CHAPTER IV. 



RELAPSE AND RETROGRESSION. 



FLOWERS which have reached a given stage in the 

 progressive scale of colouration often show a tendency 

 to fall back to a lower stage. When this tendency is 

 of the nature of a mere temporary reversion (that is 

 to say, when it affects only a few individuals, or a 

 casual variety), it may conveniently be described as 

 Relapse. When> however, it affects a whole species, 

 and becomes fixed in the species by a new and pre- 

 sumably lower adaptation, it may best be styled Retro- 

 gression. The difference between these two forms of 

 reversion will become clearer after we have examined 

 a few cases of each in detail. 



Primary yellow flowers, like the buttercups, poten- 

 tillas, and St. John's worts, show little or no ten- 

 dency to vary in colour in a state of nature. They 

 have never passed through any earlier stage to which 

 they can revert ; and they are not likely to strike out 

 a new hue for themselves except through the action 

 of some special differentiating circumstances, such as 

 those ensured by cultivation. 



Some white flowers, on the other hand, show a 

 decided tendency occasionally to revert to yellow, 



