CONTROL OF HEREDITY: EUGENICS 403 



There is some evidence, as was pointed out 

 in the last chapter, that environmental changes 

 of the right sort acting upon germ plasm at the 

 right stage may lead to permanent modifica- 

 tion of the hereditary organization. Extrinsic 

 influences acting upon germ cells at the time 

 of their maturation divisions may lead to new 

 distributions of chromosomes or even to 

 changes in the composition of individual chro- 

 mosomes, thus producing new hereditary 

 types. Certain mutants of Oenothera (Fig. 

 95) seem to be of this sort; for example O. 

 lamarckiana has 14 chromosomes, O. lata 15, 

 O. semi-gigas 21, O. gigas 28, and these vari- 

 ations in the number of chromosomes are prob- 

 ably due to abnormalities in the maturation 

 divisions. It is significant that the mutants 

 lata and semi-gigas have occurred several 

 times, whereas gigas appeared but once; this 

 may be explained by the fact that the chances 

 of the doubling of chromosomes in both germ 

 cells (gigas) are very few compared with the 

 chances of their doubling in one germ cell 

 (semi-gigas) or of their increase by one in one 

 germ cell (lata). 



