No. 602] MENBELIAN FACTOR DIFFERENCES 99 



Mendelian units making up the reaction systems. Ap- 

 parently this will account satisfactorily for the orderly 

 resemblance of these chromosome "mutants" to each 

 other and to the forms from which they arise. Conceiv- 

 ably the great body of data on which the mutation theory 

 is based would, for the most part, find a simpler expla- 

 nation along the lines thus indicated. 



Summary 



Summarizing briefly the content of this paper, the fol- 

 lowing facts have, on the experimental side, been pre- 

 sented with reference to a species hybrid: 



1. N. sylvestris when crossed with various varieties of 

 N, Tahacum gives Fj hybrids which are replicas on a 

 large scale of the particular Tahacum variety concerned 

 in the cross. 



2. The Fj hybrids of sylvestris and Tahacum produce 

 a small number of functional ovules which represent the 

 sylvestris and Tahacum extremes of a recombination 

 series, the great majority of the members of which fail to 

 function because of mutual incompatibility of the ele- 

 ments of the two systems. 



3. Back crosses with sylvestris give sylvestris and 

 al)errant forms, and of the two the sylvestris alone are 

 fertile and breed true. On the other hand, back crosses 

 with Tahacum produce apparently only Tahacmn forms 

 of which some are completely fertile and continue to pro- 

 duce only Tahacum forms. 



On the theoretical side the following conclusions have 

 been drawn and their application indicated : 



1. As a consequence of modern Mendelian develop- 

 ments, the Mendelian factors may be considered as mak- 

 ing up a reaction system the elements of which exhibit 

 more or less specific relations to one another. 



2. Strictly Mendelian results are to be expected only 

 when the contrast is between factor differences within a 

 common Mendelian reaction system as is ordinarily the 

 ease in varietal hybrids. 



