forms have been observed 

 in the tansy {Tanacetum vulgare), 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- 

 lefolium) has a red-flowered form, which occurs 



