Retrograde Varieties 153 



morphologic entities, but are to be considered 

 as physiologic units, present in all parts of the 

 organisms, and manifesting themselves where- 

 ever occasion is afforded. They are units in 

 the sense that they may appear and disappear 

 singly. But very often they are combined to 

 yield compound characters, which are capable 

 of analysis. Opportunities for such an analysis 

 are afforded by the groups of cultivated varie- 

 ties, showing a single distinguishing quality, or 

 a small group of them. 



