No. 601] MENDELIAN FACTOR DIFFERENCES 



45 



be considered, the corresponding factor A' in the other 

 system, if there be such a factor, might be just as differ- 

 ent from ^ as a lethal factor is different from its normal 

 allelomorph. Further, from a modern Mendelian view- 

 point there is no basis for assuming that recombinations 

 could be obtained involving only exchanges in isolated loci 

 in the systems. For if the behavior in segregation in the 

 Fi hybrids corresponds to that in Drosophila then such 

 combinations as could be obtained would depend on the 

 shifting of entire choromosomes or in case of crossing- 

 over of relatively large portions of chromosomes. Tlie 

 recombinations obtained in the hybrids between Tahacum 

 varieties and sylvestris, provided chromosome distribu- 

 tion takes place after the normal fashion in these hybrids, 

 would involve, therefore, for the most part, the formation 

 of systems containing either whole chromosomes or large 

 sections of chromosomes of opposing systems. Such 

 attempted reconstruction of systems might well fail in 

 cases of any marked specificity in the relations of the 

 factors of the opposing reaction systems, since any large 

 proportions of both systems in a gamete might conceiv- 

 ably destroy the continuity or the balanced relations neces- 

 sary for the continuance of system reactions. When the 

 proportions of one or the other system are relatively 

 small, the system reactions miglit be merely disturbed, 

 resulting in the production of the abnormal forms which 

 have been secured in our cultures. It is to such relations 

 between the two systems involved that we ascribe the 

 selective elimination of the greater portion of the possible 

 gametic combinations in the crosses between Tahacion 

 varieties and sylvestris, and which, therefore, results iu 

 a high degree of sterility in these hybrids. 



It is of course obvious that thevo are ninny categories 

 of sterility, in the hereditary s.-n^' n< as in the physi- 

 ological sense. The particular typo luMvin considered is 

 that which results from relatively wide crossing such as is 

 involved in species hybrids. That certain types of steril- 



