No. 546] INCONSTANCY OF UNIT-CHARACTERS 361 



ibility that if in a case such as this dominance were 

 wanting, i. e., if the cross always produced intermediates, 

 the bulk of the second-generation offspring would also be 

 intermediate, with only an occasional complete segrega- 

 tion. He suggests that size differences may involve units 

 of this sort, without dominance though fully segregating. 

 This attractive hypothesis would account for the known 

 facts of size inheritance fairly well, involving only the 

 existence of multiple units which may be perfectly stable 

 and changeless in character. Nevertheless this hypothe- 

 sis has not been established beyond question. It is quite 

 possible that we are stretching Mendelism too far in 

 making it cover such cases. Dominance is clearly absent 

 and the only fact suggesting segregation is the increased 

 variability of the second as compared with the first hy- 

 brid generation. This fact however may be accounted 

 for on other grounds than the existence of multiple units 

 of unvarying power. 



If size differences are due to quantitative variations 

 in special materials within the cell, it is not necessary to 

 suppose that these materials are localized in chunks of 

 uniform and unvarying size, or that they occur in any 

 particular number of chunks, yet the genotype hypothe- 

 sis involves one or both of these assumptions. Both are 

 unnecessary. Variability would result whether the 

 growth-inducing substances were localized or not, pro- 

 vided only they were not homogeneous in distribution 

 throughout the cell. Crossing would increase variability 

 beyond the first generation of offspring because it would 

 increase the heterogeneity of the zygote in special sub- 

 stances (though not its total content of such substances) 

 and this heterogeneity of structure would lead to greater 

 quantitative variation in such materials among the 

 gametes arising from the heterozygote. Thus greater 

 variability would appear in the second hybrid genera- 



