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THE AMERICAN NATURALIST [Vol. XL.VI 



differentiation of the germ-plasm expressed by the ap- 

 pearance in the F 2 plants of definite tendencies in dif- 

 ferent directions towards the parents of the cross. This 

 sems to the writer the essential principle of Mendel- 

 ism and does not necessarily involve the acceptance of 

 the doctrine of unit characters and their segregation in 

 either modified or unmodified form. 



'2. Certain characters of the parent species have ap- 

 pealed in the F., segregates in apparently pure condition 

 but the very large range of intermediate conditions indi- 

 cates that factors governing the form and measurements 

 of organs (if present at all) must in some cases be con- 

 cerned with characters so numerous and so small that 

 they can not he separated from the possible range of 

 fluctuating variations. If this is true such characters 

 seem beyond tlie possibility of isolation and analysis and 

 the unit character hypothesis for these cases has little 

 more than a theoretical interest. 



Both cultures certainly showed a marked progres- 

 sive advance in the range of flower size, the largest 

 flowers having petals somewhat more than 1 cm. longer 

 than those of the ffmiirU flora parent. There was a 

 -iniilar advance in the size of the leaves and the extent of 

 their crinkling. These progressive advances would 

 seem to demand on the unit character hypothesis either the 

 modifications of the old or the creation of new factors. 



4. The absence of classes among the F 2 hybrids (except 

 for the dwarfs) further works against the unit character 

 hypothesis as of practical value in the analysis of a 

 hybrid generation of this character. It should be remem- 

 bered, however, that there were in this cross no sharply 

 contrasted distinctions of color, anthocyan coloration 

 proving most unsatisfactory for the purposes of a gene- 

 tical study. 



5. Both cultures presented a class of dwarfs, very 

 different from one another and from the F x hybrid plants 

 10.30Lrt and 10.30L&. This phenomenon of nanism has 



