TWIN HYBRIDS OF OENOTHERA HOOKERI T. AND G. 279 



that the same deduction holds good for the succeeding generations. 

 Control experiments have justified this conception. The question, 

 however, why the two types never arise in equal numbers, but al- 

 ways with a large majority on the side of the Hookeri-\'\ke, remains 

 still to be answered. But analogous deviations are often seen after 

 crosses between species and mutants of the Oenotheras. 



As usual, some mutants, especially of the type of nanella, were 

 found in these cultures. 



The conception of the heterogamy of Oe. biennis, from which our 

 explanation started, may be controlled by experiments, in which the 

 pure species is replaced by its hybrid with another heterogamic 

 form. As such I have chosen Oe. syrticola Bartlett (Oe. muricata) 

 and fertilized the reciprocal hybrids, Oe. biennis xOe. syrticola and 

 Oe. syrticolaxOe. biennis, by the pollen of Oe. Hooker i. If the here- 

 ditary characters of the male gametes cannot be transmitted to the 

 female sexual cells, it is evident that the hybrids must behave, in 

 these cases, exactly like the pure species, which served as their 

 female parent. In other words the characters which they inherited 

 from their male parent, must disappear in the new cross. Of course, 

 heterogamy is hardly ever absolute and does not necessarily embrace 

 all the visible marks, and our argumentation applies only to the 

 really heterogamous characters as such. 



I made the crosses in 1913, using the hybrid races of the hetero- 

 gamous species mentioned above, which have been described in my 

 book on Gruppenweise Artbildung (p. 39—51). In 1914 I had from 

 each of the two crosses a culture of 60 plants, among which 15 were 

 allowed to flower and ripen their fruits. They reached a height of 

 about two meters and I compared them, during their whole lifetime, 

 with the hybrids between the pure species and Oe. Hookeri. 



The results were as follows: 



Table 9 



Each of the two groups was uniform, and exactly like the cor- 

 responding binary hybrid. It was evident that the characters of the 

 central parent of the formula, as I called it, were as fully eliminated 

 as is the case in the double reciprocal crosses between these two 

 heterogamous species. 



