6 Varietäten, Descendenz, Hybriden. 



tobacco arose through the segregation and recombination of the 

 Mendelian factorial differences of the „Havana" and „Sumatra" 

 tobaccos, from which it is a cross, and not as a mutation. It is 

 simply a union of the factors that stand for leaf size and height of 

 plant in the „Havana" variety with the factors that bring about leaf 

 shape and high number of leaves in the „Sumatra" variet5^ It 

 happened that the somatic characters of these varieties account for 

 all the characters of the hybrid. At the same time one must 

 remember that strains were obtained by selection that averaged 

 higher in number of leaves than did even the „Sumatra" parent. 

 We can only conclude from this fact that the difference between 

 the „Havana" and the „Sumatra" varieties in leaf number is greater 

 factorially than somatically. Besides certain factors common to the 

 two varieties, the factors for leaf number in „Havana" tobacco 

 might be represented bj^ the letters AA, and those of „Sumatra" 

 tobacco by the letters BB, CC, DD, EE. ßy recombination, this 

 would give plahts with a smaller number of leaves than the „Havana" 

 variety and plants vsrith a greater number of leaves than the 

 „Sumatra" variety. Both combinations vs'ere obtained. 



It is probably unwise to suggest too concrete a factorial analysis 

 of the cross, yet the factorial difference assumed above will account 

 for all the facts obtained, by simple recombination. We assume a 

 factor in the heterozygous condition to account for the production 

 of one leaf and a factor in the homozygous condition to account 

 for the production of two leaves. Somatically there is a difference 

 between the „Havana" and the „Sumatra" variety of 6 leaves or 

 three factorial pairs for which to account. But in order to have the 

 theory coincide with the facts there must be at least one (possibly 

 two or three) factorial difference that does not show in the two 

 varieties. The meaning of this Statement can be best shown by an 

 Illustration. The 20 leaves of the „Havana" variety and the first 20 

 leaves of the „Sumatra" variety are represented by 10 pars of 

 factors, of which nine are the same and one different in the two 

 strains. The „Havana" variety is nine leaf factors plus AA, the 

 first 20 leaves of the „Sumatra" variety are none leaf factors (the 

 same as those in the „Havana") plus BB. The additional leaf factors 

 of the „Sumatra" are CC, DD, EE. With these assumptions, the 

 recombinations of a tetra-hybrid will represent our facts fairly 

 accurately. 



The essential part of the writers's conception of this hybrid 

 type is that recombinations of characters quantitative in their nature 

 can be expected and predicted in crosses in exactly the same manner 

 as is done with qualitative characters. On the other band, it must 

 be borne in mind that here was a hybrid type that appeared to 

 be breeding true to the general characters that we have described , 

 in the F^ generation. That it was not breeding true is clear from 

 the results of the selection experiments. yet out of the small number 

 of F5 and Fß families taken under Observation at least two were 

 found to be breeding true for all practical purposes in the F^ and 

 Fg generations. We were able to reproduce the „Havana" type by 

 continued selection in Family 77 and were able to produce strains 

 breeding approximately true to 30 leaves or so by the selection of 

 mother plants in several families. But we can not say that any of 

 our families are now fixed so that no progress can be made by 

 selection. We can say that some of them are so constant that it 

 would be a loss of time for selection to be continued for economic 



