30 



GENETICS IN RELATION TO AGRICULTURE 



With animals the best known experiments on the artificial production 

 of germinal variations are those of Tower who worked with the Colorado 

 potato beetle, Leptinotarsa decemlineata, and related species. Like other 

 arthropods these beetles are more directly under the influence of tempera- 

 ture changes at least than are warm-blooded animals. Tower first de- 

 termined the period in ontogeny when external stimuli will affect the 

 germ cells. He found that in Leptinotarsa the germ cells do not 

 become susceptible to external stimuli until after the time in ontogeny 

 when the color pattern of the individuals subjected to the stimuli can be 

 influenced. He found that eggs were most susceptible just before and 

 during maturation and this observation is in agreement with those of 

 Fischer, Standfuss, Weismann and others who have conducted similar 



FIG. 13. A, Leptinotarsa decemlineata and three mutants; B, tortuosa; C, pallida; D, 

 defectopunctata. (After Tower.} 



investigations. Tower concluded that certain individuals from the germ 

 cells of a stimulated parent "show intense heritable variations, whereas 

 those not acted upon do not show these changes. " Most of the inherited 

 variations involve changes in the pigmentation of the body parts. In 

 certain cases there was an actual change in the color pattern (see Fig. 

 13). It is to these results that Tower attaches the greatest significance 

 inasmuch as most similar experiments have not succeeded in causing 

 pattern changes. In spite of the elaborateness of Tower's methods con- 

 siderable skepticism exists regarding the validity of his conclusions, and 

 this has not been lessened by the non-appearance of confirmatory data. 

 In a recent paper he reports the production of very striking germinal 

 modifications in L. decemlineata as a result of subjecting a morphologically 

 homogeneous race to an extreme change in environment. However, it 

 is still a question whether the material used may not be heterogeneous 

 as regards the germinal factors that condition certain physiological 

 characters. 



Stockard's investigations on the effect of alcohol on the progeny of 

 guinea pigs have shown that the germ cells as well as the somatic tissues 



