474 <^N TWIN HYBRIDS. 



manifest in the crosses of 0. Lamarckiana with the types of our 

 dunes. The 0. Lamarckiana x biennis has been described in my 

 Mutationstheorie (2:31) for two generations, the third having only 

 been cultivated in 1907. The 0. Lamarckiana x muricata has been 

 described and figured in the same volume (see page 29), but 1 have 

 not as yet succeeded in getting a second generation, partly on 

 account of the weakness of the type and partly on account of the 

 general phenomenon of strongly diminished fertility in all these 

 primrose hybrids. 



The reciprocals of the two hybrids just mentioned afford the 

 curious and rare phenomenon which I call that of the twin hybrids. 

 In crossing one spike of 0. biennis with the pollen of one plant of 

 0. Lamarckiana, one does not get one but two hybrid types. The 

 same holds good for 0. muricata. These forms are intermediates 

 between the parents and almost alike when seen from a distance, 

 but sharply distinguished when closely examined. These differences 

 are the same whether 0. biennis or 0. muricata is the mother plant, 

 the hybrids showing their divergent origin only in apparently sub- 

 ordinate marks. One of the twins has broad and flat leaves of a 

 bright green; I call it 0. laeta. The other has narrow, more or less 

 furrow-shaped leaves of a grayish green, which are more hairy, 

 and is therefore designated 0. velutina. These same names may be 

 used to distinguish the twins in all the hybrid combinations in 

 which they may occur. 



The twin hybrids, 0. laeta and 0. velutina, occur in numerous 

 hybrid combinations where 0. Lamarckiana or one of its mutants 

 is the pollen parent; as for instance with 0. brevistylis, 0. rubriner- 

 vis and 0. nanella. In these cases they drop, at least in the first 

 generation, the special mark of the mutant-father, and so are the same 

 in all the cases named. The mother plant may be 0. biennis, 0. 

 biennis cruciata (a form of 0. biennis with linear petals which occurs 

 in the dunes of Holland), 0. muricata, or some other species of 

 Onagra, although not all of them are capable of producing twins. 



In the cases named the twins are widely different from their 

 reciprocal hybrids, but closely related between themselves. Their 

 characters often show transgressive fluctuability, and as a result of 

 this they are more easily distinguished at some periods of their life 

 than at others. The better the culture and the stouter the plants, 

 the larger and more striking are the differences. Especially speci- 

 mens which contrive to make a large rosette of root leaves before 

 they send up their stem, and therefore begin flowering only late in 



