Sterile Varieties. 
89 
a list of further instances partly from the literature 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 application. 
DARWIN himself repeatedly cited them as objections and 
examined them minutely. 
Fig. 13. A flower of Li- 
Hum candidum plenum. 
The thalamus is changed 
into a long stalk on 
which the narrow per- 
fectly white petals are 
spirally arranged. 
