46 E. B. Babcoc\ 



some group contains one 8-chromosome species. It has been 

 pointed out by Babcock and Cameron'' that the most likely 

 process by which 8-chromosome species might arise from 10- 

 chromosome ancestors would be by reciprocal translocations 

 between nonhomologous chromosomes, followed by meiotic ir- 

 regularities leading to disappearance of one pair of chromo- 

 somes. The disappearance of a pair of chromosomes, however, 

 does not necessarily imply actual loss of any essential genes. 

 Reduplications of small sections of chromosomes are known to 

 occur and these might compensate for the loss of fragments lack- 

 ing a spindle-fiber attachment. There has come to light in recent 

 years abundant evidence that reciprocal translocations between 

 nonhomologous chromosomes occur in nature and that they 

 may have a profound effect on genetic relationships. A mecha- 

 nism is known to exist, therefore, which makes possible the ori- 

 gin of 8-chromosome species from lo-chromosome progenitors. 



The other fundamental process involved is that of interspe- 

 cific hybridization and amphidiploidy. That this phenomenon 

 may be of great importance in the evolution of the higher plants 

 has been amply demonstrated by the production, through arti- 

 ficial hybridization, of highly fertile and constant new forms 

 which might be expected to maintain themselves under natural 

 conditions. 



The classical example to be investigated cytologically is that of 

 Nicotiana digulta of Clausen and Goodspeed,"' " who obtained 

 it by crossing N. tahacum (n = 24) with A/", glutinosa (n = 12) . 

 N, digltita has 36 pairs of chromosomes, is fairly fertile, and 

 reproduces itself constantly. But when it was crossed with the 

 parental species, the investigators found that the hybrid progeny 

 were more or less fertile and displayed a tendency to revert to 

 the types of the parental species. Thus, although N. digluta 



