THE MUTATION THEORY 



361 



new characters. Three of Tower's mutants of the potato beetle 

 (Leptinotarsa decemliueata) are shown in I-'iirure 60. 



MacDougal injected into the ovules of various species of plants 

 such foreign materials as solutions of zinc salts, cane sugar, etc. The 

 seeds produced from these plants developed into plants with radicallv 



Fig. 60. — Some divergent types (mutations) of beetles produced by subject- 

 ing the germ cells to external influences. A, normal dcccmlincata; B, the form 

 pallida; C, tortuosa; D, dcfecio punctata. {From Tinccr.) 



new characters (Fig. 61) which bred true to type for four or more 

 generations. Whether or not the changes persisted longer we do not 

 know, since MacDougal has not published any further details. 



Gager discovered that the action of radium rays on the pollen 

 grains of various plants had a profound etTect uix)n the chromatin. 

 Some of the latter was apparently lost during mitotic cell division. 

 The same writer exposed the ovules of plants to radium rays and |-)ro- 

 duced marked changes in the germ cells so that they grew into various 



