Sterile Varieties. 



89 



a list of further instances partly from the hterature and 

 partly from my own observations in order to place my 

 conclusions on a broader basis 

 of facts. 



The difficulty of this task is 

 increased by the fact that it often 

 seems impossible to show how 

 those cases, in which other in- 

 vestigators believed that they had 

 detected transitional series, are 

 to be explained on the theory of 

 mutation. This is especially so 

 where the authors have simply 

 relied on comparative investiga- 

 tions. The results of these can 

 usually be explained, no doubt, 

 by the supposition of transgres- 

 sive variability, but a proof can 

 only be given if the phenomena 

 in question are investigated by 

 statistical methods. 



In strong contrast to these 

 doubtful cases, however, there is 

 a long series of observations in 

 which the absence of transitions 

 is practically certain. Perhaps 

 the most striking of these are 

 the sterile varieties which consti- 

 tute one of the most serious ob- 

 stacles to the current doctrine of 

 selection, at any rate as regards its exclusive api)lication. 

 Darwin himself repeatedly cited them as objections and 

 examined them minutely. 



Fig. 13. A flower of Li- 

 liuui candidnm plenum. 

 The thalamus is changed 

 into a long stalk on 

 which the narrow per- 

 fectly white petals are 

 spirally arranged. 



