42 



THE AMEBIC AN NATURALIST [Vol. LI 



Mendelian reaction systems and the consistent reproduc- 

 tion of all Tahacum characters, whether qualitative or 

 quantitative, indicates at one and the same time that these 

 are fundamentally of the same nature, depending essen- 

 tially for their expression on a complex set of Mendelian 

 factors, and, moreover, that the Tahacum system as a unit 

 dominates the course of somatogenesis and determines the 

 reaction end products of the two systems. 



This domination of the somatogenic processes by the 

 Tahacum reaction system is followed by important experi- 

 mental possibilities. If the species hybrids always dis- 

 play the Tahacum characteristics as completely as all our 

 present evidence indicates that they do, then they will 

 furnish a powerful method of attack on the problem of 

 Mendelian behavior in the Tahacum section of the genus 

 Nicotiana. For by crossing hybrids between Tahacum 

 varieties with sylvestris, it should be possible to secure in 

 the partially sterile hybrids resulting a phenotypic repro- 

 duction of the gametic series of the Tahacum parent. 

 This series would not be complicated by intergrading of 

 heterozygous forms, because the plants thus obtained 

 would exhibit the phenotypic characters of homozygotes, 

 and recessive factors as well as dominant ones would be 

 reproduced in their proper place in the Tahacum system. 

 Apparently it should be possible, therefore, to demon- 

 strate the fundamentally similar nature of linkage in 

 Nicotiana and Drosophila. Such an analysis will still be 

 very difficult in Nicotiana on account of its high chromo- 

 some number, but by the method of procedure outlined 

 above some distinct advance at least seems perfectly 

 feasible. These, however, are matters on which we have 

 as yet very little data. 



Since, therefore, the Tahacum reaction system domi- 

 nates the somatogenic processes in the hybrid to nearly 

 or quite the exclusion of the sylvestris system, the ele- 

 ments of the two systems must be largely mutually in- 

 compatible. Free interchanges between the two systems 

 would not, therefore, necessarily result in the formation 



