Vol. 7, 1921 
GENETICS: SETCHELL ET AL. 
55 
back crosses which he made and out crosses of extracted whites to pink 
bear out this analysis. Of his varieties his pink (Maryland Mammoth) 
appears to be identical with our virginica (Maryland), consequently we 
are forced to assume that his carmine is genetically distinct from our red. 
It follows that if we add to the two pairs of factors for flower color which 
we have demonstrated, a third, Pp, for carmine dominant versus pink 
recersive, his results are brought into accord with ours. Certain experi- 
mental results which we have just secured and which we are reserving 
for future treatment demonstrate the correctness of this suggestion. 
A general result of these investigations has been a demonstration of the 
complexity of difference from a genetic standpoint between any two of 
these so-called fundamental varieties of Tabacum. In one sense this result 
confirms the opinion of Comes and of Anastasia as to the manner of origin 
of the vast assemblage of Tabacum varieties. It goes further, however, 
and demonstrates the futility of seeking to determine affinities on the 
basis of morphological studies unaccompanied by experimental investi- 
gations. Thus, for example, we have seen such puzzling segregation 
products as the auriculata and loriifolia leaf types, the former with its 
peculiarly constricted leaf base, the lat f er with a much narrower leaf 
blade than either parent, coming out of the angustifolia-macropkylla series. 
Moreover, the demonstration of the existence of genetically distinct red- 
flowering varieties is another evidence of the limitations of purely morpho- 
logical studies and of the errors in the determination of affinities to which 
such studies are subject. The genetic studies outlined above indicate 
merely that Tabacum is a group species like corn, barley, oats, etc., pos- 
sessing a complex series of allelomorphic contrasts. The so-called funda- 
mental varieties of Tabacum intercross freely and produce fully fertile 
progenies. They cannot genetically, therefore, be regarded as represent- 
ing anything but a few very distinct genotypes. A demonstration that 
a few such varieties may contain within them the possibility by means of 
recombination of producing a host of secondary varieties does not really 
demonstrate that they are fundamental. In fact when we consider the 
fugitive nature of our Tabacum varieties, except in so far as they are kept 
isolated by natural or artificial means, the conclusion appears inevitable 
that we must regard all our varieties as fundamentally equivalent from a 
genetic standpoint. The really significant problem in considering the 
species is the determination of how these allelomorphic contrasts have 
come into existence. These investigations throw no light upon that prob- 
lem. 
We have been interested in the methodology of Mendelian analysis in 
Tabacum. Doubtless one of the reasons why our knowledge of the details 
of inheritance in Tabacum is so meager is because of the prevailingly 
quantitative or semi-quantitative nature of the character differences. 
Even in the case of flower color, the impression given by a series of varie- 
