SPECIES CROSSING 193 



is a considerable proportion of plants that are pure syl- 

 vestris in all their characters. These plants are fertile 

 and breed true to sylvestris. They must be supposed to 

 have come from ovules with a sylvestris chromosome 

 group, fertilized by sylvestris pollen. There are also 

 plants that resemble sylvestris, but contain other ele- 

 ments, probably derived from the Tabacum group of 

 chromosomes. They are sterile. 



Back-crossing to Tabacum was unsuccessful, but a few 

 hybrids have appeared in the field from open pollination 

 that are like Tabacum and have undoubtedly come from 

 Tabacum pollen. Some of them are fertile. Their descend- 

 ants never show sylvestris characters. They exhibit segre- 

 gation for whatever Tabacum genes were present. There 

 are also sterile forms in the series, and these resemble the 

 Fi hybrids between Tabacum and sylvestris. 



These remarkable results are important in another re- 

 spect. The Fi hybrid may be obtained both ways; i.e., 

 either species may be the ovule parent. It follows that 

 even with a sylvestris protoplasm the Tabacum group of 

 genes completely determines the character of the indi- 

 vidual. This is strong evidence in favor of the influence 

 of the genes in the determination of the character of the 

 individual, since this result is obtained when the proto- 

 plasm belongs to a widely different species. 



The idea of a reaction system, proposed by Clausen 

 and Goodspeed, while novel, contains nothing in principle 

 that is opposed to the general interpretation of the gene. 

 It means only that the haploid set of genes of sylvestris, 

 when placed in opposition to the haploid set of genes of 

 Tabacum, is totally eclipsed and ineffectual. The sylves- 

 tris chromosomes, nevertheless, retain their identity. 

 They are not eliminated or-injured, since from the hybrid 

 a set of functional sylvestris chromosomes may be re- 

 gained in back-crosses to a sylvestris parent. 



