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THE AMERICAN NATURALIST [Vol. XLVI 



fact suggesting segregation is the increased variability of the second as 

 compared with the first hybrid 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 

 hypothesis involves one or both of these assumptions. Both are un- 

 necessary. Variability would result whether the growth-inducing sub- 

 stances were localized or not, provided only that they were not homo- 

 geneous 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 substances (though 

 not its total content of such substances) and this heterogeneity of struc- 

 ture would lead to greater quantitative variation in such materials 

 among the gametes arising from the heterozygote. Thus greater varia- 

 bility would appear in the second hybrid generation. 



I can not agree with this statement as I understand it, 

 though this disagreement may be dne to my own limita- 

 tions. We do not stretch Mendelism and we do not make 

 it cover such cases. The facts of breeding have been ob- 

 tained and the Mendelian notation expresses them. That 

 is all that it is necessary to claim. It is not precisely 

 true, however, to say that increased variability in the 

 second hybrid generation is the only fact to be expressed. 

 It is of paramount importance that various F 2 individ- 

 uals giving F 3 populations differing in mean and in 

 variability, should be included in the Mendelian descrip- 

 tion. They are included. 



Again, Castle states that the genotype conception as- 

 sumes the localization of the hypothetical factors either 

 in chunks of uniform and unvarying size, or that they are 

 carried by a particular number of chunks. I am unaware 

 of any such assumptions. It is true that some such pic- 

 ture has been suggested as a diagram helpful to the 

 imagination in its conception of the scheme as a me- 

 chanical process, but this is purely and simply a dia- 

 gram. The real matter under discussion is that the 

 breeding facts are adequately described in a notation 

 essentially Mendelian. 



