THE COEFFICIENT OF MUTATION IN OENOTHERA BIENNIS L. 9 



as follows: The genera Jussieua, Zauschneria, Epilobium, Bois- 

 duvallia and Lopezia are wholly fertile; they show neither rudi- 

 mentary ovules nor sterile pollen grains. Only in Epilobium and 

 Boisduvallia some rare pollen tetrads may sometimes miscarry. 

 In the genera Clarkia, Eucharidium , Godetia and Gaura all the 

 ovules are fertile, but among the pollen grains about 30 per cent 

 are sterile. Kneiffia, Xylopleurum, and Lavauxia have some rudi- 

 mentary ovules as well as sterile pollen grains (10-50 per cent). 

 In the genus Oenothera, with the subgenera Onagra, Euoenothera, 

 andAnogra, the percentage of sterility is about 50 per cent in the 

 ovary as well as in the anthers. In the first group about 40 species 

 were studied, in the second 30, in the third 10 and in the last 

 40, making together about 120 species. If in the last three groups 

 some species were pure and devoid of sterile sexual cells, they 

 would no doubt have been discovered, and the supposition that 

 the remainder might be considered as their hybrids would have 

 found support. But this was not the case, and if we wish to ascribe 

 the presence of all these sterile sexual cells to ancestral crosses, the 

 crosses must be supposed to have taken place, or at least to have 

 begun, among the ancestors of the whole family, with the exception 

 of the Lopezieae, the Jussieueae and the Epilobieae. It seems 

 hard to have to suppose that the whole pedigree of the Xylo- 

 pleurinae, the Clarkiinae and the Oenotherinae should have had to 

 go through the development of partial sterility in order to produce 

 the present mutability of Oenothera Lamarckiana and half a dozen 

 or perhaps even a dozen of its nearest allies. 



The second main supposition, namely that hybridism might 

 be a cause of mutability, is dealt with by Jeffrey in a particular 

 way. He assumes "that there is every reason to suppose that it 

 has been an agency of great importance in multiplying species, 

 although it is logically inconceivable in the present state of our 

 biological knowledge that it could have presided at their origin." 

 The first of these two alternatives represents, so far as I can see, 

 a conviction which is at least very widely spread among biologists 

 ever since the time of Linnaeus. It by no means contradicts the 

 theory of natural selection, nor that of mutation, nor any other 

 evolutionary principle. It has no obvious reference to the phe- 

 nomena observed in the evening primroses, since with them the 



tiellen Sterilitat von Oenothera Lamarckiana, Amsterdam, pp. 114, mit 

 24 Tafeln, 1901; see p. 93. 



