OENOTHERA GRANDIFLORA AIT. 235 



qualities of the type of the species will be dominant, whereas the 

 lethal factor must be recessive. 



Among the living seedlings the proportion of green hybrid mu- 

 tants and yellowish ochracea must therefore be 2 : 1, and the average 

 figure for the latter was 26 per cent, although this was somewhat 

 too low on account of the loss of part of the yellow seedlings in 

 early youth. Artificial crosses between the hybrid mutants and the 

 ochracea should give about 50 per cent of either type. I found for 

 both the reciprocal crosses about 34 per cent, but the figure was 

 depressed from the same cause. A repetition of these experiments, 

 excluding the influence of these losses, is proposed; it is expected 

 to give a fuller proof. 



We assume the supposed initial mutations to have been analo- 

 gous to the mutations into lorea and gigas, which may still be ob- 

 served to occur in my garden. New mutations into ochracea may 

 occur also, but they must evidently always escape observation, 

 being different in no respect from the secondary or mass mutation. 



Bartlett has pointed out the analogy between the phenomenon 

 of mass mutation and Mendelian splitting, observing, however (1, 

 p. 452), that "there can be no doubt that mass mutation is not 

 Mendelian segregation, although the tv/o phenomena have points of 

 resemblance." In our instance this resemblance is plain enough, 

 but a splitting is called Mendelian if it is observed among the pro- 

 geny of hybrids between different species, varieties, or strains, 

 whereas the half mutants are hybrids between mutated and non- 

 mutated sexual cells of the same parent. They are produced by one 

 experimental pure line, whereas real hybrids are the result of the 

 combination of different strains. The hybrid mutants start from 

 a mutation; they can never be made use of as an argument against 

 the mutation theory. The names of mass mutation and secondary 

 mutation, therefore, seem to be very appropriate, indicating, as 

 they do, the true explanation of the phenomenon. 



G. Twin hybrids, considered as a result of mass mutation 



In Gruppenweise Artbildung (5) I have devoted a laige part to 

 the study of the twin hybrids of 0. Lamarckiana and its derivatives 

 in their crosses with other species. I was convinced that some rela- 

 tion must exist between the cause of this curious phenomenon and 

 the high degree of mutability of the species. I supposed this internal 

 cause, whatever it might be, to be responsible in a large degree, 



