THE MUTATION THEORY 361 
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 
6. ’ 
D Cc 
Fic. 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, tortuosa; D, defectopunctata. (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. 
The same writer exposed the ovules of plants to radium rays and pro- 
duced marked changes in the germ cells so that they grew into various 
