42 



THE AMEBIC AN NATURALIST [Vol. XL VIII 



gated by Mr. Shamel and Mr. Halladay from the cross 

 between " Havana" and 4 4 Sumatra" tobaccos, arose 

 through the segregation and recombination of the Men- 

 delian factorial differences of the two plants, 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" variety. It hap- 

 pened that the somatic characters of these varieties ac- 

 count 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 fac- 

 torially than somatically. Besides certain factors com- 

 mon to the two varieties, the factors for leaf number in 

 "Havana" tobacco might be represented by the letters 

 AA, and those of "Sumatra" tobacco by the letters BB, 

 CC, DD, EE. By recombination, this would give plants 

 with a smaller number of leaves than the "Havana" 

 variety and plants with a greater number of leaves than 

 the "Sumatra" variety. Both combinations were ob- 

 tained ; and further, the theory has been shown to be cor- 

 rect by the results of other crosses where both types ap- 

 peared (Hayes, '12) . It is probably unwise to suggest too 

 concrete a factorial analysis of the cross, yet the factorial 

 difference assumed above will account for all of the facts 

 obtained, by simple recombination. We assume a factor 

 in the heterozygous condition to account for the produc- 

 tion of one leaf and a factor in the homozygous condition 

 to account for the production of two leaves. The mean 

 of the " Havana" variety is about 20 leaves and the mean 

 of the "Sumatra" variety about 26 leaves. Somatically 

 there is a difference 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 (pos- 

 sibly two or three) factorial difference that does not show 

 in the two varieties. The meaning of this statement can 



