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 
Fie. 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 
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