1917] Goodspeed-Clausen: F^ Species Hybrids in Nicotiana 321 



The above evidence, therefore, although not above reproach, indi- 

 cates clearly that hybrids between species of Nicotiana often do exhibit 

 a rather close resemblance to one or the other of the parents involved, 

 instead of displaying an intermediate behavior. The suggestion has 

 therefore been made that in such cases distinct hereditary systems 

 have been contrasted, one of which apparently has gained the ascend- 

 ancy nearly or quite to the exclusion of the other. This behavior we 

 have taken merely as a clue to the type of relations which exist in 

 species crosses. It is not necessary for the sake of the argument that 

 either species should display a predominating influence in the hybrid, 

 but in the absence of such effects, it can readily be seen that the 

 behavior called for upon the assumption of relations dependent upon 

 system contrasts might be obscured beyond recognition, just as is 

 Mendelian behavior in experiments which involve a large number of 

 factor differences. 



The data bearing upon inheritance in species hybrids of Tahacum 

 and sylvcstris are reserved for a forthcoming paper. The present 

 paper has been designed merely to deal with the resemblances of 

 Tabacum-sylvestris hybrids to the particular Tahacum variety used 

 in the crosses. 



SUMMARY 



1. Crosses between sylvestris and varieties of Tahacum always dis- 

 play the characters of the particular Tahacum variety used in the 

 cross, but usually on a greatly enlarged scale. 



2. The predominating influence of Tahacum in such crosses is 

 ascribed to the dominance of the Tahacum reaction system as a unit 

 when contrasted with the hereditary reaction system of sylvestris. 



3. An examination of the literature dealing with species hybrid- 

 ization shows that the case of varieties of Tahacum- and sylvestris is 

 not unique, but that in many other crosses one or the other parent 

 exerts a more or less greatly predominating influence. 



Trati.smitfcd June JS, 1916. 



