THE MUTATION THEORY 



361 



produced a small number of offspring with distinctly changed charac- 

 ters. These turned out to be mutants because they bred true to the 

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

 {Leptinotarsa decemlineata) are shown in Figure 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 radically 



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

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

 pallida; C, tortiiosa; D, defectopimctata. {From Tower.) 



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 effect upon the chromatin. 

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



