262 Retrograde Varieties 



tried to trace this rule in the single cases, but 

 have met with nearly unsurmountable difficul- 

 ties, owing chiefly to our ignorance of the units 

 which form the differences between the parents 

 in the observed cases. 



In. the case of Oenothera muricata x bien- 

 nis the differentiating units reduce the fertility 

 to a low degree, threatening the offspring with 

 almost complete infertility and extinction. 

 But then we do not know whether these charac- 

 ters are really units, or perhaps only seemingly 

 so and are in reality composed of smaller en- 

 tities which as yet we are not able to segregate. 

 And as long as we are devoid of empirical means 

 of deciding such questions, it seems useless to go 

 farther into the details of the question of the 

 sterility. It should be stated here however, 

 that pure varietal crosses, when not accompan- 

 ied by unbalanced characters, have never showed 

 any tendency to diminished fertility. Hence 

 there can be little doubt that the unpaired units 

 are the cause of this decrease in reproductive 

 power. 



The genus Oenothera is to a large degree de- 

 void of varietal characteristics, especially in 

 the subgenus Onagra, to which biennis, mur- 

 icata, lamarckiana and some others belong. 

 On the other hand it seems to be rich in 

 elementary species, but an adequate study of 



