392 HEREDITY AND ENVIRONMENT 



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). 



It is probable that mutations are not always 

 nor even usually associated with changes in 

 the number of chromosomes. Where the num- 

 ber of chromosomes remains constant the 

 change may take place in the number or com- 

 position of the chromomeres or units of the 

 next lower order, but it would be practically 

 impossible to find such changes in bodies so 

 small and so numerous. Whatever the cellu- 

 lar changes may be which accompany muta- 



