TWIN HYBRIDS OF OENOTHERA HOOKERI T. AND G. 275 



This table shows that the three cases run parallel, that the seeds 

 do not contain empty grains due to the presence of a lethal factor, 

 and that the laeta split off velutina in the same proportion as is ob- 

 served in the first generation after the cross. In my book on Gruppen- 

 weise Artbildung I have deduced from the evidence given there that 

 the laeta-velutina qualities must be assumed to be present in the 

 Hookeri laeta in a special condition (Gruppenweise Artbildung, p. 132), 

 which, however, could not be that of a Mendelian hybrid, since 

 constant laeta failed in my experiments. The same arguments holds 

 good for the laeta from Oe. grandiflora and from Oe. mut. velutina. 

 The formula of Mendel for monohybrids would require 25 percent 

 velutina and if we would assume the presence of a lethal factor 

 killing the germs in such a manner that their seed coats could not 

 develop, it would lead us to expect 33 percent velutina. Our figures 

 are, however, so much higher than these, that it is evident that even 

 these assumptions could not explain them. 



Crosses of Oe. Lamarckiana with Oe. franciscana 



As already mentioned, this new species is so closely related to Oe. 

 Hookeri that we may confidently expect analogous results from its 

 different crosses. In order to control this deduction for this case I 

 crossed some annual plants of my culture of 1915 with my races of 

 Oe. Lamarckiana and Oe. Lamarckiana mut. nanella. The next year 

 I cultivated sixty plants from each cross and almost all of them 

 flowered in July-September. Already in April, when still in the boxes 

 and before being transplanted into the garden, the young rosettes 

 showed a difference. The larger part were almost like those of Oe. 

 franciscana, but others were stouter and with broader leaves, resem- 

 bling the laeta of the corresponding crosses of Oe. Hookeri. These 

 differences increased gradually during the summer and were sharp 

 and clear in August, without intermediates or dubious specimens. 

 There were a large majority of velutina. 1 found: 



Table 7 

 First generation. 



