BOTANY 



tions may reasonably be placed in these categories, 

 but by no means all. Occasionally new domi- 

 nant or otherwise progressive mutations appear 

 {CEnothera rubricalyx is one, CE. gigas another) 

 which will furnish a new starting-point for evolution 

 in fresh directions. If we could have been present 

 when the first composite flower appeared we should 

 undoubtedly have looked upon it as a degenerate 

 monstrosity. Yet the Composite, on account of 

 the advantages afforded by their aggregate heads 

 of flowers combined with their methods of seed 

 distribution, have become the most successful and 

 cosmopolitan family of flowering plants. 



In the last few pages I have confined myself 

 rather closely to the CEnothera story, because of 

 the part it played in connection with the develop- 

 ment of a number of the views which are now 

 current in genetics. It is one of the lines of 

 research which have helped to bring about a con- 

 vergence of the originally independent sciences 

 of cytology and experimental breeding. In other 

 fields, such as the study of the sex-chromosomes in 

 many animals and recently in certain plants, and 

 in some of the experimental breeding work with 

 Drosophila as well as in CEnothera, a definite 

 relation has been demonstrated to exist between 

 the chromosomes and the external characters and 

 hereditary behaviour of organisms. Much remains 

 to be done, but the correlation of these two fields 



l 75 



