Retrograde Varieties 131 



omilla), the yarrow {Achillea Millefolium), 

 the daisies, the Dahlia and many others. 

 Species occur in this group of plants from time 

 to time that lack the ray-florets, as in the tansy 

 {Tanacetum vulgar e) and some artemisias. 

 And the genus of the marigolds or Bidens is 

 noted for containing both of these types. The 

 smaller and the three-toothed marigold {B. 

 cernua and B. tripartita) are very common 

 plants of wet soil and swamps, ordinarily lack- 

 ing the ray-florets, and in some countries they 

 are very abundant and wholly constant in this 

 respect, never forming radiate flower-heads. 

 On the other hand the white-flowered and the 

 purple marigold {B. leucantha and B. atropur- 

 purea) are cultivated species of our gardens, 

 prized for their showy flower-heads with large 

 white or deeply colored, nearly black-purple 

 florets. 



Here we have opportunity to observe positive 

 and negative varieties of the same character. 

 The smaller, and the three-toothed marigold 

 occur from time to time, provided with ray- 

 florets, showing a positive variation. And the 

 white marigold has produced in our gardens a 

 variety without rays. Such varieties are quite 

 constant, never returning to the old species. 



Positive and negative varieties of this kind 

 are by no means rare among the compositae. 



