I9I5.] PURE SPECIES OF OENOTHERA. 239 



biennis and franciscana have in the F2 generations given abundant 

 evidence of that extensive variation interpreted as segregation. I 

 beheve that the apparent stabiHty of the very small progenies pro- 

 duced by hybrids of biennis and muricata simply means that the 

 remarkably high mortality among gametes and zygotes of these 

 hybrids, or the delayed germination of their seeds, has prevented 

 the appearance in our cultures of the diverse types which theo- 

 retically would be expected. Any general conclusions on genetic 

 behavior in the Oenotheras which fails to take into account the 

 phenomena of sterility rests upon insecure foundations. 



It is true that we do not know to what extent physiological fac- 

 tors may affect seed sterility as well as pollen and ovule abortion. 

 Nevertheless a main fact is clear, namely that seed sterility elimi- 

 nates in certain Oenothera species and hybrids immense numbers 

 of zygotes which fail to develop seeds. And, furthermore, we 

 know for Oenotheras that large classes of weak offspring are some- 

 times produced that are unable to reach maturity. Seedlings with 

 white or yellow cotyledons, which quickly die, are not uncommon 

 in my experience with Oenothera cultures ; in certain cases they 

 have appeared in very large numbers (Davis, '11, p. 222) and prob- 

 ably have important genetical significance. This situation in Oeno- 

 thera finds a close parallel in the behavior recorded for a number of 

 animals and plants. Thus Baur's " golden " variety of Antirrhinum is 

 an impure or heterozygous form which besides reproducing itself 

 throws a class of normal green plants and a class represented by 

 weak yellow seedlings that shortly die. The yellow mice studied 

 by Castle and Little although interbred always remain impure giv- 

 ing progeny heterozygous for yellow because of the death of zygotes 

 with a double dose of the factor for yellow. A dwarf wheat iso- 

 lated by Vilmorin cannot be fixed since it always remains hetero- 

 zygous throwing tails but never producing homozygous dwarfs. 

 The white female form of the clover butterfly, Colias, was found 

 by Gerould always to give yellow offspring either because of the 

 failure of the gametes carrying white to conjugate or because zy- 

 gotes homozygous for white fail to develop. A form of Drosophila 

 characterized by confluent wings has been found by Metz only in the 

 heterozygous condition, always throwing normals and never breed- 



