1922] Setchell-Goodspeed-Clausen : Nicotiana Tahacum 459 



and selection attempting to secure constant races exhibiting various 

 recombinations of the parental characters. The work thus conceived 

 has been carried out in detail in a certain few but seemingly charac- 

 teristic cases. Several different crosses were made in 1909, the first 

 filial generations were grown in 1910, and each year since that time 

 has seen successive filial generations in the field. 



Although the Nicotiana investigations were originally designed to 

 attack experimentally a comparatively simple and definite problem, 

 they have since been greatly amplified in scope. At the present time 

 three rather distinct lines of investigation are actively in progress, viz., 



1. Mendelian inheritance in N. Tahacum. 



2. Inheritance of quantitative characters. 



3. Studies of interspecific hybrids. 



The recent appearance of bud variations in hybrid lines favorable for 

 an analytic study of that phenomenon has resulted in the addition 

 of another research project. Now it has been found that, although 

 seemingly distinct, progress in these separate lines of investigation is 

 more or less interdependent. In particular it has been found that 

 certain of the phenomena exhibited in interspecific hybrid populations 

 from crosses between N. sylvestris and varieties of N. Tahacum require 

 for satisfactory analysis and explanation an accurate and detailed 

 knowledge of the Mendelian differences which exist among the par- 

 ticular varieties of N. Tahacum that have been used in those investi- 

 gations. Accordingly in later years these studies of hybrids between 

 varieties of N. Tahacum, originally designed merely to test experi- 

 mentally the interrelationships existing among such varieties, have 

 been directed toward a specific Mendelian analysis of the germinal 

 differences existing in a selected set of varieties. 



With this change in emphasis has come a full appreciation of the 

 difficulties of Mendelian studies in N. Tahacum. It has been very 

 evident that, for the most part, the character differences among varie- 

 ties of N. Tahacum do not rest upon a simple genetic basis; on the 

 contrary, they often depend upon very complex and involved Men- 

 delian differences ; so that in segregating populations it is often im- 

 possible to demonstrate the existence of definite, discontinuous char- 

 acter classes. Not uncommonly the members of such populations maj'' 

 be arranged in series connecting by imperceptible differences the most 

 extreme character expressions in the population. 



But although complex intergrading segregation has often been 

 observed in F2, it has not been found that such complex segregation 



