300 Retrograde Varieties 



But here we will limit ourselves to normal 

 cases. 



The first example to be considered is obvious- 

 ly the assumption that the parents of a cross 

 differ from each other in respect to two charac- 

 ters. A good illustrative example is afforded 

 by the thorn-apple. I have crossed the blue- 

 flowered thorny form, usually known as Datura 

 Tatula, with the white thornless type, desig- 

 nated as D. Stramonium inermis. Thorns and 

 blue pigment are obviously active qualities, as 

 they are dominant in the hybrids. In the 

 second generation both pairs of characters are 

 resolved into their constituents and paired anew 

 according to Mendel 's law. After isolating my 

 hybrids during the period of flowering, I counted 

 among their progeny : 



128 individuals with blue flowers and thorns 

 47 " " " " without " 



54 " white " and " 



21 " " " " without " 



250 



The significance of these numbers may easily 

 be seen, when we calculate what was to be ex- 

 pected on the assumption that both characters 

 follow Mendel's law, and that both are inde- 

 pendent from each other. Then we would have 

 three-fourths blue offspring and one-fourth in- 

 dividuals with white flowers. Each of these 



